Science Microscope

7/28/2019 Science Microscope

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A. COMPONENTS OF MICROSCOPE

MICROSCOP

A.COMPONENTS

MICROSCOPE

B.HOW TO USE A

MICROSCOPE

C.TYPES OF

MICROSCOPE

D.MICROSCOPE C& HANDLING


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The simplest optical microscope is the magnifying glass and is good to aboutten times (10X) magnification. The compound microscopehas two systemsof lenses for greater magnification, 1) the ocular, or eyepiece lens that onelooks into and 2) the objective lens, or the lens closest to the object. Before

purchasing or using a microscope, it is important to know the functions ofeach part.

Eyepiece Lens: the lens at the top that you look through. They are usually10X or 15X power.

Tube: Connects the eyepiece to the objective lenses


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Arm: Supports the tube and connects it to the base

Base: The bottom of the microscope, used for support

Illuminator: A steady light source (110 volts) used in place of a mirror. If

your microscope has a mirror, it is used to reflect light from an external lightsource up through the bottom of the stage.

Stage: The flat platform where you place your slides. Stage clips hold theslides in place. If your microscope has a mechanical stage, you will be ableto move the slide around by turning two knobs. One moves it left and right,the other moves it up and down.

Revolving Nosepiece or Turret: This is the part that holds two or more objectivelenses and can be rotated to easily change power.

Objective Lenses: Usually you will find 3 or 4 objective lenses on a microscope. Theyalmost always consist of 4X, 10X, 40X and 100X powers. When coupled with a 10X(most common) eyepiece lens, we get total magnifications of 40X (4X times 10X),100X , 400X and 1000X. To have good resolution at 1000X, you will need a relativelysophisticated microscope with an Abbe condenser. The shortest lens is the lowestpower, the longest one is the lens with the greatest power. Lenses are color coded andif built to DIN standards are interchangeable between microscopes. The high powerobjective lenses are retractable (i.e. 40XR). This means that if they hit a slide, the endof the lens will push in (spring loaded) thereby protecting the lens and the slide. Allquality microscopes have achromatic, parcentered, parfocal lenses.

Rack Stop: This is an adjustment that determines how close the objective lens can getto the slide. It is set at the factory and keeps students from cranking the high powerobjective lens down into the slide and breaking things. You would only need to adjustthis if you were using very thin slides and you weren't able to focus on the specimen athigh power. (Tip: If you are using thin slides and can't focus, rather than adjust the rackstop, place a clear glass slide under the original slide to raise it a bit higher)

B. HOW TO USE THE MICROSCOPE

1. When moving your microscope, always carry it with both hands (Figure 1). Graspthe arm with one hand and place the other hand under the base for support.

2. Turn the revolving nosepiece so that the lowest power objective lens is "clicked"into position.


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3. Place the microscope slide on the stage and fasten it with the stage clips. Youcan push down on the back end of the stage clip to open it.

4. Using the coarse adjustment, lower the objective lens down as far as it will gowithout touching the slide!Note: Look at the slide and lens from the side when

doing this (see Figure 2).

5. Look through the eyepiece and adjust the illuminator (or mirror) and diaphragm(Figure 3) for the greatest amount of light.

6. Slowly turn the coarse adjustment so that the objective lens goes up (away fromthe slide). Continue until the image comes into focus. Use the fine adjustment, ifavailable, for fine focusing.

7. Move the microscope slide around so that the image is in the center of the field ofview and readjust the mirror, illuminator or diaphragm for the clearest image.

8. You should be able to change to the next objective lenses with only slightfocusing adjustment. Use the fine adjustment, if available. If you cannot focus onyour specimen, repeat steps 4 through 7 with the higher power objective lens inplace. DO NOT ALLOW THE LENS TO TOUCH THE SLIDE!

9. The proper way to use a monocular microscope is to look through the eyepiecewith one eye and keep the other eye open (this helps avoid eye strain). If you

have to close one eye when looking into the microscope, it's ok. Remember,everything is upside down and backwards. When you move the slide to the right,the image goes to the left!

10. Do not touch the glass part of the lenses with your fingers. Use only special lenspaper to clean the lenses. (read the page on keeping your microscope clean)


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11. When finished, raise the tube, click the low power lens into position and removethe slide.

