Constructive and Destructive Interference Pt.1

This week while editing a drum track on Protools, I decided to import some previously recording kick-drum samples to add a bit more ‘punch’ to my audio.

This procedure sounds simple enough, as many of you know importing audio isn’t exactly ‘Maxwell’s theory’, but its how you manage your new audio that can lead to some trick and sometimes-surprising turn of events. The result of this unusual occurrence with adding numerous sound waves together is called Interference, and it can be split into two categories:

Constructive interference

From looking at the image below, you can see 2 sine waves going in the same direction with the same frequency over the same time (start and end). These waves are in phase and will result into a larger amplitude, although my imported samples wont have exactly the same wave if they was displaced in the same place I can get the ‘punch’ I need:

interference images

Destructive Interference

In the above image I have the same waves, going in the same direction, with the same frequency. But there is a slight different; the starting point of each wave is at a different point, as they are out of phase, in fact completely at opposite displacement. This leads sum of these two waves being less that the original wave, or zero.

When we import or record multiple sounds into Protools, it is essential to further inspect the audio waves we are about to employ because that ‘punch’ can easily end up in complete silence.

This isn’t always the biggest issue; if two waves have slightly different frequencies and are out of phase, beat frequencies can occur. This is a pulsing noise that goes up and down in amplitude as the waves as the varying constructive and destructive interferences go in and out of phase. This can sound pretty horrific, but don’t take my word for it, have a listen:

Courtesy of Jeff Regester, Acoustics Beats.

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