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Microphone theory

Mic Theory

Transducer- device that changes one form of energy into another corresponding form.

Audio Signal- changes in voltage that represent sound.

Mic- transducer that converts sound (acoustic energy) into signal (electrical energy). Means small sound.

Micro- small
Phone- sound

A mic converts sound into a low level signal(sometimes only a few thousandths of a volt).

Our systems operating level, called line level, is much higher that most mic’s output(zero dB on analog gear’s meters is 1.23 volts).

We use a mic pre amp to boost level.
A mic preamp is in the mixer, but we may use an external preamp or a tape machine may have one (usually portable machines).

Amplitude- the amount of change in sound or signal’s intensity. Generally measured in dB.

If sound’s amplitude changes, the amplitude of the mic’s output voltage must change by the same ratio.

Frequency- the number of times per second a sound or signal completes a cycle.
Measured in cps(cycles per second) or hz(hertz).
If the sound’s frequency changes, the mic’s output must change to the rate.
Any changes in the sound must be represented in the signal.

No mic is entirely accurate. Somtimes we want accuracy, as in recording classical, acoustic, ambient. Many times, we use the mic to change or “color” the sound intentionally.

Mic Spec’s

Polar pattern (pickup pattern)- describes how a mic responds to sounds from different directions. A polar plot is a visual indication of how the mic responds.

Omnidirectional- picks up equaly as well from all directions often used when we want to pick up more room reverb and we mic at a distance.

Unidirectional- more sensitive to sounds entering from the front. Discriminates against sounds entering from sides and rear. There are 3 types: Cardiod, Supercardiod, Hypercardioid.

Cardiod- heart shaped, rejects most from the rear and some from the sides.

Supercardiod- Rejects more from sides than cardiod. Does not reject from rear as well as cardiod.

Hypercardiod- Rejects even more from the sides and less from rear.


Directional mic’s can pick up hi freq’s at a greater distance than omni mics.

Hypercardioids are the best at this, supercardiods are second best.

Cardioid
Most common in a studio
Minimize pickup of unwanted sounds
Wider pattern than super or hyper
Allows more movement from performer

Super & Hypercardiod
Most common in production sound (film/video)
Since we can’t place the mic’s in filming as close as we might like, their superior pickup of hi freq’s at a distance adds clarity.


Bidirectional or Figure8- picks up equally well from front and rear, rejects from sides. Picks up more room sound that a cardiod, less than omni. Used less often than other patterns.

Some mic’s have a selectable polar pattern, multipattern mic’s.

On-Axis Pickup- When a sound source is at 0degrees to microphone diaphragm’s center, it is said to be on-axis. Placing a sound source on-axis gives us the highest level and most accurate pickup.

Diaphragm- area of mic that responds to sound.

Off-Axis Pickup- When sound source is other than o degrees to mic’s diaphragm.
The polar pattern usually shows mic’s pattern at 1khz. Some charts show mic’s response at other freq’s as well. Uneven response from sides and rear is called off-axis coloration.
Unidirectional mic’s are usually less directional at lower freq’s, meaning off-axis sound will be dark and muddy.

Frequancy response: The range of freq’s that a device will produce and its accuracy over that range.

Flat response: No variations in amplitude over a device’s range. Flatter response and a wider range makes a device more accurate.

Range of freq’s is in hertz.
Amount of variation is in decibels, above or below flat resonse.

Presence Peaks: an increase in upper midrange.

Proximity effect: An increase in low’s when mic is close to sound source. Any mic other that an omnidirectional mic will have proximity effect(bidirectional mics have the most). Some freq’s response charts will show response at a distance and up close.

Sensitivity: How many volts output a mic produces for a given sound pressure level(spl) input. Does not affect sound quality. Affects audibility of mic preamp noise (a low sensetivity requires a greater boost from preamp, increasing noise). This is only important when recording quiet sounds.

Absolute Zero: at -460 degrees ferinhit all meleculare motion stops. NO NOISE!

Maximum SPL: The sound pressure at which mic begins to audibly distort. Only inportant when miking loud sources. A mic that is sensitive may not handle high SPL.

Self noise: The electrical noise which the mic itself produces (also called equivalent noise level). Only important when recoridng quiet sounds. We record the noise of the mic and the preamp. A noisy mic with higher sensitivity may produce less total noise than a quieter but less sensitive mic.

Transient response: A sudden, momentary rise in level.

MIC FEATURES

Mic’s are often categorized as large or small diaphram. Under 1” diameter is usually called small.

Small: More sensitive, move faster, so high freq’s and transients are reproduced better.

Large: Better bass response.

Front address: Diaphram (pickup area) is at 90 degree angle to mic’s body length.

