You play a note on a bass. (Here we’ll only look at the note and not any of the higher tones that add to the timbre of the instrument.) The bass string vibrates back and forth, it pushes together air in a radiating pattern of compressed and uncompressed regions that reach our ears and cause the eardrum to vibrate in time. Drawing pockets of air is difficult so we draw a waveform that shows pressure over time.
All is simple so long as that bass note reaches our ears undisturbed. But suppose we had two people playing exactly the same note on the bass. They are slightly different distances from our ear.
Depending on the distance to the source the two waves arrive more or less out of phase. The more out of phase they are, the less volume we get as they add together, as the peaks and troughs cancel out each other. So what chance that two basses play exactly the same note? Very little. However the same bass recorded on two mics or coming out of stereo speakers… entirely likely.
Areas where the signals cancel out we call a node. You will find multiple nodes in a stereo listening space where bass is going to disappear or be muted. If you are micing up a kick or bass you need to avoid placing mics that cancel out some or all of the sound. Monitoring the combined mic inputs in mono can help detect phase problems. But better use just one microphone. Having bass coming out over stereo speakers is going to create nodes as well and make your mix sound bad as people move around – big problem with live shows. But we can try create a large ’sweet spot’ by limiting the stereo spread of low frequencies. Make your kick come out the centre of the stereo space, so that both speakers are moving as one. Basically treat low frequencies as a mono recording and avoid stereo treatments.
Why only bass? Because as you go up in frequency these nodes get so close together that you are less likely to notice them. Also the richer the sound (in terms of multiple overtones and sounds in the instrument) the more complex the pattern of interference becomes. That can be an advantage if we try to use higher pitched sounds within the instrument.
Because we usually place the bass frequencies in the middle (unless you are going for a particular effect of course), the instruments start their arrangement out from a central ’shaft’. This diagram is going to be refined greatly as we go along, but you can already see how the bass instruments push the higher pitched ones left and right in a tree shape.
In the early days of stereo people would mix all the bass guitar in one speaker. That works as well. But if you are going to have it coming from two places then better to have two different sounds – e.g. the deeper part of the sound here and the slap over there or use effects such as phasing that introduce radical changes in the phase of the instrument. Recording the same part twice is good too.
And this is yet another reason why mixing on headphones can be dangerous … no phase cancellation between the left and right phone. Use speakers to check for this.