Theory of Operations - Airmar Technology

Theory of Operations

35 Meadowbrook Drive, Milford, New Hampshire 03055Tel 603 -673-9570 • Fax 603-673-4624• www.airmar.com AIRMAR

TECHNOLOGY CORPORATION17-256-01 Rev. 01

AIRMARTECHNOLOGY CORPORATION

Contents

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

A. Airmar Technology Corporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

B. Echosounder Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

C. Transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

D. Frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

E. Piezoceramic Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

F. Sound Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

G. The Structure of Transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

H. Performance Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

I. Airmar Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

J. Glossary of Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19



The purpose of this booklet is to inform you about the products that are designed and manufactured by Airmar and give you a basic understanding of how they work and how they are used. Our customers depend upon us for products that meet theirrequirements. As a member of the Airmar Team, your quality work is a vital part ofmeeting those requirements and contributes to maintaining Airmar’s leadership position in the transducer industry.

The information in this booklet will help you to better understand how significant whatyou do is, and how it fits into the overall picture. Included is a glossary of frequentlyused scientific terminology. If you have any questions not answered by this booklet,please submit them to your manager.

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What is Airmar?Airmar Technology Corporation was founded in Amherst,New Hampshire in 1982 by two engineers, Stephen G.Boucher and Robert K. Jeffers. Airmar moved to Milford in1987 where it continues to grow as a leader in the trans-ducer industry.

At Airmar, we pride ourself on our ability to work hand-in-hand with our customers to fulfill their specific require-ments with the highest quality products. This may bedone with one of our existing products, a slight modifica-tion to a standard product, or an entirely new design.

Airmar’s first product in 1982 was a simple, marine, ultra-sonic transducer. Today, marine transducers remain thecore of our product line with over 1,000 transducer partnumbers available. We now offer:

• Recreational sensors for echosounders, fishfinders, andpersonal watercraft

• Commercial fishing transducers for vessels of all sizes

• Large navigation and survey transducers for use on shipsand research vessels

• Aquaculture systems

• Air transducers for industrial uses

Airmar holds over twenty-five United States and foreignpatents. We have won the important Innovation Awardpresented by the International Marine Trade ExhibitionConference for the first truly low-cost phased array trans-ducer developed for use in marine environments. Thistransducer allows electronic steering of the ultrasonicbeam giving the user the ability to gather more information.

Designers, engineers, and scientists develop the products.Assemblers on the manufacturing floor produce the sen-sors and a full line of parts and accessories. All this activityis supported by shippers, receivers, maintenance people,and business personnel.

Who are Airmar’s customers?Airmar has a world-wide customer base of OriginalEquipment Manufacturers (OEMs). Our sensors arebought to be coupled with the OEM’s electronics. Usually,the OEM sells the system to the public with its name onthe label.

While many of our customers are echosounder producers,we have customers in other industries as mentioned earli-er. Some of our customers include:

• Raytheon

• Furuno

• Yamaha

• Odom

• Hycontrol

• Lowrance

• Standard Communications

• Interphase

• Kawasaki

• Bombardier

We also sell to one after-market distributor, Gemeco. Thiscompany sells replacement transducers as well as marineaccessories to the public.


Airmar Technology Corporation

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What is a transducer? The transducer is the heart of an echosounder system. It isthe device that changes electrical pulses into sound wavesor acoustic energy and back again. In other words, it is thedevice that sends out the sound waves and then receivesthe echoes, so the echosounder can interpret or “read”what is below the surface of the water.

How does a transducer work?The easiest way to understand how a transducer functionsis to think of it as a speaker and a microphone built intoone unit. A transducer receives sequences of high voltageelectrical pulses called transmit pulses from the echo-sounder. Just like the stereo speakers at home, the trans-

ducer then converts the transmit pulses into sound. Thesound travels through the water as pressure waves. Whena wave strikes an object like a weed, a rock, a fish, or thebottom, the wave bounces back. The wave is said toecho—just as your voice will echo off a canyon wall.

When the wave of sound bounces back, the transduceracts as a microphone. It receives the sound wave duringthe time between each transmit pulse and converts it backinto electrical energy. A transducer will spend about 1% ofits time transmitting and 99% of its time quietly listeningfor echoes. Remember, however, that these periods of timeare measured in microseconds, so the time between pulsesis very short.

What is an echosounder system?An echosounder system is specialized equipment whichgives a boater or fisher the ability to see below the sur-face of the water. It gives information about the depthand shape of the bottom and of any fish present.

An echosounder system is made up of two major parts.The first is the echosounder. This includes a display screento present the information, the transceiver to drive thetransducer and receive echo informa-tion, and a microcomputer to processthe information.

The second key part is the transducer. Itgenerates sound waves and receives theechoes of those sound waves. The infor-mation is fed from the transducerthrough cables to the echosounderwhich interprets and presents the infor-mation in an understandable form onthe display screen.

How does an echosoundersystem work?All echosounder systems work in essen-tially the same way. The echosounder

transceiver generates high voltage electrical pulses andsends them to the transducer. The transducer convertsthese pulses into sound waves that bounce off objectsunderneath the boat, then echo back to the transducer.The transducer converts the sound energy of the echo toan electrical pulse which is returned to the echosounder.The echosounder measures the time between the begin-ning of a pulse of sound and the return of the echo. Themicroprocessor then “reads” this information, translates it,and presents it on a display screen in a way that depicts

the bottom, any objects, and the location ofany fish.

Different echosounders display information indifferent ways. A flasher type echosounderdisplays illuminated bars of varying intensityto depict the depth of the water and anyobjects in the water. A digital echosounderdisplays the bottom depth in numbers or let-ters. Echosounders with LCD and CRT screensdisplay the information in a “picture” form.

What part of the echosoundersystem does Airmar produce?Airmar produces the transducer—the under-water sensor for the echosounder system.


Echosounder Systems

Transducers

3

The echosounder can calculate the time differencebetween a transmit pulse and the return echo and thendisplay this information on the screen in a way that canbe easily understood by the user.

How does a transducer know how deep the water is?The echosounder measures the time between transmittingthe sound and receiving its echo. Sound travels throughthe water at about 4,800 feet per second, just less than amile per second. To calculate the distance to the object,the echosounder multiplies the time elapsed between thesound transmission and the received echo by the speed ofsound through water. The echosounder system interpretsthe result and displays the depth of the water in feet forthe user.

How does a transducer know what the bottom looks like?As the boat moves through the water, theechoes of some sound waves return morequickly than others. We know that allsound waves travel at the same speed.When a sound wave in one section ofthe sound field returns more quickly thananother, it is because the wave hasbounced off something closer to thetransducer. These early returning soundwaves reveal all the humps and bumps inthe underwater surface. Transducers areable to detect whether a bottom is soft orhard and even distinguish between a

clump of weeds and a rock, because the sound waves willecho off of these surfaces in a slightly different manner.

How does a transducer see a fish?The transducer can see a fish, because it senses the airbladder. Almost every fish has an organ called an air blad-der filled with gas that allows the fish to easily adjust tothe water pressure at different depths. The amount of gasin the air bladder can be increased or decreased to regu-late the buoyancy of the fish.