Side address: Diaphram is parallel to body lenth.

MIC CABLE CONNECTORS

XLR: All pro mic’s use this connector. Balanced connector (balanced systems can carry a signal over a longer cable with less noise pickup). The connectors lock.

Male= output
Female= input

Pro mics are low impedance. They don’t have high freq with increased cable length. High impedance gear does. All other pro gear is lo impedance as well.

Vibration coming up the stand from the floor can produce noise. Shock mounts are used to minimize noise.

Mic Pad: Reduces mic capsule’s output level by a preset amount. Used to prevent overload of mic’s internal electronics due to hi level sound source.

FAMILIES OF MICS COMMONLY USED IN A STUDIO

Electomagnetic: There are 2 types.

moving coil (dynamic)
Ribbon (technically dynamic too)
Most engineers mean moving coil when they say dynamic.

Electrostatic: 1 type

condenser

Changes in air pressure cause movement of diaphram and voice coil in a magnetic field, causing a changing voltage (positive or negative) to be created in the voice coil.

Characteristics of a moving coil mic: Generally cardioid (some are super or hyper and a few are omni but never figure8). Proximity effect (all unidirectional). Presence peaks emphasized upper mids. Overload resistant. Rugged & durable. Respond slowly to transients. Low self noise. Not sensitive. Used most on hi level freq’s and very close to source.

Changes in air velocity cause movement of conductive metal ribbon in a magnetic feild, which causes corresponding changes in voltage.

Ribbon acts as both diaphram and voice coil.

Characteristics of a Ribbon mic: Generally bi-directional/figure8 (can be unidirectional). “Warm sounding”, more emphasized low mids that most other types. More fragile than moving coil. Better transient response than most moving coil mics. Low self noise. Low out put level. Very lo impedance (some models work best with very lo impedance preamps). A few ribbon mics have internal preamps to boost weak signal and require power.

Condensor mics have no magnets. A thin, conductive diaphram is separated by an air space from a conductive backplate. Both are charged by DC voltage. Sound waves cause diaphram to move, which causes a change in an electrical property called capacitance, creating a flow of AC current (audio signal).

Characteristics: Requires external power supply ie; Battery, AC(wall current), Phantom Power.

Vacum tube mics have their own power supply that is conected to AC outlet. Each type of mic may require its own specific supply (this adds to the cost of the mic).

Tube mics need to warm up. Allow 15min or more for unit to stabalize or tune and level will change during recording.

More characteristics: Has internal preamp (boosts low level signal from capsule). Any polar pattern may be made available.

Selectable Patterns: Capsules (diaphram & backplate assemblies) may by inetchangable allowing pattern selection. Mice may have 2 diaphrams and by changing the current suplied to rear diaphram, change the pattern. Widest freq range (extended hi & low). Flattest freq respone. High sensetivity (light weight diaphram). Fast transient response (light weight). Fragile. Higher self noise (active electronics).

Vacum tube vs. Solid State

Either type may be amde to be high quality. The main differance is their distortion characteristics. Tubes distort gradually and emphasize less harsh sounding harmonics. Tubes are usually noisier.




Other types of condensers

Electret condensers- permanently polarized diaphram & backplate. Require a lower voltage than true condenser. Not as sensitive nor as responsive to transients.

Back Electrets- Have a permanently charged backplate only. May be as accurate as true condenser.

Special uses for Condensers

Shotgun mic- A highley directional condenser mic generally used in fillm/video production. Have a very narrow angle of pickup and are generally used a few feet away from sound source.

Parabolic Mic- A mic facing into a dish which picks up an area approximately the same diameter as the dish. Also increases the sound’s amplitude. Often used for sports events & recording bird calls & surveillance.

Contact mics- Mics that pickup vibrations from a solid surface (often that of a musical instrument). These mics may be condesners or other designs.

Hydrophone- Designed for underwated use.

Pressure Zone Mic (PZM) Also know as boundry mic. A condenser mic element faces away from the sound source, directly into a plate a fraction of an inch away. The only sound entering the mic is reflected off the plate. A boundry mic is designed to be placed on a large, flat surface. We usually avoid placing conventional mics close to a reflective surface.

Hemispherical pickup pattern
Eliminates phase distortion from surface on which is mounted.
No Delay
Whith conventional mic’s, phase cancelation due to delay occurs when mic is placed close to a reflective surface. The size of the surface on which it is mounted determines low freq limit.

Miniature condeser/Lavaliers: Often used in film/t.v., concerts. Used when mic needs to be hidden, more portable, or have minimal visual impact. Don’t sound as good as conventional condensers, rarley used in studio. Some are hidden in actors hair(live theater). Not used in studio do to lower sound quality.