628
15
Dynamic Range of Sound
The other definition of dynamic range describes the natural range over which signals fluctuate.
The signals under discussion are electrical representations of sounds, so it follows that sound has
dynamic range. The dynamic range of the human voice, from a whisper to a shout, is over 100 dB.
An accurate microphone converts the sound pressure of a voice going from a whisper to a shout
into an electrical voltage having the same dynamic range.
Why Audio Compressors are Our Friends
To keep audio signals below distortion and above the noise floor, their natural dynamic range must
remain within the specified dynamic range of the circuits through which they pass. Unfortunately,
the actual dynamic range of real world signals exceeds the available dynamic range of even the best
audio processing equipment.
For example, the dynamic range of the best analog tape recorders calculates to around 80 dB, while
digital recorders top out around 96 dB. As good as these machines are, theres still not quite
enough room for the very wide dynamic range of unprocessed audio. In order keep signals 60 dB
above the noise floor, the dynamic range of audio recorded on the analog tape recorder would have
to be restricted to 20 dB of dynamic range (80 dB minus 60 dB equals 20 dB). Signals recorded on
the digital recorder would have to be restricted to 36 dB of dynamic range (96 dB minus 60 dB
equals 36 dB).
A compressor or limiter reduces the dynamic range by setting an upper limit on the larger signals.
In some cases, its better to put processing to work on the lower end of the dynamic range rather
than on the upper end. In other words, instead of reducing the amount of change at the upper end
of the dynamic range with a compressor or limiter, we could increase the amount of change at the
lower end of the dynamic range with a downward expander or gate. In practice we throw away
some of the low level audio down near the noise floor.
Compressors are to Limiters as Downward Expanders are to Gates
Compressors reduce the dynamic range of their output whenever the input signal rises above the
set threshold, while downward expanders increase the dynamic range of their output whenever the
input signal falls below a set threshold.
Compressors, limiters, expanders and gates increase or decrease signal levels by some ratio. Good
compressors have an adjustable ratio, a ratio of the input level to the output level which is user-
adjustable. A compressor operating with a 2:1 ratio allows only a 1 dB increase in output level for
every 2 dB increase in input level.
Limiters operate at high ratios (greater than 10:1). At 10:1, the limiter allows only a one dB increase
in the output level for every 10 dB increase in the input level. Limiters can be thought of as high-
ratio, high-threshold compressors. They stay out of the way until the level goes above the
threshold. Above threshold their action becomes pronounced.
The Threshold Concept
A dynamic range processors activity begins once the input signal rises to, or falls below, a
predetermined level. In operation, the dynamic range processors sensing circuitry constantly
looks at the incoming signal and compares it to a reference level, the threshold level. The
operator sets that reference level via the threshold control. Remember, compressors and limiters
respond when signals at the input rise above threshold, while downward expanders and gates
respond only when signals at the input fall below the defined threshold.
Attack and Release Timing
If a compressor responded instantly to each waveform which crossed the threshold we would have
one very busy device. A signal passing through it would become a roar of constant level audio