Because the air bladder contains gas, it is a drastically dif-ferent density than the flesh and bone of the fish as wellas the water that surrounds it. This difference in densitycauses the sound waves from the echosounder to bounceoff the fish distinctively. The transducer receives theechoes and the echosounder is able to recognize thesedifferences. The echosounder, then, displays it as a fish.


Transducer transmitting pulses Transducer receiving echoes

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What is frequency?Frequency is the number of complete cycles or vibrationsthat occur within a certain period of time, typically onesecond. Sound waves can vibrate at any one of a widenumber of frequencies. The easiest way to understand fre-quency is to think of it in terms of sounds that are familiar.For example, a kettle drum produces a low-pitched sound(low frequency). That is, it vibrates relatively few times persecond. Whereas, a flute produces a high-pitched sound(high frequency). It vibrates many more times per secondthan a kettle drum.

The frequency of sound waves is measured in a unitcalled a Hertz. A Hertz is one cycle per second. For exam-ple: a 150 kHz transducer operates at150,000 cycles per second.

Is the frequency of alltransducers the same?No, transducers can be designed tooperate efficiently at any number ofspecific frequencies depending uponthe application and performancerequirements of the customer. Airmartransducers are often designed for 50kHz (50,000 cycles per second) or200 kHz (200,000 cycles per sec-ond).

Can fish hear the soundwaves produced by atransducer?No, the sound waves are ultrasonic.They are above (ultra) the sound(sonic) that human ears are able to

hear. Humans can hear sound waves from 10 Hz to 20kHz. Most fish are unable to hear frequencies higher thanabout 500 Hz to 1 kHz. The ultrasonic sound waves sentout by Airmar transducers have frequencies ranging from10,000 kHz to 2 Megahertz (200,000,000 Hz), clearlybeyond the hearing of fish. However, most people canhear the transmit pulses of our 10 kHz transducers; theysound like a series of clicks.

How does the frequency of a transducer determine what we see?A higher frequency transducer will put out quicker, short-er, and more frequent sound waves. Like the ripples made

when a small pebble is thrown intostill water, small waves of soundmove evenly out and away from thesource. Because they are just smallwaves, they will not travel far andsmall obstacles will cause them tobounce back. Higher frequenciesare more sensitive to small objectsand will send back detailed informa-tion which will show as crisp, high-resolution pictures on theechosounder screen. The range ofhigh frequency sound waves, how-ever, is short. In fact, sound wavesemitted by a 200 kHz transducerhave a limited range of about 600feet.

Now, think of the large waves creat-ed by a large boulder thrown intostill water. Low frequency soundwaves are like these large waves;they travel much farther than high


Frequencies

Higher Frequency Lower FrequencyNumber of waves or cycles per second more fewer

Wave length shorter longer

Detail more detail, small objects less detail, large objects

Depth Capacity shallow to moderate deeper

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frequency waves. But because low frequency waves areso large, they wash right over small obstacles. Low fre-quency sound waves are not as sensitive in detectingsmall fish or other small obstacles as are high frequencywaves, and although they can see to greater depths, they will not send back detailed information or clear crisp pictures.

Why is it important to know the lengthof a sound wave?Knowing the length of sound waves is particularly impor-tant, because it determines where the sound waves willbounce. A sound wave will bounce strongly off some-thing that is larger than itself. If the object is smaller, thenthe sound wave will almost wash over the object, and theecho will be very weak.

The length of a sound wave is determined by the frequen-cy of the sound vibrations and the density of the mediumthat the sound is traveling through. At Airmar, wavelength is calculated by dividing the speed of sound inwater by the frequency.

The speed of sound in water is4,800 feet per second. If wehave a 200 kHz transducer thenour equation would look likethis:

4800 ft/sec ÷ 200,000 cyc/sec =0.024 ft/cyc = 0.29 inches/cyc

One sound wave at 200 kHz isslightly longer than 1/4 of aninch, so a 200 kHz sound wavewill be able to detect fish asshort as a quarter of an inch.

Let us compare the 200 kHztransducer to the size of a wavelength of a 50 kHz transducer:

4800 ft/sec ÷ 50,000 cyc/sec =0.096 ft/cyc = 1.15 inches/cyc

One sound wave at 50 kHz isslightly over one inch, so a 50

kHz sound wave will only detect fish if their air bladdersare large, slightly longer than an inch.

How does a customer decide what frequency is needed?A higher frequency sound wave will give the user a high-er resolution picture of what is present under the water,but the range will be short. Fishers in more shallow lakeswho want a crisp clear picture of the bottom need a high-er frequency transducer.

Low frequency sound waves will not give the user as cleara picture of the bottom, but they have greater range forvery deep areas where high frequency sound waves can-not reach. A low frequency unit will work well in thedepths of Lake Michigan or the ocean. You may find thechart on page 5 helpful.


Higher frequency waves

Lower frequency waves

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What are piezoceramic elements?The main component of a depth transducer is the piezo-ceramic element. It is the part that converts electrical puls-es into sound waves, and when the echoes return, thepiezoceramic element converts the sound waves back intoelectrical energy.

Airmar does not manufacture its own piezoceramic ele-ments, because their manufacture is a very specializedprocess. Instead, we buy piezoce-ramics from companies that makethem to our specifications.

What does a piezoceramic element look like?Piezoceramic elements are mostoften in a disk form, but they mayalso be in the shape of a bar or aring. A transducer may containone element or a series of ele-ments linked together called an array.

What are piezoceramicelements made of?Most of the piezoceramic elementsthat we use in our transducers aremade of Barium Titanate (BT) orLead Zirconate Titanate (PZT).

How are piezoceramic elements made?The BT and PZT elements go through several processesbefore Airmar receives them.

Pressing—Both BT and PZT begin in powdered form. Thepowder is pressed into the desired shape.

Firing—The pressed shapes are baked in a kiln just like wemight fire a clay pot made in an art class. The tempera-ture of the kiln depends upon the element’s maximumheat tolerance. It is important to fire the piezoceramic atprecisely the right temperature.

Like a piece of china that has been fired in a kiln, thepiezoceramic element is very strong, yet brittle and easilycracked or broken. Any piezoceramic element that hasbeen cracked or chipped, even slightly, will not functionproperly in a transducer.

Coating—After pressing, the piezoceramic element is coat-ed on two opposite sideswith a layer of silver andbaked a second time, sothe silver actually bakesonto the element. This sil-ver functions as the elec-trode, the material thatwill conduct electric cur-rent through the element.

Polarizing—Next thepiezoceramic element ispolarized. Piezoceramicelements are made up ofindividual crystals that have a positive (+) and negative (–)electric charge on respective ends. These crystals are nor-mally resting in a haphazard way in the piezoceramic ele-ment. But if a high voltage electric current is applied tothe element, the crystals will adjust their alignment untilnearly all are positioned in straight columns with theirpositive (+) and negative (–) poles lying in the same direction.

