Phase Cancellation

SeamlessR returns with a new lesson all about phase cancellation.

Phase is a change in the starting position of a sound. Changing the starting point of the waveform against a copy of itself can do many interesting things as well as adding waveforms together. The center of a waveform is called zero crossing because it has no amplitude. A waveform has a positive (+1) and negative peak (-1). Sound is created by waveforms. A waveform represents the motion that is being sent to a speaker cone to move air and that is how sound is heard.

When adding two waveforms together that are identical and placed together, the result is double the waveform or double the amplitude. However, when slightly out of phase it does not add together quite as much and if it is 100% out of phase, there is silence. The waveforms are cancelling each other out; phase cancellation.

Phase cancellation does not occur often. For example, accidentally routing one speaker monitor out of phase (i.e. place the positive in the negative and vice versa) odd noises are heard because the routing is sending the opposite phase in the left channel and right channel. It is possible to put a mono signal through both channels and not hear anything because it is phase cancelling. It is rare to run into this situation unless purposefully using phase cancellation for the purpose to cancel a waveform or leave something out.

One of the most ingenious uses for phase cancellation is the use of TRS cables (balanced audio cables). Essentially, for any length of cable, noise can be introduced into the signal regardless of being shielded properly. The longer the cable, the more noise produced. A balanced cable has two-pronged rings and is not a stereo cable. The polarity is shifted in a certain way at the beginning and end of the cable so that the signal goes through and is cancelled against itself in such a way that the noise generated on both channels of the same polarity gets cancelled, leaving the original signal in pristine condition. This is not used in basic sound design but is still an intriguing use of phase cancellation.

Phase cancellation is used in Reese bass. Pitch changes alternate being out of phase to completely in phase. Combining the pitch changes produce rhythmic phase cancellation or “beating”. When the waveforms are almost completely out of phase the amplitude is greatly diminished and when almost completely in phase the amplitude is maximized. The pitch does not change and the rhythmic phase cancellation stays the same speed.

Harmonics occur when adding saturation to a waveform. Harmonics are not just higher tones from the original waveform. If a harmonic is played on the same single channel, the tones will phase cancel and the brain processes one tone. The farther the interval, the faster phase cancellation will happen. Harmonics are multiple tones that are not phase cancelling. Harmonics are multiples of the fundamental bass tone and fit perfectly inside each other, therefore there is no phase cancelling that occurs. Each new octave has double the number of harmonics to the point where there will be no real notes, only frequencies. When this doubling phenomenon occurs, a SuperSaw sound wave is created.

Phase cancellation is used in many ways in sound production. Bass resampling, phasing, and pitch differencing are used because they create additional movement in the spectrum through phase cancellation. Further, phase cancellation is important to understand when working in sound design because it is a core fundamental and application for all other sound production concepts.