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audio-frequency meter • audio mixer 47
audio-frequency meter An instrument to measure
frequencies in the audio-frequency spectrum (approximately
20 Hz to 20 kHz). Three types are
commonly used:
• Analog Gives direct indications of frequency on
the scale of a D’Arsonval meter; the usual range
is 20 Hz to 100 kHz.
• Digital Gives direct indications of frequency by
means of readout lamps; the usual range is 1 Hz
to 15 MHz. This instrument is useful also as a
radio-frequency meter.
• Bridge Consists of a frequency-sensitive bridge,
such as a Wien bridge, with a null-indicating
meter. The operator balances the bridge and
reads the unknown frequency from the dial of
the balance control.
audio-frequency noise Any electrical noise signal
causing interference within the audio-frequency
spectrum.
audio-frequency oscillator See AUDIO OSCILLA-
TOR.
audio-frequency peak limiter Any circuit or device,
such as a biased diode, that performs the
function of audio limiting.
audio-frequency-shift keying Abbreviation, AFSK.
Frequency-shift keying that is done at audio
frequencies (below approximately 20 kHz) rather
than at radio frequencies. There are two audio
sine-wave signals, one for logic 1 (high or mark)
pulses and the other for the logic 0 (low or space)
pulses. This scheme is commonly used with
telephone modems where the signal bandwidth is
severely limited by circuit characteristics. At
typical data speeds in twisted-pair telephone
lines (usually 28.8 or 57.6 kbps), signals of this
type sound like a hiss or roar. Compare
FREQUENCY-SHIFT KEYING.
audio-frequency-shift modulator A modulator for
audio-frequency-shift keying of a signal.
audio-frequency spectrum The band of frequencies
extending from roughly 20 Hz to 20 kHz.
High-fidelity component specifications extend
this range somewhat in both directions (e.g., from
10 Hz to 50 kHz).
audio-frequency transformer Abbreviation, AF
transformer. A device used for the purpose of
matching impedances at frequencies within the
range of human hearing (up to approximately 20
kHz). This ensures the most efficient possible
transfer of power between stages of audio amplification,
between an amplifier and a speaker or
headset, or between a microphone and an audio
preamplifier. These transformers are available
with various power ratings and impedancematching
ratios. Some devices are tailored to
have a certain attenuation-versus-frequency response.
At audio frequencies, transformers are
physically similar to the alternating-current
transformers used in power supplies. They are
wound on laminated or powdered-iron cores.
Compare RADIO-FREQUENCY TRANSFORMER.
audio-frequency transistor A transistor that is
usually used only at audio frequencies.
audiogram A graph used to rate hearing, used by
audiologists and audiometrists.
audio image In a direct-conversion receiver, a response
to a signal on one side of (above or below)
the local-oscillator (LO) frequency, when the operator
is listening to a signal on the other side of
the LO frequency. These responses are reduced
or eliminated in single-signal receivers.
audio-level meter An ac meter for monitoring signal
amplitude in an audio-frequency system. It
can indicate in volts, decibels, volume units (VU),
or arbitrary units, and is often permanently connected
in the circuit.
audio limiter A limiter or clipper operated in the
audio-frequency (AF) channel of a receiver or
transmitter to hold the output-signal amplitude
constant, or to minimize the effect of noise peaks.
audiologist A person skilled in testing hearing (i.e.,
in using audiometers and other electronic instruments)
and evaluating their indications for the
fitting of hearing aids.
audiometer An instrument used for hearing tests,
which consists of a specialized audio-frequency
(AF) amplifier with calibrated attenuators, output
meter, and signal source.
audiometrist A person skilled in the use of audiometers
and other electronic instruments that
measure sound and human hearing, and who
deals with attendant health and behavior problems.
Compare ACOUSTICIAN and AUDIOLOGIST.
audio mixer An amplifier circuit for blending two
or more audio-frequency (AF) signals, such as
those delivered by microphones or receivers.