AKG Acoustics WMS 40 - Manual (Page 90)

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D
EFINITIONS 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