Note: Since this process is done in an oil bath, it is veryimportant that the piezoceramic element has all of the oilcarefully removed or the potting material will not bond to it. A weak bond will result in poor transducer perfor-mance and poor reliability.

How do piezoceramic elements work?Remember, transducers work by taking electrical pulsesfrom the echosounder and changing them into soundwaves. This process is reversed when the transducer isacted upon by the pressure of the returning echoes whichis called transduction.

The internal arrangement of the piezoceramic element’scrystals with their positive (+) and negative (–) poles lying


Piezoceramic Elements

7

in the same direction is the keyfactor. Pulses of alternating cur-rent (AC) from the echosounderactivate the piezoceramic ele-ment. The AC changes its direc-tion of flow back and forth.[Which is why it is said to alter-nate, and this change in thedirection of the flow is noted as(+) and (–).] Because the piezoce-ramic elements are polarized, theywill expand when a positive volt-age is applied and contract whena negative voltage is applied.

The piezoceramic’s expansion andcontraction changes the electricalpulse into sound waves that will travel through the wateruntil they bounce off an object or weaken and finally dissipate.

When an echo returns to the transducer, the pressure ofthe sound waves act on the piezoceramic element caus-ing it first to contract and then toexpand as each cycle in the echo hits it.This alternating pressure on the elementcreates a small voltage which is thensent back to the transceiver and micro-processor.

The element expands and contracts atthe frequency of the electrical pulse. Thisoccurs very rapidly, faster than can beseen by the eye. The frequency of theexpansion and contraction is controlled by the frequencyof the pulse generator in the echosounder.

How do the engineers know whichpiezoceramic element to use?When an electrical voltage is applied to a piezoceramicelement, it will vibrate best at a certain frequency.Piezoceramic materials can be thought of as bells. When abell rings, it produces a tone. Each bell has its own natur-al resonant frequency. Those who cast bells know the sizeand shape necessary to create a bell that produces a cer-tain tone.

Like bells, every piezoceramic material has its own natural

resonant frequencies. The size,shape, and thickness of thepiezoceramic element determinethe frequency at which it willvibrate best. Engineers very care-fully control these factors to pro-duce transducers that resonate atthe correct frequency to meet thecustomers’ needs.

Most of the piezoceramic ele-ments that Airmar uses are thick-ness resonant. The thicknessdimension of the piezoceramicelement, rather than its diameteror shape, determines the resonant frequency.

A transducer can be designed with one piezoceramic thatoperates at two frequencies. Our popular 50/200 kHztransducer houses a piezoceramic element that canvibrate efficiently at two separate frequencies. It resonatesat 200 kHz in the thickness mode and at 50 kHz across its

diameter which is called the radial mode. A transducerthat can operate at two frequencies will have the charac-teristics of both frequencies—the ability to “see” well inboth shallow and deep water with good bottom definition.

What is capacitance?Capacitance comes from the word capacity. It is the abilityof a material to store an electrical charge. Piezoceramicelements are first class capacitors, able to hold a largeelectrical charge. In fact, the larger the piezoceramic thelarger the charge that can be stored.

Knowing that piezoceramic elements store a charge and


Piezoceramic expandswhen + voltage applied

Piezoceramic contractswhen - voltage applied

Vibrates across the thickness

Vibrates across the diameter

Vibrates across the thickness

and the diameter

8

that even slight cooling and heating will build-up an elec-trical charge, means that all piezoceramics must be han-dled with great care. Production workers must alwaysshort a piezoceramic before handling it. If this is not done,the piezoceramic may discharge, damaging other compo-nents in a multi-sensor transducer or even giving the han-dler a very nasty, albeit harmless, shock.

In spite of the precautions that need to be taken with

piezoceramics, their ability to store an electrical chargecan be used to our advantage. With a simple capacitancemeter, we can test the piezoceramic after wires have beensoldered in place and the cable has been attached. If awire connection is faulty, only a small capacitance willshow on the capacitance meter. A much larger capaci-tance will show if the piezoceramic is wired properly. Thisis an easy check of the manufacturing process.


Sound Waves

How do the sound waves travel in the water?Understanding wave motion can help you understand the way in which transducers work. When you throw apebble into still water, you can see small ripples or wavesform around the spot where the pebble entered the water. If you watch closely, you will see these circular ripples or waves move evenly away from the center. Whenwaves move in this manner, they are said to move in concentric circles.

When sound waves are transmitted, they too begin tospread out as they make their way deeper into the water.Because the sound waves have been transmitted underthe boat, they are moving downward and outward in con-centric circles. Since the downward and outward move-ment is happening at the same time, the waves are actual-ly making a cone shape. This is referred to as the radiationpattern. Thinking of the sound waves transmitted from atransducer as an ice cream cone turned up side down willhelp visualize the sound field for a typical single-ceramic transducer.

How wide is the radiation pattern made by a transducer?We refer to the widest part of the cone-shaped radi-ation pattern as the beamwidth. It is the diameterof the outer most circle of sound waves. Engineersare able to increase or decrease the beamwidth andtherefore the area that the transducer can “see”.

One way to change the beamwidth is to use piezo-ceramic elements of different diameters. The largerthe piezoceramic, the smaller and more concentrat-ed is the sound beam; the smaller the piezoceram-

ic, the wider andless concentratedis the soundbeam. For exam-ple, the beam-width producedby the smaller,one inch, piezo-ceramic will “see”fish over a largearea, whereas alarger, two inch,piezoceramic willprovide a narrow-er beamwidthgiving better bottom definition and detection of small fishin shallow water.

How much underwater area can be seenby an echosounder?Engineers determine how wide anarea will be seen on the water’sbottom by knowing the depth ofthe water and the transducer’scone angle. If you have a mathe-matical calculator or trigonometrytables, you can calculatebeamwidth using the followingformula:

(2 x depth) x (tangent of 1/2 coneangle) = diameter of the beam(beamwidth)

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The chart on the bottom of this page shows this concept.

There are several ways to specify beamwidth. The com-mercial fishing and naval industry frequently providebeamwidth information by measuring the sound beam at–3 dB. The marine recreation industry, however, providesbeamwidth information measured at–6 dB, giving the impression that theirtransducers have a wider beam field.

What is target masking?There are areas within the transducer’srange which seem to be invisible tothe echosounder. This is known as tar-get masking. It can happen if the lakeor sea bottom drops off suddenly orcontains a large rock. The soundwaves will bounce off all of the seabottom within the sound beam andreturn as strong echoes. The echoesfrom the highest point, the rock ordrop-off, return to the transducer first, falsely indicatingthe apparent depth of the bottom. Small fish below thehighest point will produce relatively small echoes whichwill return after the larger ones. Therefore, fish can beswimming around the sides of a large rock or a drop-off,be in the sound field, and yet remain invisible to theechosounder.

What are sidelobes?Our engineers are concerned about a phenomenon calledsidelobes. You can see sidelobes for yourself by using aflashlight. If you shine a flashlight against a wall, you willsee an area at the center where most of light is concentrated. Around the edge of the flashlight beam,

the rings of light willbe dimmer. These ringsare the sidelobes of theflashlight beam.

