AKG MP 40 User Manual download pdf (Page 90)

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DEFINITIONS FROM A TO Z

GLOSSARY

Feedback

When a microphone picks up amplified

sound from a loudspeaker this signal will

be reamplified, picked up again, etc., until

the commonly known shrill howling (some-

times a lower midrange rumbling) sets in.

In small rooms, feedback is usually caused

by reflections. In this case, acoustic treat-

ment of the walls should help. On stages

with correctly set up FOH speakers it is the

monitor speakers that may cause feedback.

A very good hypercardioid microphone (e.g.

a D 3900) may sometimes provide a few

extra dB's of gain-before-feedback. Place

the monitors slightly off-axis (135-) where

the microphone is least sensitive.

Frequency Management

Organization of frequency resources.

Frequency Modulation

A technology that alters (modulates) carrier

frequencies to transmit information.

Frequency Range

The frequency range of a microphone is

usually stated as the upper and lower fre-

quency limits within which the microphone

delivers a useful output signal.

Frequency Response

Microphones are not equally sensitive to all

notes. The frequency response indicates

the relationship between sensitivity and

pitch. The 0-dB reference being the output

voltage at 1 kHz, the frequency response is

measured at constant sound pressure level,

from about 20 Hz (lowest note) to 20 kHz

(above the upper limit of human hearing).

Hum Sensitivity

Magnetic fields from amplifiers, long power

cables, and lighting systems in particular

may induce hum in microphones. A micro-

phone's hum sensitivity gives an indication

of how susceptible it is to this kind of inter-

ference. Values are 3 µV/5 µT for dynamic

microphones with hum suppression coil,

30 µV/5 µT for dynamics with no suppres-

sion coil (D 90, D 95, D 190), and up to

10 µV/5 µT for condenser microphones.

In practice, though, it is the microphone

cables, most of all unbalanced ones, and

mixer inputs, that are most likely to pick up

hum.

Impedance

Frequency dependent AC resistance of a

microphone. Always quoted at 1 kHz the

actual impedance at other frequencies may

differ slightly from this reference value.

Also known as “source impedance”.

Intercept Point

The Intercept Point (IP) provides a measu-

re for an amplifier's resistance to intermo-

dulation distortion. IP 3, for example, is

the reciprocal value of the third-order coef-

ficient of an amplifier's nonlinear transmis-

sion polynomial.

Interference

Disturbance in transmission caused by

extraneous signals.

Intermodulation

A nonlinear (multiplicative) combination of

signals with different carrier frequencies

that will produce completely new frequen-

cies, called intermodulation products.

Limiter

Electronic circuit that prevents subsequent

circuits being overloaded by excessive sig-

nal levels that would also cause distortion.

Line Microphone

The directivity factor of conventional unidi-

rectional microphones is limited by the

laws of physics. This can be overcome by

installing a slotted tube in front of the dia-

phragm (“interference tube”). Off-axis

sounds are canceled through interference,

which results in an ultradirectional polar

pattern.

Matching

Microphones should operate in an open cir-

cuit. This is the case if the input impe-

dance of the preamplifier or mixer is at least

2 to 5 times as high as the microphone's

rated impedance. The appropriate value is

quoted in the specifications of each micro-

phone as “recommended load impedance”.

Maximum SPL

The highest sound pressure level (loud-

ness) a microphone can handle without

introducing more than a specified amount

of “Total Harmonic Distortion” (1 %), in

other words, without distorting the signal.

Usually measured at 1 kHz, except for the

C 460 B ULS Series where it is quoted

from 30 Hz to 20 kHz.

Mechanical Noise

See “Vibrational Noise”.

Memory Effect

The loss of capacity which occurs in nickel-

cadmium storage batteries if they are not

completely discharged prior to recharging.

Modulation/demodulation

A sine-wave carrier starting at a time of

minus infinity and ending at a time of plus

infinity contains no information. However,

any change in amplitude or frequency at

any time (e.g., a pulse-like change) adds

information to the carrier.

This process is called “modulation”. The

process by which a receiver detects and

extracts this information from the carrier is

called “demodulation”.

Multichannel System

A wireless microphone system that allows

several radio microphones to be operated

simultaneously in the same room.

Noise Burst

Brief disruption of the desired signal by

noise from a transient interference source

(e.g., ignition spark).

Noise skirt

An ideal carrier spectrum would be a line.

As the carrier is modulated, the noise inhe-

rent in the switching signals makes the tran-

sients look ragged. This raggedness ultima-

tely frequency-modulates the carrier with

noise. Once that happens, the carrier spec-

trum is no longer a line but a noise spec-

trum that tapers off to either side of the

wanted frequency, which is why this part of

the spectrum is called a “noise skirt”.

Phantom Power

to IEC 2681 5/DIN 45596

Condenser microphones require an opera-

ting voltage. It can be fed to the micro-

phone either by a-b powering or phantom

powering. In a-b powering, the operating

voltage is fed to the balanced audio wires

without using the shield. a-b powering is

incompatible with dynamic microphones

since the operating voltage would flow

through the moving coil and destroy it.

In phantom powering, the negative terminal

is connected to the cable shield and the

positive terminal is split via decoupling

resistors to the balanced audio wires. Since

both audio wires carry the same potential,

no current will flow through the coil of a

dynamic microphone so there is no risk of

destroying it even if the phantom power is

accidentally left on.

When adding phantom power to a single

ended (grounded) input or an input with no

front-end transformer, either capacitors or

an optional transformer need to be wired

into the audio lines as shown below, to pre-

vent leakage currents from entering the

input stage.

Polarity

If you use more than one microphone for a

recording, they should be of the same pola-

rity. This means that if the diaphragms

move in the same direction, the output vol-

tages of all microphones should have the

same polarity. If they don't there will be sig-

nal cancellation effects causing sound

coloration – particularly in the bass range –

as soon as you mix the microphone output

signals together.

Polar Pattern

The “polar pattern” of a microphone indi-

AKG WMS GLOSSARY

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AKG MP 40 User Manual Page 90

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