Remember, microscopes are expensive scientific instruments. Handle themproperly and carefully and they will last for many years!

C. TYPES OF MICROSCOPE


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I. The Compound Light Microscope

Commonly binocular (two eyepieces), the compound lightmicroscope, combines the power of lenses and light to enlarge the

subject being viewed.

Typically, the eyepiece itself allows for 10X or 15X magnificationand when combined with the three or four objective lenses, whichcan be rotated into the field of view, produce higher magnificationto a maximum of around 1000X generally.

The compound light microscope is popular among botanists forstudying plant cells, in biology to view bacteria and parasites as wellas a variety of human/animal cells.

It is a useful microscope in forensic labs foridentifying drug structures.

Compound light microscopes are one of themost familiar of the different types ofmicroscopes as they are most often found inscience and biology classrooms.

For this reason, simple models are readily available and areinexpensive.

II. The Stereo Microscope

The Stereo microscope, also called a dissecting microscope, hastwo optical paths at slightly different angles allowing the image tobe viewed three-dimensionally under the lenses.


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Stereo microscopes magnify at low power, typically between 10X

and 200X, generally below 100x.

With this type of microscope you generally have the choice ofpurchasing the fixed or zoom variety from a manufacturer and are

relatively inexpensive.

Uses for this type of microscope include looking at surfaces,microsurgery, and watch making, plus building and inspectingcircuit boards.

Stereo microscopes allow students to observe plant photosynthesisin action.

III. The Digital Microscope

Step into the 21st century with a digital microscope and enter aworld of amazing detail.

The digital microscope, invented in Japan in 1986, uses the powerof the computer to view objects not visible to the naked eye.

Among the different types of microscopes, this kind can be foundwith or without eyepieces to peer into.
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It connects to a computer monitor via a USB cable, much like

connecting a printer or mouse. The computer software allows the

monitor to display the magnified specimen. Moving images can be

recorded or single images captured in the computers memory.

An advantage of digital microscopes is the ability to email images,as well as comfortably watch moving images for long periods.

The popularity of the digital microscope has increased at schoolsand among hobbyists.

IV. The USB Computer Microscope

Although not well suited to the same scientific applications as otherlight microscopes, the USB Computer microscope, among thedifferent types of microscopes, can be used on almost any objectand requires no preparation of the specimen.

It is essentially a macro lens used to examine images on acomputer screen plugged into its USB port.
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However, the magnification is restricted and is not comparable toyour standard compound light microscope at only up to 200X with arelatively small depth of field.

Great for hobbyists and kids, it is an inexpensive device with apurchase price usually under $200US.

V. The Pocket Microscope

In examining the different types of microscopes available on themarket, the pocket microscope may be tiny but its abilities areimpressive.

This is a device which is a great gift for a child or your student. It isused by scientists for hand-held imaging of a variety ofspecimens/objects in the field or in the laboratory.

It is small, durable and portable with a magnification ranging from25x to 100x. There are many different models available.

VI. The Electron Microscope

Among the different types of microscopes, the ElectronMicroscope(EM) is a powerful microscope available and usedtoday, allowing researchers to view a specimen at nanometer size.

The transmission electron microscope(TEM), the first type of EM, iscapable of producing images 1 nanometer in size.

The TEM is a popular choice for nanotechnology as well as

semiconductor analysis and production.

A second type of electron microscope is the scanningelectron microscope(SEM)are approximately 10 timesless powerful than TEMs, they produce high-resolution, sharp, black and white 3D images.
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The Transmission Electron Microscopes and Scanning ElectronMicroscopes have practical applications in such fields as biology,chemistry, gemology, metallurgy and industry as well as provideinformation on the topography, morphology, composition and

crystallographic data of samples.

VII. The Scanning Probe Microscope (SPM)

Among the different types of microscopes and microscopytechniques, scanning probe microscopy is used today in academicand industrial settings for those sectors involving physics, biology

and chemistry. These instruments are used in research anddevelopment as standard analysis tools.

Images are highly magnified and are observed as three-dimensional-shaped-specimens in real time. SPMs employ a delicateprobe to scan the surface of the specimen eliminating the limitationsthat are found in electron and light microscopy.

D. Microscope Care & Handling

1.