When engineers designtransducers,they calculatewhat sidelobesare present. Infact, 60% to70% of thesound waveswill be concen-trated in thecone area andthe remaining waves will escape in all directions.

Engineers generally like to minimize sidelobes,although they may actually be desirable in somesituations. As with a flashlight, a transducer doesnot “see” as well in the sidelobes, because fewerof the sound wave echoes return to the transduc-

er. Fish might be present in this area, but a fisher wouldbe unaware of them. However, in the case of a narrowbeam transducer, sidelobes can be useful, since they effec-tively widen the coverage.

Sidelobes are typically presented as part of the transduc-er’s radiation pattern. The lower, smaller, and narrower thesidelobes, the better the transducer performs, becausemore of the sound waves are focused in the main beam.

Sidelobes are watched carefully during product develop-ment. Each transducer model creates its own particularsidelobe pattern as the sound waves travel through thewater. This information is very important to those whodesign the echosounders to be used with our transducers.


Cone Angle

Depth in Feet 9° 16° 18° 20° 32° 45° 53°

5 0.8 1.4 1.6 1.7 2.8 3.9 4.6

10 1.6 2.8 3.1 3.5 5.6 7.9 9.2

25 3.9 7.0 7.9 8.7 14.0 19.6 23.1

35 5.5 9.8 11.0 121.2 19.5 27.5 32.4

50 7.9 14.0 15.7 17.4 27.9 39.3 46.2

Diameter of Viewable Area or Beamwidth

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What goes into the making of a transducer?It is clear that the transducer would not work without thepiezoceramic element, however, other parts are also need-ed. A transducer is made up of six separate components:

• Piezoceramic element or an array of elements

• Housing

• Acoustic window

• Encapsulating material

• Sound absorbing material

• Cable

What is the housing of a transducer?The housing is the container that covers, protects, andsupports the component parts of the transducer. Thepiezoceramic element is very brittle, and the wire connec-tions to the piezoceramic are fragile. Because of this, thehousing needs to be sturdy as well as resistant to thechemical, mechanical, and electrical forces in the environ-ment where it will be used.

Our housings are made of a variety of materials and takeon different shapes depending upon the customer’s needand intended type of installation. Some of the housingmaterials are:

• Molded plastic • Stainless steel

• Bronze • Urethane (SEALCAST™)

The plastic and ure-thane housings areproduced in our ownmolding department.Airmar’s custom SEAL-CAST ™ transducersfeature a seamlesshousing and a com-pression fitting at thecable exit. This com-pression fitting securesthe flexible cable atthe point of attach-ment to the housing

and provides a water-tight seal to minimize any possibilityof water entering the unit.

What is an acoustic window?The acoustic window is the surface through which thesound waves travel. It occupies the space between thepiezoceramic assembly and the water. Any material usedto create the acoustic window will absorb some of thesound waves that pass through it. Therefore, engineerscarefully choose the least absorbent materials. Theacoustic window is sometimes referred to as the acoustic face.

Epoxy, plastic, and urethane are the three materials Airmaruses most often for our acoustic windows. These materialshave sound wave carrying capabilities or acoustic properties between those of the piezoceramic elementand water.

Acoustic window materials fall into two categories.Soft, rubbery, elastic materials like urethane carry soundwaves in almost the same manner as water. So, water andurethane are said to have similar acoustic properties.Because of this close match, the thickness of acoustic win-dows made from urethane does not need to be tightlycontrolled in our product designs.

Hard materials like plastic and epoxy have acoustic proper-ties somewhere between those of piezoceramic elements

and water. In other words,the plastic or epoxy actslike an intermediateacoustic step between thefluid water and the rigidpiezoceramic element.

A plastic or epoxy acousticwindow is called a match-ing layer. Layer thicknessesare carefully calculatedand produced to matchthe sound wavelength atthe operating frequency.


The Structure Of Transducers

11

How will air bubbles interferewith transducer function?Because air is much less dense than water, air bubblesscatter and reflect sound waves. Any air bubbles in theacoustic window material or in the water will interferewith the proper working of the transducer, greatly reduc-ing its performance. To minimize the chance for tiny, fleck-like, micro-bubbles in the acoustic window material, it isplaced under a vacuum for a specific amount of time.

Air bubbles must, also, be carefully guarded against bythe installer and user. If the transducer is glued to theinside of the hull of a boat, even the glue cannot have airbubbles in it. Indeed, a transducer needs to be placedaway from anything, including the propeller, that willcause air bubbles to form while the boat is underway.

What will interfere with an acoustic window’s ability to become wet?In order for a transducer to work correctly, the surface ofthe acoustic window must be thoroughly wet. How quick-ly the acoustic window becomes wet depends, in part,upon the type of material used to make it.

Glossy surfaces, such as our plastic and SEALCAST™ ure-thane acoustic windows, wet almost instantly, becausethey are smooth.

Our sanded ure-thane acousticwindows takemuch longer tobecome thor-oughly wet. Although the sanded urethane does not seem rough to the touch, the sandingprocess leaves microscopic peaks and valleys which trap air bubbles keeping the water from touching the urethane.

Because of this, transducers with sanded urethaneacoustic windows would have to soak in water for a mini-mum of one hour before testing their ability to transmitand receive. In order to shorten this process, our testerslightly scrub the urethane window with alcohol in orderto hasten the wetting process. Both the scrubbing andthe alcohol help to quickly displace the microscopic airbubbles.

In what other ways do acoustic windows differ?Acoustic windows can be “hard” or “soft.” Soft acousticwindows made of urethane provide excellent sensitivity toechoing sound waves, therefore soft windows can “read”through deeper water with better clarity of detail. Thismaterial is extremely stable in water, therefore providingexcellent reliability for years. Because the acoustic proper-ties of urethane are similar to water, the acoustic windowcan be made in the shape of a dome, wedge, or an arc.

Hard plastic and epoxy acoustic windows are especiallygood for boats that are trailered or often in and out of thewater, because these windows become wet quickly. Theyalso have characteristics which are good in shallow waterand in fishfinding.

What is the encapsulant?The encapsulant is the material that encases the parts ofthe transducer within the housing. It can act as anacoustic window material, filler, or sealant. At Airmar theencapsulant is often called potting material and includesepoxies and urethanes. The choice is based upon its abili-ty to meet the performance and application requirementsof the particular transducer design.

What is sound absorbing material?Sound absorbing material is any material that inter-rupts or stops the flow of sound waves. In ourcase, it is the material used to dampen anyunwanted vibrations of the piezoceramic element

in a transducer.

When AC voltage is applied to a piezoceramic element, all surfaces of the element vibrate. This means it sends off or radiates sound waves in all directions. Therefore,the piezoceramic element could pick-up echoes returning from all directions giving false information to the echo-sounder. This effect is called spurious radiation and mustbe avoided.

To reduce spurious radiation as much as possible, thepiezoceramic element is surrounded with sound absorb-ing material, usually a layer of cork or foam. In this wayvibrations of the piezoceramic element are dampened onall the surfaces except the surface facing the acoustic win-


Sanded, urethane, acoustic window (magnified)

12

dow. As a result, the sound waves are given directionality.They are concentrated in one direction only. Directionalityis also effected by the piezoceramic’s shape and frequency.

