AKG Acoustics WMS 4000 Feedback, Frequency Management, Frequency Modulation, Frequency Range

AKG WMS GLOSSARY

GLOSSARY

DEFINITIONS FROM A TO Z

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-

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