Understanding The Effect Of Attack On The Dynamic Range Of A Compressed Signal

Question

When downward compression is applied with a slower attack, I'm struggling to understand how the dynamic range of the signal as a whole is reduced. If the signal is a bass hit for example, where the signal crosses the threshold very quickly and the loudest part of the original signal is let through by the slower attack before compression is applied, then surely that peak will still exist in the compressed signal. Even if the compressor then clamps down and reduces the gain on the rest of the signal that has crossed the threshold, that original part of the signal isn't compressed, so surely the dynamic range of the signal as a whole isn't reduced, because neither the loudest part of the signal (the initial part of the hit that slipped through) or the quietest part of the signal have changed in volume.

So would it be true to say that compression only reduces the dynamic range of a signal if the loudest part of the signal doesn't happen before the compressor kicks in (due to a slower attack). Because in this instance it would appear that the Perceived dynamic might actually be increased because the gain of everything other than the peak will be reduced) which might well be a desirable effect in the case of a kick.

Answer 1

You should consider that any signal is a combination of different frequency components all across the spectrum. Each of these components will have a differing amount of energy and will affect the attack trigger in a different way. For instance with a Kick drum, there is likely to be a lot of energy in the high-frequency part of the spectrum that may well sneak past the compressor due to a long attack delay. Consider experimenting with multi-band compressors with different attack and threshold settings. See how they behave in this context as opposed to using a single (c1) band compressor.

Answer 2

One part I will respond to is where you said that it shouldn't change the signal if the attack doesn't catch the transient fast enough. But you have to think of that threshold as a trigger. Once it is triggered, it will act.

So in your example:

Bass hit, compressed with slow attack

Transient surpasses threshold

Compressor starts to slowly clamp down (transient has passed by now)

Compressor keeps clamping

Different compressors work differently, but there are release times that also effect how quickly it stops attenuation.

To respond directly to this: "So would it be true to say that compression only reduces the dynamic range of a signal if the loudest part of the signal doesn't happen before the compressor kicks in (due to a slower attack). "

False, once the threshold is passed, the compressor begins it's cycle, and will attenuate the following signal, which may or may not be above the threshold.

Hopefully that clarifies that point.