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So why do I need to

know how to use the

microscope?

Because microscopes cost several hundred

dollars it is very important to make them last

for a long time.

What happens if I

break a microscope?

You break it, you buy a new one.....

How long will a

microscope last if I

take good care of it?

They can last for at least 10 years if you care

for the "scope" as well as you care for your hair

2. Care and Handling

Transporting:

When you pick up the microscope and walk with

it, grab the arm with one hand and place your

other hand on the bottom of the base.

DON'T SWING THE MICROSCOPE !

Handling &

Cleaning:

Never touch the lenses with your fingers. Your

body produces an oil that smudges the glass.

This oil can even etch the glass if left on too

long. Use only LENS PAPER to clean the glass.

TOILET PAPER, KLEENEX, AND PAPER

TOWELS HAVE FIBERS THAT CAN SCRATCH

THE LENSES.

Storage:

When you are finished with your "scope"

assignment, rotate the nosepiece so that it's on

the low power objective, roll the nosepiece so

that it's all the way down to the stage, then

replace the dust cover.

DON'T FORGET TO USE PROPER

TRANSPORTING TECHNIQUES!

Clean up:

Clean all slides, materials, and work area when

you're done. Please, be careful with the slides

and cover slips. They are made of glass and if

broken, you will get cut and you will bleed.
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DON'T CUT YOURSELF, THERE ARE NO BAND

AIDS IN THIS ROOM.

Microphones

I. How They Work. II. Specifications.

III. Pick Up Patterns

IV. Typical Placement

V. The Microphone Mystique

I. How They Work.
http://artsites.ucsc.edu/ems/music/tech_background/te-20/teces_20.html#I.http://artsites.ucsc.edu/ems/music/tech_background/te-20/teces_20.html#II.http://artsites.ucsc.edu/ems/music/tech_background/te-20/teces_20.html#III.http://artsites.ucsc.edu/ems/music/tech_background/te-20/teces_20.html#IV.http://artsites.ucsc.edu/ems/music/tech_background/te-20/teces_20.html#V.http://artsites.ucsc.edu/ems/music/tech_background/te-20/teces_20.html#I.http://artsites.ucsc.edu/ems/music/tech_background/te-20/teces_20.html#II.http://artsites.ucsc.edu/ems/music/tech_background/te-20/teces_20.html#III.http://artsites.ucsc.edu/ems/music/tech_background/te-20/teces_20.html#IV.http://artsites.ucsc.edu/ems/music/tech_background/te-20/teces_20.html#V.


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A microphone is an example of a transducer, a device thatchanges information from one form to another. Soundinformation exists as patterns of air pressure; the microphonechanges this information into patterns of electric current. The

recording engineer is interested in the accuracy of thistransformation, a concept he thinks of as fidelity.

A variety of mechanical techniques can be used in buildingmicrophones. The two most commonly encountered in recordingstudios are the magneto-dynamic and the variable condenserdesigns.

THE DYNAMIC MICROPHONE.

In the magneto-dynamic, commonly called dynamic, microphone,sound waves cause movement of a thin metallic diaphragm andan attached coil of wire. A magnet produces a magnetic fieldwhich surrounds the coil, and motion of the coil within this field

causes current to flow. The principles are the same as those thatproduce electricity at the utility company, realized in a pocket-sized scale. It is important to remember that current is producedby the motion of the diaphragm, and that the amount of currentis determined by the speed of that motion. This kind ofmicrophone is known as velocity sensitive.


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THE CONDENSER MICROPHONE.

In a condenser microphone, the diaphragm is mounted close to,but not touching, a rigid backplate. (The plate may or may nothave holes in it.) A battery is connected to both pieces of metal,which produces an electrical potential, or charge, between them.The amount of charge is determined by the voltage of thebattery, the area of the diaphragm and backplate, and thedistance between the two. This distance changes as thediaphragm moves in response to sound. When the distancechanges, current flows in the wire as the battery maintains thecorrect charge. The amount of current is essentially proportioinalto the displacement of the diaphragm, and is so small that itmust be electrically amplified before it leaves the microphone.

