When layering drum sounds it is often with the intent of using certain frequencies from one sound that are appealing and layering those frequencies with the frequencies of another sound that are also appealing. However, when these frequencies are not separated and are simply layered one on top the other then a number of anomalies will appear.
Clashing: this occurs when certain frequencies from source layers do not marry well or complement each other to form a harmonic resultant layer. They end up sounding ‘wrong’. This is called clashing, i.e. frequencies that sound out of place when used together. This can reveal itself in the form of noise, frequency mush, phase etc.
You often hear this term used when certain sounds in a mix simply do not sit together well.
Summing: when two shared (the same) frequencies (from different layers) of the same gain value are layered you invariably get a boost at that particular frequency. This form of summing can be good if intended or it can imbalance a layer and make certain frequencies stand out that were not intended to be prominent.
A good way around this problem is to leave ample headroom in each waveform file so that when two or more files are summed they do not exceed the ceiling and clip.
In terms of an audio signal, headroom is the difference between the maximum signal level and the maximum limit of its environment/device.
In the digital domain we know that the ceiling is 0 dBFS, and anything beyond this incurs digital clipping.
When using extracted ADSR components the headroom is not affected as much as entire waveform layering simply because each component is already at a pre determined level and placed at different timeline location in the resultant waveform. However, as ADSR components often overlap it is still important to take into account the headroom available and not to eat into it too much.
Additionally, ample headroom is required when we come to applying dynamics to a waveform. Dynamics, by their very nature, control gains and invariably are used to boost levels (compression, EQ etc). Any form of a gain boost will eat into the headroom.
Masking: when two shared frequencies are layered and one has a higher gain value than the other then it can ‘hide’ or ‘mask’ the lower gain value frequency.
How many times have you used a sound that on its own sounds excellent, but gets swallowed up when placed alongside another sound? This happens because the two sounds have very similar frequencies and one is at a higher gain; hence one ‘masks’, or hides, the other sound. This results in the masked sound sounding dull, or just simply unheard. A common example, in a mix context, is when a high range piano sound might be masked by a high range string sound. The same problem applies when layering drum sounds. The sustain (body) of one waveform might dominate and mask the body of another waveform when layered together.
Sensible use of filters and dynamics with a good understanding of gain management usually resolves this problem. We will come to this later in this book as dynamics and effects have a pronounced effect on frequencies and gain values.
Excerpt taken from the book Art of Drum Layering.