Compression and Drums
A recording with a strong pulse, such as drums or percussion, with a regularly repeating transient will trigger gain reduction in a compressor and can serve as a useful type of sound to highlight the effect of a dynamics processing.
By processing a stereo mix of a full drum kit through a compressor at a fairly high ratio of 6:1, attack and release times can be adjusted to hear their effect on the sound of the drums.
On a typical recording of a snare drum that has not been compressed, there is a natural attack or onset, perhaps some sustain, and then a decay.
The compressor can influence all of these properties depending on how the parameters are set. The attack time has the greatest influence on the onset of the drum sound, allowing an engineer to reshape this particular feature of the sound.
Increasing the attack time from a very short time to a much longer time, the initial onset of each drum hit is audibly affected.
A very short attack time can remove the sense of a sharp onset. By increasing the attack time, the onset sound begins to gain prominence and may in fact be accentuated slightly when compared to the uncompressed version.
Let us explore the sonic effect on a drum kit when listening through a compressor with a low threshold, high ratio, and very short attack time (e.g., down to 0 milliseconds).
With such a short attack time, transients are immediately brought down in level, nearly at the rate at which the input level rises for each transient.
Where the rate of gain reduction nearly matches the rate at which a transient signal rises in level, a signal’s transient nature is significantly reduced.
So with very short attack times, transients are lost because the gain reduction is bringing a signal’s level down at nearly the same rate that the signal was originally rising up during a transient.
As a result, the initial attack of a transient signal is reduced to the level of the sustained or resonant part of the amplitude envelope. Very short attack times can be useful in some instances such as on limiters that are being used to avoid clipping.
For shaping drum and percussion sounds, short attack times are quite destructive and tend to take the life out of the original sounds.
Lengthening the attack time to just a few milliseconds, a clicking sound emerges at the onset of a transient. The click is produced by a few milliseconds of the original audio passing through as gain reduction occurs, and the timbre of the click is directly dependent on the length of the attack time. The abrupt gain reduction reshapes the attack of a drum hit.
By increasing the attack time further, the onset sound begins to gain prominence relative to sustain and decay portions of the sound, and it may be more accentuated than without processing.
When compressing low-frequency drums such as bass drum or kick drum, an increase in the attack time will increase the presence of low-frequency harmonics.
Because low frequencies have longer periods, a longer attack time will allow more cycles of a low-frequency sound to occur before gain reduction and therefore low-frequency content to be more audible on each rhythmic bass pulse.