A common varient of this design uses a material with apermanently imprinted charge for the diaphragm. Such a materialis called an electret and is usually a kind of plastic. (You oftenget a piece of plastic with a permanent charge on it when youunwrap a record. Most plastics conduct electricity when they arehot but are insulators when they cool.) Plastic is a pretty goodmaterial for making diaphragms since it can be dependablyproduced to fairly exact specifications. (Some popular dynamicmicrophones use plastic diaphragms.) The major disadvantage of


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electrets is that they lose their charge after a few years andcease to work.

II. Specifications

There is no inherent advantage in fidelity of one type ofmicrophone over another. Condenser types require batteries orpower from the mixing console to operate, which is occasionally ahassle, and dynamics require shielding from stray magneticfields, which makes them a bit heavy sometmes, but very finemicrophones are available of both styles. The most importantfactor in choosing a microphone is how it sounds in the required

application. The following issues must be considered:

Sensitivity.

This is a measure of how much electrical output is produced by agiven sound. This is a vital specification if you are trying to recordvery tiny sounds, such as a turtle snapping its jaw, but should beconsidered in any situation. If you put an insensitive mic on aquiet instrument, such as an acoustic guitar, you will have toincrease the gain of the mixing console, adding noise to the mix.

On the other hand, a very sensitive mic on vocals might overloadthe input electronics of the mixer or tape deck, producingdistortion.

Overload characteristics.

Any microphone will produce distortion when it is overdriven byloud sounds. This is caused by varous factors. With a dymanic,the coil may be pulled out of the magnetic field; in a condenser,

the internal amplifier might clip. Sustained overdriving orextremely loud sounds can permanently distort the diaphragm,degrading performance at ordinary sound levels. Loud sounds areencountered more often than you might think, especially if youplace the mic very close to instruments. (Would you put your earin the bell of a trumpet?) You usually get a choice between high


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sensitivity and high overload points, although occasionally thereis a switch on the microphone for different situations.

Linearity, or Distortion.

This is the feature that runs up the price of microphones. Thedistortion characteristics of a mic are determined mostly by thecare with which the diaphragm is made and mounted. Highvolume production methods can turn out an adequatemicrophone, but the distortion performance will be a matter ofluck. Many manufacturers have several model numbers for whatis essentially the same device. They build a batch, and then testthe mics and charge a premium price for the good ones. The

really big names throw away mic capsules that don't meet theirstandards. (If you buy one Neumann mic, you are paying forfive!)

No mic is perfectly linear; the best you can do is find one withdistortion that complements the sound you are trying to record.This is one of the factors of the microphone mystique discussedlater.

Frequency response.

A flat frequency response has been the main goal of microphonecompanies for the last three or four decades. In the fifties, mics

were so bad that console manufacturers began adding equalizersto each input to compensate. This effort has now paid off to thepoint were most professional microphones are respectably flat, atleast for sounds originating in front. The major exceptions aremics with deliberate emphasis at certain frequencies that areuseful for some applications. This is another part of themicrophone mystique. Problems in frequency response are mostly


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encountered with sounds originating behind the mic, as discussedin the next section.

Noise.

Microphones produce a very small amount of current, whichmakes sense when you consider just how light the moving partsmust be to accurately follow sound waves. To be useful forrecording or other electronic processes, the signal must beamplified by a factor of over a thousand. Any electrical noiseproduced by the microphone will also be amplified, so even slightamounts are intolerable. Dynamic microphones are essentiallynoise free, but the electronic circuit built into condensor types is a

potential source of trouble, and must be carefully designed andconstructed of premium parts.

Noise also includes unwanted pickup of mechanical vibrationthrough the body of the microphone. Very sensitive designsrequire elastic shock mountings, and mics intended to be held inthe hand need to have such mountings built inside the shell.

The most common source of noise associated with microphones isthe wire connecting the mic to the console or tape deck. A micpreamp is very similar to a radio reciever, so the cable must beprevented from becoming an antenna. The basic technique is tosurround the wires that carry the current to and from the micwith a flexible metallic shield, which deflects most radio energy. Asecond technique, which is more effective for the low frequency

hum induced by the power company into our environment, is tobalance the line:


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Current produced by the microphone will flow down one wire ofthe twisted pair, and back along the other one. Any currentinduced in the cable from an outside source would tend to flowthe same way in both wires, and such currents cancel each otherin the transformers. This system is expensive.