How do Airmar cables function?A transducer cable is the vehicle which carries the electri-cal current between the echosounder and the transducer.It is usually made of several conductors or wires. Cablesare carefully engineered to carry a specified voltage andcurrent from the echosounder to the sensor(s) in thetransducer.

Inside the jacket of each cable is shielding to protectagainst electrical pulses from other electrical equipmentthat could interfere in the workings of the transducer. Thisinterference is called electrical noise. Commonly heard asstatic, electrical noise could come froma ship-to-shore radio, navigation equip-ment, ignition impulses, or evenanother transducer. Interference hasthe same effect on the echosounderdisplay screen as it does on a televi-sion screen; the picture becomes

snowy—the clarity and sharpness of the image is lost.

Depending upon the application and performancerequirements for the transducer, several types of shieldingare used such as:

Aluminum foil on Mylar

Tinned braided shield

Spiral shield

The cables outer jacket protects the inner conductors andprovides strength and flexibility.

Jackets materials include:

PVC—polyvinyl chloride

TPR—thermoplastic rubber

Neoprene

Polyurethane


Performance Testing

What is performance testing?Airmar products are designed to a cus-tomer’s specifications or to fill a marketneed. Engineers most often begin with arequired frequency and cone angle. Afterthe transducer is designed, a sample isbuilt in the lab where it is extensively test-ed. Data is collected on:

• Frequency

• Beamwidth

• Transmitting Voltage Response (TVR)

• Receiving Voltage Response (RVR)

• Figure-of-Merit

• “Q” (Bandwidth)

• Ringing

• Impedance

This information is known as per-formance data which is madeavailable to customers and usedas the standard for final testingof the product. The figures areextremely important to our OEMsas this data is used to determinethe frequency at which theechosounder must be set.

Airmar products are tested manytimes during their manufactureand are subjected to a carefulfinal inspection. A wide variety oftest equipment, including ourtest tanks, is used.

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What are the TransmittingVoltage Response and the Receiving VoltageResponse?Each Airmar transducer model is test-ed to measure the strength of thetransmitted sound wave and thereceived sound wave (what we havebeen calling the echo).

The Transmitting Voltage Response(TVR) is a measure of the acousticpressure of the sound wave at a dis-tance of one meter when one volt ofelectricity is applied to the transducer.If we could hear the sound waves,we might say the TVR is an indicationof how loud a sound the transducerproduces when one volt is applied to it.

The Receiving Voltage Response (RVR) is a measure ofthe voltage produced within the transducer when a 1mPa (micropascal) of acoustic pressure is received. AnmPa is a unit for measuring sound pressure. The pres-sure of the echoing sound waves causes the piezoce-ramic to produce voltage, the RVR. Think of a lamp thatcan be turned on by the clap of hands. It is the loudnessof the clap, the pressure of the sound wave, that turnson the lamp. If we could see the transducer’s responseto the echoing sound waves, we might say the RVR isan indicator of how brightly the lamp shines.

Comparing the TVR and RVR shows that the differencebetween the actual voltage used for transmitting andthe actual voltage generated by the returning echo istremendous. Sound waves are emitted by voltage mea-suring in the hundreds (100s) of volts, yet the returningechoes are measured in hundredths (1/100s) of a volt.

What is the Figure-of-Merit?The Figure-of-Merit is a measure of how well a transducerworks when used for both transmitting and, then, receiv-ing its own echoes. It is the algebraic sum of theTransmitting Voltage Response and the Receiving VoltageResponse. The Figure-of-Merit is sometimes referred to asthe Insertion Loss.

As we know from the ReceivingVoltage Response, the returningsound wave will be far weaker thanthe original sound wave that wassent out. This is because the transmit-ted sound wave looses energy by thetime it travels through the water,bounces, and returns.

Engineers test each transducermodel and graph the results of theTransmitting Voltage Response, theReceiving Voltage Response, and theFigure-of-Merit. The graphed curve ofthe Figure-of-Merit will usually peaksomewhere between the peak of theTransmitting Voltage Response and

the peak of theReceiving VoltageResponse.

What is “Q”?“Q” stands for quali-ty and is a measureof the sharpness ofthe response of thepiezoceramic ele-ment to the fre-quency that is sup-plied to it. In otherwords, “Q”describes how pre-cisely the frequencymust be output toachieve the bestperformance from

the transducer. It answers the questions: “What is thepiezoceramic element’s best or resonant frequency?”,“How well does the piezoceramic element work on eitherside of its resonant frequency?”, and “How long will thetransducer continue to ring after a transmit pulse?”

Engineers have a standard method for determining “Q”. Itis the operating frequency divided by the bandwidth. Fornon-engineers, it is helpful to think of frequency andbandwidth in terms of volume and tuning-in your favorite


14

radio station. The operating frequency is that spot on theradio dial where your favorite station comes in the loudestand clearest.

The bandwidth includes the frequencies slightly aboveand below the best spot, but where the station can stillbe heard. Engineers use the standard of three decibelsbelow the peak Transmitting Voltage Response, shown as–3 dB. If we could hear the transducer sound waves, the–3 dB point is where the sound would be half as loud. Aswe turn the dial above and below our station, the signalbegins to fade, so we can’t hear it as well. The pointsabove and below the radio station on the dial at whichour radio is half thevolume of the correctradio station frequencydetermines our band-width.

The “Q” of a transducermodel can be deter-mined by analyzing itsTransmitting VoltageResponse graph. Theresonant frequencyand peak TVR is at 50kHz. At 3 dB belowTVR, the frequenciesare 47.6 kHz and 52.8kHz, giving us a band-width of 5.2 kHz (52.8kHz – 47.6 kHz = 5.2kHz). To determine “Q”,the resonant frequency is divided by the bandwidth giv-ing us a “Q” factor of 9.6. (50 kHz ÷ 5.2 kHz = 9.6).

“Q” factors range between 1 and 40. At 9.6 this model’s“Q” factor is relatively low. The OEM need not be as pre-cise in setting the echosounder’s drive frequency when atransducer has a lower “Q”.

What is Ringing?Ringing is the continued vibration of the piezoceramic ele-ment after each transmit pulse. Imagine the ringing of alarge church bell. After the church bell is struck by theclapper, it continues to ring for a time if the vibrations arenot dampened.

This phenomenon also occurs in piezoceramic elements.The vibrations of the element continue after the transmitpulse. These vibrations decrease in amplitude (or “loud-ness” if we could hear them) just as the ring of the churchbell gets softer over time. The tapering off of the vibra-tions is called the ring down.

In effect, ringing causes a “stretching” of the transmitpulse, because it generates unnecessary sound waves.These additional sound waves add additional microsec-onds to the dead band, interfering with the reception ofechoes.

If a desired echo arrives during the ring down it willappear on the echosounder screen as asmear or it may even be hidden by the ringdown and not appear on the echosounderscreen at all. Ringing, therefore, reduces theclarity of the display on the echosounderscreen.

