Acoustic TransducersAcoustic Transducers

Fateme Mohandespour

Amirkabir University of TechnologyAutumn2003

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OverviewOverview

Sound to voltage transducers

Voltage to sound transducers

Ultrasound as an instance

References

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Sound to Voltage TransducersSound to Voltage Transducers

Carbon transducers

Dynamic transducers

Condenser transducers

Piezo transducer

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Carbon TransducersCarbon Transducers

The simplest type of modern microphone is the CARBON microphone, used in telephones. This microphone consists of a metallic cup filled with carbon granules. A movable metallic diaphragm mounted in contact with the granules covers the open end of the cup. Sound waves vibrate the diaphragm, varying the pressure on the carbon granules. The electrical resistance of the carbon granules changes with the varying pressure, causing the current in the circuit to change accordingto the vibrations of the diaphragm.

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Carbon TransducerCarbon Transducer

Fig.1-1

These type of transducers are not widely used because of :

Poor frequency response

Bad signal-to-noise ratios

need Power supply

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Dynamic TransducersDynamic TransducersIn the magneto-dynamic, commonly called dynamic

transducer, sound waves cause movement of a thin metallic diaphragm and an attached coil of wire. According to the faraday's electromagnetism law, motion of the coil within the field causes current to flow.

The principles are the same as those that produce electricity at the utility company, realized in a pocket-sized scale. The amount of current is determined by the speed of that motion. This kind of microphone is known as velocity sensitive.

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Dynamic TransducersDynamic Transducers

Fig.1- 2

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Dynamic TransducersDynamic Transducers

Dynamic microphones are renowned for :Can withstand extremely high sound levels without damage or excessive distortion

They need no batteries or external power supplies

Their ruggedness and reliability

They are capable of smooth, extended response

Can be easily "tailored“ in response for special applications

Reasonable care will maintain their performance for many years

The disadvantage is that it doesn't respond well to extreme frequencies.

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Dynamic TransducersDynamic Transducers

Element:Neodymium dynamicimpedance: 200 ohmspolar pattern: Hypercardioidsignal to noise ratio:0.00db, 1 kHz at 1 pafrequency response:70 - 16,000 Hzopen circuit sensitivity:-56 db (1.5 mV) re 1V at 1 pa

Fig.1- 3

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Ribbon Ribbon TransducersTransducersIt is a form of dynamic, with a thin metallic ribbon (which serves as

both voice coil and diaphragm) suspended between the poles of a magnetic circuit. While it is capable of excellent performance, the ribbon element must be protected against high acoustic pressures, since it is relatively fragile.

Wide frequency range

Very sensitive to physical abuse.

Fig.1- 4

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Condenser TransducersCondenser TransducersIn a condenser microphone, the diaphragm is mounted close to, but not touching, a rigid

backplate. (The plate may or may not have holes in it.) A battery is connected to both pieces of metal, which produces an electrical potential, or charge, between them. As the distance changes, the diaphragm moves in response to sound and the current flows in the wire.The amount of current is essentially proportional to the displacement of the diaphragm.

Extremely wide frequency.

Good pick up sensitivity.

Condenser microphones are easy to miniaturize, while dynamic microphones cannot be miniaturized.

The down side is that it is easy to overload and distorts if placed too close to a high intensity sound source.

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Condenser TransducersCondenser TransducersElement: condenser

phantom power requirements: 9 -52V, 2 ma typicalpolar pattern: Hypercardioidbattery type: 1.5V AA/UM3frequency response: 30 - 20,000 Hzopen circuit sensitivity: phantom: -38 db (12.5 mV) re 1V at 1 pa, battery: re 1V at 1 paswitch: flat/roll-offimpedance: phantom - 500 ohmslow frequency roll-off: 180 Hz, 12 db per octavemaximum input sound level:phantom- 130 db SPL, 1 kHz at 1% T.H.D., Battery- db SPL, 1 kHz at 1% T.H.D.Signal to noise ratio: 70.00db, 1 kHz at 1 paFig.1- 6

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Electret Condenser Transducers Electret Condenser Transducers The electret class of microphones

are condenser microphones which use a permanently polarized electret material for their diaphragms, thus avoiding the necessity for the biasing DCvoltage required for the conventional condenser. They are the typical microphones on portable tape recorders.

Fig.1- 7(a)Fig.1- 7(b)

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Electret Condenser Transducers Electret Condenser Transducers

Very uniform frequency response.Ability to respond with clarity to transient sounds.No external power supply is needed(although an FET impedance matching circuit is typically required, powered by a small low-voltage battery in the microphone itself.)Very inexpensivelyThe low mass of the diaphragm permits extended high frequency performance,while the nature of the design also insures outstanding low frequency response.

Element type: Electret condenser frequency response: 40 - 18,000 Hz polar pattern: half-Supercardioid impedance: 150 ohms sensitivity: 22 mV/pa

Fig.1- 8

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Electret Condenser Transducers Electret Condenser Transducers

GLM-100 electret-condenser type omnidirectional

Powered by 12 to 48V phantom power.

Smooth frequency response from 20 Hz to 20 kHz

Uniform off-axis response Sensitivity : 3.2 mV/pa maximum

SPL for 3% THD shall be 150 db

Fig.1- 9

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Piezo TransducersPiezo Transducers

The crystal microphone uses a thin strip of piezoelectric material attached to a diaphragm. The two sides of the crystal acquire opposite charges when the crystal is deflected by the diaphragm. The charges are proportional to the amount of deformation and disappear when the stress on the crystal disappears.

Fig.1- 10

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Piezo TransducersPiezo Transducers

Early crystal microphones used Rochelle salt because of its highoutput, but it was sensitive to moisture and somewhat fragile. Later microphones used ceramic materials such as barium titanate and lead zirconate.