Microphone Levels

As I said, microphone outputs are of necessity very weak signals,generally around -60dBm. (The specification is the powerproduced by a sound pressure of 10 uBar) The output impedancewill depend on whether the mic has a transformer balancedoutput . If it does not, the microphone will be labeled "highimpedance" or "hi Z" and must be connected to an appropriateinput. The cable used must be kept short, less than 10 feet or so,

to avoid noise problems.

If a microphone has a transformer, it will be labeled lowimpedance, and will work best with a balanced input mic preamp.The cable can be several hundred feet long with no problem.Balanced output, low impedance microphones are expensive, andgenerally found in professonal applications. Balanced outputsmust have three pin connectors ("Cannon plugs"), but not allmics with those plugs are really balanced. Microphones with

standard or miniature phone plugs are high impedance. Abalanced mic can be used with a high impedance input with asuitable adapter.

III. Pick Up Patterns


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Many people have the misconception that microphones only pickup sound from sources they are pointed at, much as a cameraonly photographs what is in front of the lens. This would be a nicefeature if we could get it, but the truth is we can only

approximate that action, and at the expense of other desirablequalities.

MICROPHONE PATTERNS

These are polar graphs of the output produced vs. the angle ofthe sound source. The output is represented by the radius of thecurve at the incident angle.

Omni

The simplest mic design will pick up all sound, regardless of itspoint of origin, and is thus known as an omnidirectionalmicrophone. They are very easy to use and generally have goodto outstanding frequency response. To see how these patternsare produced, here's a sidebar on directioal microphones.

Bi-directional

It is not very difficult to produce a pickup pattern that acceptssound striking the front or rear of the diaphragm, but does notrespond to sound from the sides. This is the way any diaphragmwill behave if sound can strike the front and back equally. Therejection of undesired sound is the best achievable with any
http://artsites.ucsc.edu/ems/music/tech_background/te-20/Directional_Microphones.htmlhttp://artsites.ucsc.edu/ems/music/tech_background/te-20/Directional_Microphones.html


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design, but the fact that the mic accepts sound from both endsmakes it difficult to use in many situations. Most often it is placedabove an instrument. Frequency response is just as good as anomni, at least for sounds that are not too close to the

microphone.

Cardioid

This pattern is popular for sound reinforcement or recordingconcerts where audience noise is a possible problem. The conceptis great, a mic that picks up sounds it is pointed at. The reality isdifferent. The first problem is that sounds from the back are notcompletely rejected, but merely reduced about 10-30 dB. This

can surprise careless users. The second problem, and a severeone, is that the actual shape of the pickup pattern varies withfrequency. For low frequencies, this is an omnidirectionalmicrophone. A mic that is directional in the range of bassinstruments will be fairly large and expensive. Furthermore, thefrequency response for signals arriving from the back and sideswill be uneven; this adds an undesired coloration to instrumentsat the edge of a large ensemble, or to the reverberation of theconcert hall.

A third effect, which may be a problem or may be a desiredfeature, is that the microphone will emphasize the low frequencycomponents of any source that is very close to the diaphragm.This is known as the "proximity effect", and many singers andradio announcers rely on it to add "chest" to a basically lightvoice. Close, in this context, is related to the size of themicrophone, so the nice large mics with even back and side

frequency response exhibit the strongest presence effect. Mostcardioid mics have a built in lowcut filter switch to compensate forproximity. Missetting that switch can cause hilarious results.Bidirectional mics also exhibit this phenomenon.
http://artsites.ucsc.edu/ems/music/tech_background/te-20/Proximity_Effect.htmlhttp://artsites.ucsc.edu/ems/music/tech_background/te-20/Proximity_Effect.html


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Tighter Patterns

It is posible to exaggerate the directionality of cardioid typemicrophones, if you don't mind exaggerating some of the

problems. The Hypercardioid pattern is very popular, as it gives abetter overall rejection and flatter frequency response at the costof a small back pickup lobe. This is often seen as a goodcompromise between the cardioid and bidirectional patterns. A"shotgun" mic carries these techniques to extremes by mountingthe diaphragm in the middle of a pipe. The shotgun is extremelysensitive along the main axis, but posseses pronounced extralobes which vary drastically with frequency. In fact, the frequencyresponse of this mic is so bad it is usually electronically restricted

to the voice range, where it is used to record dialogue for filmand video.