Ringdown also keeps the transducer from“seeing” in very shallow water.

Ringing can never be totally eliminated. Withthe proper engineering, however, it can begreatly reduced. A transducer with a high “Q”factor is one which will ring for a long timeafter being struck with a transmit pulse.Conversely, a low “Q” transducer exhibits lessringing.

How are the deadband and blanking zone related?

As you have learned, during the transmit pulse the piezo-ceramic is vibrating, so no echoes can be received—in thesame way that you cannot listen when you are talking.The microseconds when the transducer is transmitting isthe deadband for the reception of echoes.

When something very close to the transducer (usuallybetween one and three feet), the bouncing echoes will


15

How are Airmar products unique?Airmar’s products incorporate the most advanced technol-ogy; often pioneered by Airmar. Our attention to detailmakes our sensors more accurate and reliable than thecompetition. Careful design, manufacture, and testinginsures that our transducers perform to specifications. Weuse waterproof connectors, multiple O-rings, seamlessconstruction, and strong materials to produce sensors thatcan withstand harsh environments.

What products does Airmar manufacture in each product line?Recreational Sensors—Our recreational transducers canbe manufactured to perform several different functionswhen used with the appropri-ate information display unit.

Airmar sensors can measure:

• Temperature only

• Depth only

• Depth with Speed

• Depth with Temperature

• Depth with Speed and Temperature (TRIDUCER ®multisensor)

• Speed only

• Speed with Temperature

Recreational transducer housings come in several shapesdepending upon their intended method of installation.

In-hull—This transducer is installed against the inside of aboat hull bottom and sends its signal through the hull.Some people prefer this method, because the unit cannotbe damaged when the boat is trailered. Also, drillingthrough the hull is not necessary. In-hull transducers canwork better than other models at high boat speeds.

Because in-hull units “shoot” through the layers of thehull, there is a loss in performance. In addition, this instal-

lation will not work on a boat made of anymaterial containing air bubbles, because the airbubbles reflect and scatter the sound before itreaches the water. Wooden hulls and fiberglasshulls with foam, balsa wood, or plywood layerssandwiched between the inner and outer skinsare not recommended for in-hull installation.

Transom Mount—This transducer style is mount-ed to the back (transom) of a boat hull. Somepeople prefer this style, because it affords easy

return before the piezoceramic has stopped ringing. Sincethe echo is in the deadband, these echoes cannot bereceived—the object will be invisible. The minimum dis-tance between the transducer housing and an object thatcan be “seen” is called the blanking zone. OEMs typicallywant a small blanking zone, so their echosounders can“see” objects close to the transducer.

What is Impedance?Impedance stems from the word impede and meanssomething that hinders progress. In the field of electricity,it refers to the limitation of the amount of current that canflow through a material. Impedance is technically the ratioof voltage to current. Airmar measures the amount ofelectrical force applied to the transducer and the amountof current that actually runs through the transducer. The

quotient is the impedance.

The echosounder must be designed to match the imped-ance of the transducer to deliver the correct power to it. Ifthey do not match, the output from the transducer will bereduced lessening the performance of the entire system.

Airmar chooses materials that are natural conductors ofelectricity. Silver is used on our piezoceramic elements,because it is an excellent conductor and is easy to applyand bake onto our piezoceramic elements. Copper is usedin all electric wiring, because it is an excellent conductorand less expensive than silver.

Some materials will not allow electrons to flow throughthem at all. These materials are called insulators. They areused around all electrical wires to keep electricity flowingthrough the conductor(s) and prevent electricity fromflowing elsewhere.


Airmar Products

16


installation and removal of the unit—especially if a kick-upbracket is used. A kick-up bracket allows the transducer to be moved out of the way to prevent damage from aboat trailer. A transom mount installation gives better per-formance than the in-hull installation at boat speeds up to 30 MPH.

Thru-hull—This transducer is installed through a hole inthe boat and protrudes into the water. It is most oftenused on boats 25 feet or longer and usually provides thebest performance. The transducer is low enough in thewater to avoid most of the air bubbles caused by themotion of waves.

Thru-hull units are not recommended in two situations:

• Plastic thru-hull housings should not be used on awooden boat. Wood swells as it absorbs water, so itmay crack the housing.

• Bronze thru-hull housings should not be used in alu-minum boats. The interaction between the aluminumand the bronze especially in the presence of salt water,will eat away the aluminum hull and/or bronze housing.

Temperature SensorsTemperature sensors are desirable for several reasons.Anglers know that fish prefer certain water temperatures,so a temperature sensor can help the fisher determinewhere the fish may be hovering. Boaters may wish toknow the water temperature before jumping in for aswim, but more importantly, it can be a navigational aid.For example, the water in the Gulf Stream is much warmerthan the surrounding Atlantic Ocean. Boats traveling northare wise to locate and ride the current while those travel-ing south need to avoid the Gulf Stream entirely.

Temperature is most commonly read by a temperature sen-sor called a thermistor. The Airmar thermistor is a 10,000ohm resistor which varies in value according to its temper-ature. This means that the amount of electricity flowingthrough the resistor is directly related to the temperatureof the resistor. The warmer the resistor the more electricityis able to flow through it. Conversely, less current is able toflow through the resistor when it is cold. The amount ofelectrical current flowing through the resistor, therefore,tells us the temperature of the water.

Most of Airmar’s temperature sensing devices are built rightinto the transducer housing.

In our bronze transducer the temperature sensor is placedinside the housing cavity because the metal housing itselfis a good thermal conductor. The bronze quickly takes onthe temperature of the water and, as a result, the tempera-ture sensor doesn’t require direct water contact.

In a few other types of transducers a semiconductor tem-perature sensor is also used. Because the electrical voltageput out by the semiconductor changes slightly with eachdegree of temperature, the temperature of the water canbe determined by measuring the electrical voltage put outby the semiconductor.

Speed SensorsBoat speed is measured in nautical miles per hour calledknots. A nautical mile is 6,076 feet, approximately 1.15land miles.

The component used to determine boat speed is called aHall Cell and works like an on/off switch. Within the sen-sor are magnetic parts that can be turned on and off by apassing magnet. The Airmar paddlewheel blades are mag-netized. As the boat moves through the water, the paddle-wheel turns. Each time a magnetized blade of the paddle-wheel passes by the Hall Cell, it is turned on or off, creat-ing a pulse of electricity. Electronics in the echosoundersystem count the frequency of these pulses, then convertthem into knots. A paddlewheel may create as many as22,000 electrical pulses per nautical mile.

TRIDUCER® MultisensorThe Airmar TRIDUCER® multisensor is a combination ofthree sensing devices built into one housing to measuredepth, speed, and temperature.

Phased Array TransducerA phased array transducer can function as both a down-ward looking and a forward looking transducer. It doesnot point straight down only. Rather, the echosounder cansteer the beam about 45 degrees to either side. Thesetransducers are able to “see” both the immediate underwa-ter depths and what is ahead of the boat. It can see botha large schools of fish toward which a boater would like tosteer and large objects or a shallowing bottom to beavoided.

