Polycom SoundStructure C16 Design Manual page 120

For example, if the expansion ratio is 4:1, the threshold is -30 dBFS, and the input signal level is -35 dBFS,
then the expander applies 20 dB of attenuation (4 x (35-30) = 20). When the signal is above the expander
threshold, a gain of 1 is applied to the signal, therefore, the input signal is left unchanged.
Input Signal Attenuation, Expansion Ratio, and Signal Level
The "attack" portion of the expander is when the attenuation is reduced toward 0 dB, and the "decay" portion
is when the attenuation is increased.
Gates perform like expanders, but are typically set with higher expansion (that is, gate) ratios and have a
longer hold time. The gate does not decay until the signal is lower than the threshold for longer than the hold
time. This prevents the gate from attenuating the signal between short pauses in speech.
The gate threshold is the RMS level in dBFS of the input signal below which the gain turns on. The level
must be below this threshold longer than the gate hold time before the gain begins to apply a gain change.
The gate ratio is the multiplier applied to the difference between the current input signal level and the gate
threshold. For example, if the gate ratio is 10:1 and the input signal level is 6 dB below the gate threshold,
the gate applies 60 dB of attenuation.
The gate attack is the amount of time it takes the gate to ramp the gain to the target gain once the input
signal level surpasses the gate threshold.
The gate decay controls how quickly the gain ramps down once the signal level is lower than the gate
threshold and the gate hold time has expired.
The expander threshold is the RMS level in dBFS of the input signal that when below this threshold, the
expander engages. The expander ratio is the multiplier applied to the difference between the current input
signal level and the expander threshold. For example, if the expander ratio is 2:1 and the input signal level
is 3 dB below the expander threshold, the gate applies -6 dB of gain (equivalently 6 dB of attenuation). If
the input signal level is above the expander threshold, a gain of 1 (0 dB), is applied to the input signal.
The expander attack time is the amount of time (in milliseconds) it takes the expander to ramp the gain up
to the target level once the input signal exceeds the expander threshold.
Polycom, Inc.
Threshold
Input Level (dB)
No Expansion
2:1 Expansion
4:1 Expansion
10:1 Expansion
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