Stereo microphones

You don't need a special microphone to record in stereo, you justneed two (see below). A so called stereo microphone is really twomicrophones in the same case. There are two kinds: extremelyexpensive professional models with precision matched capsules,

adjustable capsule angles, and remote switching of pickuppatterns; and very cheap units (often with the capsules orientedat 180 deg.) that can be sold for high prices because they havethe word stereo written on them.

IV. Typical Placement

Single microphone use

Use of a single microphone is pretty straightforward. Havingchosen one with appropriate sensitivity and pattern, (and thebest distortion, frequency response, and noise characteristics youcan afford), you simply mount it where the sounds are. Thepractical range of distance between the instrument and the


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microphone is determined by the point where the soundoverloads the microphone or console at the near end, and thepoint where ambient noise becomes objectionable at the far end.Between those extremes it is largely a matter of taste and

experimentation.

If you place the microphone close to the instrument, and listen tothe results, you will find the location of the mic affects the waythe instrument sounds on the recording. The timbre may be odd,or some notes may be louder than others. That is because thevarious components of an instrument's sound often come fromdifferent parts of the instrument body (the highest note of apiano is nearly five feet from the lowest), and we are used tohearing an evenly blended tone. A close in microphone willrespond to some locations on the instrument more than othersbecause the difference in distance from each to the mic isproportionally large. A good rule of thumb is that the blend zonestarts at a distance of about twice the length of the instrument. Ifyou are recording several instruments, the distance between theplayers must be treated the same way.

If you place the microphone far away from the instrument, it willsound as if it is far away from the instrument. We judge sonicdistance by the ratio of the strength of the direct sound from theinstrument (which is always heard first) to the strength of thereverberation from the walls of the room. When we are physicallypresent at a concert, we use many cues beside the sounds tokeep our attention focused on the performance, and we are able

to ignore any distractions there may be. When we listen to arecording, we don't have those visual clues to what is happening,and find anything extraneous that is very audible annoying. Forthis reason, the best seat in the house is not a good place torecord a concert. On the other hand, we do need somereverberation to appreciate certain features of the music. (That is


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why some types of music sound best in a stone church) Closemicrophone placement prevents this. Some engineers prefer touse close miking techniques to keep noise down and add artificialreverberation to the recording, others solve the problem by

mounting the mic very high, away from audience noise but whereadequate reverberation can be found.

Stereo

Stereo sound is an illusion of spaciousness produced by playing arecording back through two speakers. The success of this illusionis referred to as the image. A good image is one in which eachinstrument is a natural size, has a distinct location within the

sound space, and does not move around. The main factors thatestablish the image are the relative strength of an instrument'ssound in each speaker, and the timing of arrival of the sounds atthe listener's ear. In a studio recording, the stereo image isproduced artificially. Each instrument has its own microphone,and the various signals are balanced in the console as theproducer desires. In a concert recording, where the point is todocument reality, and where individual microphones would beawkward at best, it is most common to use two mics, one for

each speaker.


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Spaced microphones

The simplest approach is to assume that the speakers will be

eight to ten feet apart, and place two microphones eight to tenfeet apart to match. Either omnis or cardioids will work. Whenplayed back, the results will be satisfactory with most speakerarrangements. (I often laugh when I attend concerts and watchpeople using this setup fuss endlessly with the precise placementof the mics. This technique is so forgiving that none of theirefforts will make any practical difference.)

The big disavantage of this technique is that the mics must berather far back from the ensemble- at least as far as the distancefrom the leftmost performer to the rightmost. Otherwise, thoseinstruments closest to the microphones will be too prominent.There is usually not enough room between stage and audience toachieve this with a large ensemble, unless you can suspend themics or have two very tall stands.


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Coincident cardioids

There is another disadvantage to the spaced technique thatappears if the two channels are ever mixed together into a

monophonic signal. (Or broadcast over the radio, for similarreasons.) Because there is a large distance between the mics, itis quite possible that sound from a particular instrument wouldreach each mic at slightly different times. (Sound takes 1millisecond to travel a foot.) This effect creates phase differencesbetween the two channels, which results in severe frequencyresponse problems when the signals are combined. You seldomactually lose notes from this interference, but the result is anuneven, almost shimmery sound. The various coincident

techniques avoid this problem by mounting both mics in almostthe same spot.