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Commercial Fishing TransducersThese transducers are available in frequencies from 24 kHzto 200 kHz. Units feature high-efficiency designs produc-ing superior fish finding and clear and distinct images ofboth the bottom and closely spaced fish.

Navigation and Ocean Survey TransducersThis broad product group features transducers rangingfrom 10 kHz to 2 MHz. Airmar offers transducers sizedfrom small portable units for harbor survey to multi-fre-quency arrays for deep sea sounding. Arrays of more than100 piezoceramic elements have been designed and man-ufactured by Airmar. We can produce dual beam and splitbeam transducers and linear phased, and multi-beamtransducer arrays.

One hydrographic product specifically designed for a cus-tomer is the Swath bathymetry transducer. It’s transmittingtransducer uses a fan beam which is very narrow in onedirections and very broad in the other direction. Echoesare received by 30 separate transducer elements whichform 30 separate beams. This provides 30 sets of datayielding a great amount of detail about the bottom. OneSwath is used to monitor the ever shifting channels in theMississippi River.

dB PLUS II™ Acoustic Deterrent SystemThe dB PLUS II™ Acoustic Deterrent System uses Airmar’sskill at producing sound waves, to scare away marine liferather than to find it. Our popular acoustic deterrent sys-tem is designed to protect fish farms from seals and othermarine predators who find the fish an inviting target.

The acoustic deterrent system has a series of four trans-ducers positioned around the fish pens which are madeof net. The transducers work as projectors only, giving outsound waves for 2.5 seconds in turn. It takes 18 secondsto complete one full four-transducer transmission cycle.The amplifier for creating the electrical power supplied tothe transducers is centrally located among the net pens.

Research tells us a sound wave frequency of 10 kHz is notharmful to seals, but it is definitely irritating. Their desireto get away from the sound overcomes their desire for an

easy meal. The sound has been likened to the scratchingof fingernails on a blackboard.

The dB PLUS II™ Acoustic Deterrent System has aunique “soft-start” feature. The system takes 25 seconds toreach full power. This gradual sound increase provides awarning to divers and allows seals and sea lions theoption of leaving the area before the sound reaches itshighest volume.

This product has been successfully used in waters off ofthe U.S., Canada, Chile, New Zealand, and Europe. Theseals are keeping their distance, so the customers are very happy.

Air TransducersAir transducers work in a way that is very similar tomarine transducers. Air, however, is not as good a con-ductor of sound waves as water. In fact, sound travelsonly 25% as fast in air as in water. Think of the time delaybetween seeing lightning and hearing the thunder orbetween your shout and its echo. Speed is not the deter-mining factor in the loss of efficiency. Temperature,humidity, and wind are the determining factors.

Air transducers can be used in a variety of ways in bothcommercial and industrial applications:

• Silo or tank level detection of liquids such as oil; or of solids such as grain, coal, flour, or anything storedin bulk

• Proximity measurement

• Process control

• Object detection

• Volume of liquid flow through weirs

• Sensing through foams

• Level detection in waters with sediment such as septicplants or in environments that are dusty

• Liquid column measurement

What are the characteristics of air transducers?Frequencies from 25 kHz to 225 kHz are available in ourair transducers.

Airmar’s models are designed to meet criteria set by haz-ardous environmental certification agencies. Because air


18

has relatively low density, air transducers are susceptibleto ringing. The dead band specification is very importantto the OEMs and our engineers strive to design air trans-ducers with low “Q” and, therefore, low ringing.

Parts and AccessoriesWhat is a Fairing?A fairing is a structure which is added to the transducer atthe mounting location to improve its performance. If thehull slopes, a fairing orients the transducer so the soundbeam will aim straight down. The chance of water withair bubbles flowing across the acoustic window is reducedby mounting the transducer deeper in the water. Also,Airmar carefully designs the shape its fairings to direct

water around the transducer, so that drag on the boat isminimized. All Airmar fairings are made from a polymerresin which will not swell or rot.

Our parts and accessory line includes:

• Housings, hull nuts, and cap nuts

• Switch boxes

• Mounting kits

• Cables and connectors

• Speed sensor parts

• Fairings

• Diplexer

Glossary of TermsAC the abbreviation for AlternatingCurrent. Electrical current that reversesdirection at regular intervals.

AC voltage used to cause the piezoce-ramic element to vibrate.

Acoustic relating to sound and soundwaves.

Acoustic Energy when work can bedone because energy is provided by thephysical pressure of a sound wave.

Acoustic Face see Acoustic Window

Acoustic Property the ability of a mate-rial to carry sound waves through it.

Acoustic Window that part of the trans-ducer through which the ultrasonic vibra-tions from the piezoceramic assemblytravel to the water or other transportingmedium. It occupies the space betweenthe piezoceramic assembly and the water.

Air Bladder an organ in a fish whichallows it to adjust easily to changes inwater pressure at different depths. It isthe organ that allows the echosounder todetect the fish.

Amplitude the degree of intensity (pres-sure) of a sound wave. If we could hearthe sound wave, the amplitude would beits “loudness.”

Angler a person who fishes.

Application Requirement the use thatthe sensor is designed for.

Array a series of piezoceramic elementsin a transducer.

Bandwidth the range of frequencies overwhich the transmitting sensitivity (TVR) isno less than one half of the peak sensitiv-ity at –3dB.

Beamwidth the diameter of the circle in which 50% to 70% of the soundwaves emitted by a transducer are con-centrated.

Cable the wire that carries powerbetween the echosounder and the trans-ducer. It is usually constructed of severalconductors.

Capacitance the ability to store an elec-trical charge.

Ceramic a commonly used name for thepiezoceramic element.

Concentric Circles a series of circles ofdifferent sizes having a common center.

Conductor anything that carries electri-cal current. This may be a copper wire orany other material that readily allowselectrons to pass through it.

Cone Angle the measurement ofbeamwidth in degrees. It is an indicationof how large an area is covered by atransducer’s sound beam. The larger thecone angle the larger the area covered.

Current the measure of the number ofelectrons that flow past a point in a spe-cific unit of time.

dB an abbreviation for decibel. A unit formeasuring the power of a sound wave.

Deadband the time during and immedi-ately after a transmit pulse when a trans-ducer cannot receive echoing soundwaves and therefore cannot “see” thearea below the surface of the water.

Deadzone the minimum distancebetween the transducer housing and anobject that can be “seen” by anechosounder.

Directionality the controlled emission ofsound waves in the desired direction.

Display Screen the part of theechosounder unit on which an image ofthe underwater area is projected.

Drag the retarding force exerted on amoving object.


19

Echosounder an instrument comprisedof a display screen and electronic circuit-ry. It provides necessary power to thetransducer, interprets the informationreceived from the transducer, and displays this information in a readableformat.

Echosounder System a system com-prised of two major components—anechosounder and a transducer. This sys-tem measures the depth of the waterand the distance to any objects in itsfield of vision, and displays this informa-tion in a readable format.

Electrical Energy when work can bedone because energy is provided by anelectrical force.

