Auditory masking occurs when a sound is obscured by the presence of others. This can be either simultaneous or non-simultaneous. When masking occurs, the sound you wish to hear is called the target signal and the sound/s that is/are obscuring is/are called the masking signal.
Simultaneous masking is also known as frequency-domain masking or spectral masking. This is when two signals compete with each other in a similar frequency range at the same time.
Non-simultaneous masking is called temporal or time-domain masking. This is when two signals are competing with each other, but are not occurring at the same time. Temporal masking is particularly problematic between transient sounds (percussive sources like kicks, snares and claps). There are two varieties of temporal masking: forward and backward.
Forward masking is when the quieter target signal occurs after the masking signal; for forward masking to occur there needs to be less than 200 milliseconds between the sounds (although, most of the time, the delay between sounds is much smaller).
Backward masking is less common, but can occur when the quieter target sound arrives within 20 milliseconds before the target signal.
In reality, most live instrumentation is going to vary in timing slightly so that you could consider that there is the potential for both spectral and temporal masking to occur. However, it is the spectral masking that is the most obvious and destructive, especially for sustained instruments.
The rest of this article will focus on frequency masking. Most audio engineers will automatically resort to EQ to resolve masking problems. While this isn’t a problem, it may not be the best approach and you may want to try a few simpler solutions first:
1) The first line of defence to avoid masking is to have a suitable arrangement and instrumentation. By having differing instruments occupying different registers, you can alleviate most of the problematic effects of masking. For instance, a guitar-playing an octave above a piano is likely to cause less masking than them playing in the same register.
2) Secondly, re-assessing the balance of the sources can go a long way to solve masking issues. A lot of the time you need to prioritize the intelligibility of the important elements of a mix. For instance, the kick, snare and lead vocals are all likely to be more important than a shaker.
3) Finally, you can use the stereo-field to reduce problem areas by panning two competing elements to opposite sides. Please note that you will still have these collisions if the mix is folded to mono, so this approach is often best when used in conjunction with other techniques in this article.
If you have tried to elevate these problems with the steps above and still have a problem, you may want to resort to processing the audio. There are two common ways to approach this:
1) Complimentary equalisation. This is where you would EQ the signals to avoid competition in the frequency areas where the destructive masking is occurring. Sometimes, all you need to do is remove frequencies from the masking signal. On others, you will want to remove frequencies from the masking signal and add a similar frequency to the target signal, but this is much less common.
2) Side-Chain compression. This is where you use compression to give each sound a moment in time to dominate over the other(s). This is most commonly used when the target signal is a percussive sound (like a kick drum) and the masking signal is a sustained source (like a bass guitar). Triggering compression (using a keyed sidechain input) on the bass guitar whenever a kick drum occurs will give a pocket of time where the bass is ‘ducked’ out of the way of the kick. Our ears can then hear both sources more clearly.
One final word of warning here is that masking is normal. Frequencies between elements will overlap and this is a crucial aspect of getting a mix that gels and feels like its components are working together. If you try and remove all masking from occurring, the end result will be a mess. The key here is to make sure that you do just enough so that you achieve an appropriate balance.
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