This is most often done with two cardioid microphones, onepointing slightly left, one slightly right. The microphones are oftenpointing toward each other, as this places the diaphragms withina couple of inches of each other, totally eliminating phaseproblems. No matter how they are mounted, the microphone that

points to the left provides the left channel. The stereo effectcomes from the fact that the instruments on the right side areon-axis for the right channel microphone and somewhat off-axis(and therefore reduced in level) for the other one. The anglebetween the microphones is critical, depending on the actualpickup pattern of the microphone. If the mics are too parallel,there will be little stereo effect. If the angle is too wide,instruments in the middle of the stage will sound weak, producinga hole in the middle of the image. [Incidentally, to use this

technique, you must know which way the capsule actually points.There are some very fine German cardioid microphones in whichthe diaphragm is mounted so that the pickup is from the side,even though the case is shaped just like many popular endaddressed models. (The front of the mic in question is marked bythe trademark medallion.) I have heard the results where an


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engineer mounted a pair of these as if the axis were at the end.You could hear one cello player and the tympani, but not muchelse.]

You may place the microphones fairly close to the instrumentswhen you use this technique. The problem of balance betweennear and far instruments is solved by aiming the mics toward theback row of the ensemble; the front instruments are therefore offaxis and record at a lower level. You will notice that the height ofthe microphones becomes a critical adjustment.

M.S.

The most elegant approach to coincident miking is the M.S. ormiddle-side technique. This is usually done with a stereomicrophone in which one element is omnidirectional, and theother bidirectional. The bidirectional element is oriented with theaxis running parallel to the stage, rejecting sound from thecenter. The omni element, of course, picks up everything. Tounderstand the next part, consider what happens as instrument ismoved on the stage. If the instrument is on the left half of the

stage, a sound would first move the diaphragm of thebidirectional mic to the right, causing a positive voltage at theoutput. If the instrument is moved to center stage, themicrophone will not produce any signal at all. If the instrument ismoved to the right side, the sound would first move thediaphragm to the left, producing a negative volage. You can thensay that instruments on one side of the stage are 180 degreesout of phase with those on the other side, and the closer they areto the center, the weaker the signal produced.

Now the signals from the two microphones are not merely kept intwo channels and played back over individual speakers. Thesignals are combined in a circuit that has two outputs; for the leftchannel output, the bidirectional output is added to the omni


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signal. For the right channel output, the bidirectional output issubtracted from the omni signal. This gives stereo, because aninstrument on the right produces a negative signal in thebidirectional mic, which when added to the omni signal, tends to

remove that instrument, but when subtracted, increases thestrength of the instrument. An instrument on the left suffers theopposite fate, but instruments in the center are not affected,because their sound does not turn up in the bidirectional signal atall.

M.S. produces a very smooth and accurate image, and is entirelymono compatabile. The only reason it is not used moreextensively is the cost of the special microphone and decodingcircuit, well over $1,000.

Large ensembles

The above techniques work well for concert recordings in goodhalls with small ensembles. When recording large groups indifficult places, you will often see a combination of spaced andcoincident pairs. This does produce a kind of chorusing when the

signals are mixed, but it is an attractive effect and not verydifferent from the sound of string or choral ensembles any way.When balance between large sections and soloists cannot beacheived with the basic setup, extra microphones are added tohighlight the weaker instruments. A very common problem withlarge halls is that the reverberation from the back seems latewhen compared to the direct sound taken at the edge of thestage. This can be helped by placing a mic at the rear of theaudience area to get the ambient sound into the recording

sooner.

Studio techniques


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A complete description of all of the procedures and tricksencountered in the recording studio would fill several books.These are just a few things you might see if you dropped in onthe middle of a session.

Individual mics on each instrument.

This provides the engineer with the ability to adjust the balanceof the instruments at the console, or, with a multitrack recorder,after the musicians have gone home. There may be eight or ninemics on the drum set alone.

Close mic placement.