Electrical Noise interference in an elec-tronic signal, often called static.

Electrode a solid material which con-ducts electricity and is used at the pointwhere electrical current enters or leavesan object.

Encapsulant the material that acts as afiller in and around the transducer partswithin the housing. It may also functionas an acoustic window or a sealant.Encapsulant is often called potting material.

Energy a measure of a system’s abilityto do work.

Figure-of-Merit (FOM) the algebraicsum of the Transmitting VoltageResponse and the Receiving VoltageResponse. The Figure-of-Merit usuallyindicates a net loss of energy, and issometimes referred to as Insertion Loss.

Frequency the number of completecycles or vibrations that occur within aspecific time frame, typically one second.It is usually measured in Hertz.

Hall Cell a wafer of silicon altered byelectrical current so that it respondsmagnetically.

Hertz (Hz) a measure of one cycle orcomplete vibration per second.

Housing something that covers, sup-ports, or protects the mechanical parts.

Impedance the limitation of theamount of flow of electrical current. It iscommonly used as a comparisonbetween the amount of voltage neededto get a specific amount of current.

In-hull Installation the method ofinstalling a transducer by attaching it tothe inside of the hull.

Injection Molding the process of creat-ing parts by forcing melted material(usually plastic) into a mold in thedesired shape. The material is thencooled and released from the mold.

Insertion Loss the algebraic sum of theTransmitting Voltage Response and theReceiving Voltage Response. This usuallyindicates a net loss of energy. Oftenreferred to as the Figure-of-Merit.

Insulator any material that will notallow electrons to pass through. Thesematerials are used to surround conduc-tors to keep the current flowing in thedesired direction.

Jacket the covering that surrounds acable and provides strength.

Kilo-Hertz (kHz) one thousand cyclesor complete vibrations of sound per sec-ond.

Knots nautical miles per hour. A nauticalmile is 6,076 feet or roughly 1.15statute miles.

Matching Layer an acoustic windowmaterial like plastic or epoxy that hasacoustic properties somewhere betweenthose of the piezoceramic element andthe water. It acts as an intermediateacoustic step and facilitates the travel ofsound waves from the piezoceramic ele-ment to the water.

Nautical Mile 6,076 feet or 1.15 statutemiles.

Ohm a unit of resistance to an electricalcurrent. Some materials conduct electric-ity readily while others are poor conduc-tors. The poorness of the conductivity isthe resistance of the molecular structureto carrying the electrical current.

Original Equipment Manufacturer(OEM) These are the businesses that buyour product for use with their manufac-tured products.

Performance Requirement the workthat a customer needs the transducer todo, e.g. see a narrow but deep area ormeasure speed and temperatures in saltwater below 32° F.

Personal Water Craft (PWC) a jet-skitype recreational water vehicle.

Phased Array a series of piezoceramicelements in a transducer. The piezoce-ramic elements are wired in a mannerwhich allows them to fire in timedelayed sequence, so the echosoundercan electronically steer the array.

Piezoceramic Element a materialmade of crystals with positive and nega-tive charges. Frequently referred to as apiezoceramic or ceramic.

Polarize the ability to align electricallycharged crystals placing like poles in thesame direction.

Pole one end of polarized material, i.e.the positive pole of a piezoceramic ele-ment.

Potting Material see Encapsulant

Projector when the transducer acts asa transmitter only. This is the case withthe dB PLUS™ II Acoustic DeterrentSystem.

“Q” an abbreviation for quality. A mea-sure of how tolerant a transducer is tochanges in frequency.

Radiation Pattern outline of soundwaves as they travel outwards from atransducer, usually represented as acone shape.

Receiving Voltage Response (RVR)the measure of the voltage producedwithin the transducer when returningechoes are received.

Resistor an electrical component usedto limit the amount of an electrical cur-rent passing through it. The level of thislimitation is measured in ohms.

Glossary of Terms

20

Resolution the ability to show finedetail. Better resolution provides betterdiscrimination among individual objects.

Resonant the natural tendency of amaterial to vibrate at its own favoredrate (frequency).

Ring Down the tapering off of the vibra-tions from a transmit pulse. The vibra-tions may interfere with the reception ofaccurate information by the transducerand thereby distort the information dis-played by the echosounder screen.

Ringing the continued vibration of apiezoceramic element beyond the elec-trical transmit pulse. This causes distort-ed information to be displayed on theechosounder screen.

SEALCAST™ an Airmar trademarkedname for a transducer housed in oneseamless unit.

Sensor any device that detects andresponds to a stimulus such as tempera-ture, speed, motion, light, various chem-icals, etc.

Shielding material used to preventinterference from other electrical equip-ment on the boat or in the area thatcould interfere with the workings of thetransducer.

Sidelobes those portions of the acousticsignal that are located outside the mainsound beam.

Smear/Smearing distorted informa-tion, specifically a running together offish images displayed by theechosounder system because of unwant-ed sound waves.

Sonar derived from the words soundnavigation ranging. An apparatus thatuses reflected sound waves to detectand locate underwater objects.

Sound Field the total area where soundwaves travel from a transmit pulse. Thesound field includes the main beam andsidelobes as well as spurious radiation.

Sound Isolating Material usually alayer of cork or foam that surrounds thepiezoceramic element except where itcomes in contact with the acoustic win-dow. This is done to focus the soundwaves in one direction and prevent spu-rious radiation.

Speed Sensor a device that detects therate of speed of a water craft.

Spurious Radiation sound waves thatescape in non-desired directions. Thisproduces false readings by theechosounder system.

Target Masking the areas within atransducer’s range which seem to beinvisible to the transducer.

Temperature Sensor a device thatdetects the temperature of water.

Thermal Conductor a material with theability to let heat travel through it. Agood thermal conductor adjusts quicklyto the temperature of its environment.

Thermistor a unit for sensing and mea-suring temperature.

Thru-hull Installation a method forinstalling a transducer through a hole inthe hull of a boat.

Transducer a device that changes elec-trical energy to acoustic energy andback again. This function is performedby a piezoceramic element(s).

Transmit Pulse a usually brief sequenceof sound waves sent out by the trans-ducer. Following the transmit pulsethere is a longer period of time whenthe transducer stops transmitting andreceives the echoes.

Transmitting Voltage Response (TVR)the pressure or “loudness” of a soundwave produced by one volt of electricity.

Transom Mount Installation a methodfor installing a transducer on the back(transom) of a boat hull.

TRIDUCER® Multisensor an Airmartrademarked name for a sensor that canmeasure three functions: depth, speed,and temperature in one unit. Airmar hasbeen granted a patent for TRIDUCER®multisensors and has sole rights to thisdesign.

Ultrasonic sound waves of high fre-quency than cannot be heard byhumans. Sound waves higher than20,000 Hertz.

Volt a unit of electrical force.

Voltage a measure of electrical force orthe potential for current to flow.

Work the effect of force acting on abody. It is the relationship between theapplication of a force to an object andthe distance that object moves whenthe force is applied. It is calculated bymultiplying the force times the distancethe object is moved.

Glossary of Terms

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