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Glossary
smaller dynamic range. A compressed signal
has higher average level, and therefore may
have more apparent loudness than an un-
compressed signal, even though the peaks are
no higher in level. Compression is achieved
with a compressor, a special type of am.plifier
that decreases its gain as the level of the input
signal increases. The amount of compression is
expressed as a ratio of the input dynamic range
to the output dynamic range; thus , a compres-
sor that takes a program input with lOOdB of
dynamic range and yields an output program
of SOdB dynamic range may be said to have a
2:1 compression ratio.
Compressor
A compressor is an amplifier that decreases its
gain as the level of the input signal increase to
reduce the dynamic range of the program (see
"Compression"). A compressor may operate
over the entire range of input levels, or it may
operate only on signals above and/ or below a
given level (the threshold level).
Crossover Frequency
In loudspeake r systems and multi-amplifier
audio systems, the transition frequency (actu-
ally a frequency range) between bass and mid-
range or midrange and treble speakers or
amplifiers.
Glossary
or 1/io of a Bel. The decibel is a ratio, not an ab-
solute number, and is used to express the differ-
ence between two power, voltage or sound
pressure levels. (dB is 10 times the logarithm of
a power ratio or 20 times the logarithm of a
voltage or sound pressure ratio.) If the number
of "dBs" are referenced to a given level, then
the value of the dB number becomes specific:
dBu expresses a voltage ratio. OdBu is usually
referenced to 0.775Vrms. Thus, OdBu = 0.775V,
+6dBu = 1.55V (twice OdBu), +20dBu = 7.75V
(ten times OdBu), etc.
dBv expresses a voltage ratio that is often used
interchangeably with dBu.
dBV expresses a voltage ratio and is similar to
dBu, but OdBV is usually referenced to lVrms.
Thus OdBV is 2.22dB higher than OdBu.
dB SPL expresses a Sound Pressure Level ratio.
dB SPL is a measure of acoustic pressure (loud-
ness), not acoustic power, that would be meas-
ured in acoustic watts. OdB SPL is equal to
0.0002 dynes/ square centimeter (the threshold
of human hearing at lkHz). As with dBV, an in-
crease of 6dB SPL is twice the sound pressure,
and increase of 20dB SPL is an increase of 10
times the sound pressure.
Crossover Network
A circuit that divides the audio spectrum into
two or more frequency bands for distribution
to different speakers (high level crossover) or
different amplifiers that then feed different
speakers (low level crossover).
High level crossovers are usually built into the
speaker cabinet, and are passive (they require
no power supply). Low level crossovers are
used in biamplified or triamplified sound sys-
tems. They are usually self-contained, and
come before the power amplifiers. Low level
crossovers may be passive or active; active low
level crossovers are known as "electronic cross-
overs."
Damping Factor
The ratio of loudspeaker impedance to the
amplifier's output source impedance. Dampin g
describes the amplifier's ability to prevent un-
wanted, residual speaker movement. The
higher the numerical value, the better the
damping.
dB (Decibel) also dBv, dBV, dB SPL, dBu,
dBm,dB
One dB is the smallest change in loudness the
average human ear can detect. OdB SPL is the
threshold of human hearing, whereas the
threshold of pain is between 120 and 130dB
SPL. The term dB is an abbreviation for decibel,
dbx 160XT
dBm expresses a power ratio. OdBm is 1 milli-
watt (.001 watts), or 0.775Vrms delivered to a
600-ohm load. +3dBm = 2 milliwatt s, or 1.55V
into 600 ohms (twice OdBm), +20dBm = 100 mil-
liwatt s, or 7.75V into 600 ohms (ten times
OdBm), etc. dBu and dBm are numericall y
equal when dealing with 600-ohm circuits.
However, when the impedance is other than
600 ohms, the value of dBu remains the same if
the voltage is the same, whereas the value of
dBm decreases with increasing impedan ce.
dB alone, without any suffix, doesn't mean any -
thing unles s it is associated with a referen ce. It
may express the difference between two levels.
Thus, the difference between lOdBV and
15dBV, the difference between OdBm and
SdBm, and the difference between 90dB SPL
and 95dB SPL are all differences of SdB.
Decay Time
Decay time has different meanings in different
contexts. A compressor's decay time is also
known as its release time or recovery time. Af-
ter a compresso r (or expander) changes its gain
to accommodate an incoming signal, and the
signal is then removed, the decay time is the
amount of time required for the circuitry to re-
turn to "normal." More precisely, the decay
time is the interval (usually measured in micro-
seconds) during which the compressing or ex-
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