The microphones will usually be placed rather close to theinstruments. This is partially to avoid problems that occur whenan instrument is picked up in two non-coincident mics, andpartially to modify the sound of the instruments (to get a "honky-tonk" effect from a grand piano, for instance).

Acoustic fences around instruments, or instruments in

separate rooms.

The interference that occurs when when an instrument is pickedup by two mics that are mixed is a very serious problem. You willoften see extreme measures, such as a bass drum stuffed withblankets to muffle the sound, and then electronically processed tomake it sound like a drum again.

Everyone wearing headphones.

Studio musicians often play to "click tracks", which are not

recorded metronomes, but someone tapping the beat with sticksand occasionally counting through tempo changes. This is donewhen the music must be synchronized to a film or video, but isoften required when the performer cannot hear the othermusicians because of the isolation measures described above.


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20 or 30 takes on one song.

Recordings require a level of perfection in intonation and rhythmthat is much higher than that acceptable in concert. The finished

product is usually a composite of several takes.

Pop filters in front of mics.

Some microphones are very sensitive to minor gusts of wind--sosensitive in fact that they will produce a loud pop if you breath onthem. To protect these mics (some of which can actually bedamaged by blowing in them) engineers will often mount a nylonscreen between the mic and the artist. This is not the most

common reason for using pop filters though:Vocalists like to move around when they sing; in particular, theywill lean into microphones. If the singer is very close to the mic,any motion will produce drastic changes in level and soundquality. (You have seen this with inexpert entertainers using handheld mics.) Many engineers use pop filters to keep the artist atthe proper distance. The performer may move slightly in relationto the screen, but that is a small proportion of the distance to themicrophone.

V. The Microphone Mystique

There is an aura of mystery about microphones. To the generalpublic, a recording engineer is something of a magician, privy toa secret arcana, and capable of supernatural feats. A few modernday engineers encourage this attitude, but it is mostly a holdoverfrom the days when studio microphones were expensive andfragile, and most people never dealt with any electronics morecomplex than a table radio. There are no secrets to recording; the

art is mostly a commonsense application of the principles alreadydiscussed in this paper. If there is an arcana, it is anaccumulation of trivia achieved through experience with thefollowing problems:


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Matching the microphone to the instrument.

There is no wrong microphone for any instrument. Every engineerhas preferences, usually based on mics with which he is familiar.

Each mic has a unique sound, but the differences between goodexamples of any one type are pretty minor. The artist has aconception of the sound of his instrument, (which may not beaccurate) and wants to hear that sound through the speakers.Frequency response and placement of the microphone will affectthat sound; sometimes you need to exaggerate the features ofthe sound the client is looking for.

Listening the proper way.

It is easy to forget that the recording engineer is an illusionist-the result will never be confused with reality by the listener.Listeners are in fact very forgiving about some things. It isimportant that the engineer be able to focus his attention on themain issues and not waste time with interesting but minortechnicalities. It is important that the engineer know what themain issues are. An example is the noise/distortion tradeoff. Mostlisteners are willing to ignore a small amount of distortion on loud

passages (in fact, they expect it), but would be annoyed by theextra noise that would result if the engineer turned the recordinglevel down to avoid it. One technique for encouraging thisattention is to listen to recordings over a varitey of soundsystems, good and bad.

Learning for yourself.

Many students come to me asking for a book or a course of studythat will easily make them a member of this elite company. Thereare books, and some schools have courses in recording, but theydo not supply the essential quality the professional recordingengineer needs, which is experience.


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A good engineer will have made hundreds of recordings usingdozens of different microphones. Each session is an opportunityto make a new discovery. The engineer will make careful notes ofthe setup, and will listen to the results many times to build an

association between the technique used and the sound achieved.Most of us do not have access to lots of professionalmicrophones, but we could probably afford a pair of generalpurpose cardioids. With about $400 worth of mics and a reliabletape deck, it is possible to learn to make excellent recordings.The trick is to record everything that will sit still and make noise,and study the results: learn to hear when the mic is placed badlyand what to do about it. When you know all you can about yourmics, buy a different pair and learn those. Occasionally, you will

get the opportunity to borrow mics. If possible, set them up rightalongside yours and make two recordings at once. It will not belong before you will know how to make consistently excellentrecordings under most conditions.

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