The electric output of crystal microphones is comparatively large, but the frequency response is not comparable to a good dynamic transducer.

Impedance: 1 meg ohm or more.

This unit could be destroyed with a bit of humidity !

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Piezo HydrophonePiezo HydrophoneDesigned to handle the high sound

pressure levels and the high static ambient pressure in water and other fluids,the the 8011 hydrophone uses a piezoelectric sensing element, which is frequency compensated to match the special acoustic conditions under water. The output is electronically balanced and offers more than 100db dynamic range. It is the ultimate choice for professional sound recordings in water or under other extreme conditions where conventional microphones would be adversely affected

Fig. 1- 11

8011 hydrophone

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Characteristics ComparisonCharacteristics Comparison

Carbon

ElectretsPiezo

CarbonDynamic

PiezoElectrets

ImpedanceFrequency response

Fig. 1- 12

Direction patterns

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Characteristics ComparisonCharacteristics Comparison

Fig.1-13 Fig.1-14

Fig.1-15

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Acoustic MEMSAcoustic MEMS

Akustica's microphone chips combine MEMS microphones with software and microelectronics onto a single, standard CMOS chip.

Fig.1-16 Fig.1-17

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Smart SensorsSmart Sensors

Acoustic emission technology detects the sound of the wheel touching the part with accuracy of less than one micron.

Detect, track, and classifyground/air vehicles- 4’ aperture, 5 mic array, DSP- Hand emplace or air deploy w/optional parachute- Self mapping via GPS- Separate long haul and short hauldata radios

Fig.1-20 Fig.1-21 Fig.1-19

Fig.1-18

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Voltage to Sound TransducersVoltage to Sound Transducers

Piezo transducers

Dynamic transducers

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Piezo TransducersPiezo Transducers

The working component in an audible sound transducer usually is a thin disc of piezoelectric ceramic bonded to a similarly thin metal membrane . when it is excited at low frequency, a piezoceramic material vibrates; At high frequencies it also produces sound, as a transducer does. The resonant frequency of the ceramic is too high to produce an audible tone by itself, so a metal plate must be attached that vibrates with the contraction and expansion of the piezoceramic.

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Piezo TransducersPiezo Transducers

Both audible and silent alerts can be generated from the same source by exciting it with two different frequencies.

Fig.2-1

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Piezo Transducers ApplicationsPiezo Transducers Applications

Receiver/microphonePiezoelectric vibrator Ultra-sound transmitterUltrasound receiver Piezoelectric oscillator Clock/watch Computer

Transmitter/handy-phonePiezoelectric transformer Liquid crystal displayPiezoelectric optical modulator Beam scanner Semi-conductor device

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Dynamic TransducersDynamic TransducersThe speaker is essentially the same as the dynamic microphones except

the current carrying the sound to be recovered is fed trough the coil, which creates a moving magnetic field around the coil. The interaction between the magnetic field about the coil and the field of the permanent magnetic cause the diaphragm to move back and forth. The motion of the diaphragm creates the original sound wave. Additionally implying an ac voltage to a piezoelectric material makes the piezo vibrating by the same frequency.

Fig.2- 2

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Dynamic TransducersDynamic Transducers

In fact, many intercom systems use small speakers (with lightweight cones) as both speaker and microphone, by simply switching the same transducer from one end of the amplifier to the other.

Fig.2-3

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Dynamic TransducersDynamic Transducers800/32/din specifications

• Receiver type: dynamic 36mm speakerimpedance: 32 ohm +/-15% at 1khzS.P.L: 99 db +/- 4 db at 1khzfrequency range: (1mw input) 150hz - 10khzinput power: 10mw (max. 100mw)

Fig.2- 4

Fig.2- 5

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SuppliersSuppliers

And so on…

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UltrasoundUltrasound

Introducing ultrasound

Ultrasound transducers

Ultrasound applications

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UltrasoundUltrasoundPrinciple:a short pulse of ultrasound is transmitted by the PZT transducer a booster

and a sonotrode (meant to increase vibratory speed). The waves reflected by the object , can be picked up by the receiver. For economy, one transducer is normally used for both transmitting and receiving. The system is electronically switched between transmitting and receiving functions.(Fig 3-1)

Fig.3-1

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UltrasoundUltrasound

The interval between transmitting and receiving pulses provides a measure of the distance to the object and can be shown on a display.This operating principle requires a minimum distance between transducer and object.The transducer cannot receive properly before the vibrations caused by the transmission have subsided.The ultrasound pulse should therefore be as short as possible, which calls for a high operating frequency and a large bandwidth.A high operating frequency also means that the transducer can be smaller and more compact. In practice, an echo sounder operating at 200 kHz will have A minimum detection distance of 0.2 m in air. Modern ultrasound devices, like those used in the medical field,operate at several megahertz.These can detect down to the millimeter region, but because of much higher attenuation at this frequency, their maximum range is limited.

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Medical ApplicationsMedical ApplicationsThe technique is now widely used in virtually every branch of medicine . it is

used to :SonographCardiology

Retinal problems Heat jointsRelieving arthritic joint painLithotripsy, in which shock waves break up kidney stones, eliminating the need for surgeryIs noninvasiveInvolves no radiation

Avoids the possible hazards—such as bleeding, infection, or reactions to chemicals—of other diagnostic methods.

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Industrial Applications Industrial Applications

Positioning sensing

Acoustic power and intensity measurement

Non-destructive material evaluation

Concrete tube inspection

Bubble and particle detection

Flow metering (speckle tracker)

Leakage testing

Level metering

Acoustic microscopy

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Ultrasound ReceiverUltrasound Receiver

Fig.3-3Fig.3- 2

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The End…