Phase & Complex Soundwave Interaction

Soundwaves can add together and subtract from one another (cancelling each other out). This property of sounds can be unwanted but can also be used creatively.

What is Phase?

Imagine a flat pool of water. If you drop a pebble into the middle of the pool, ripples (small waves) will flow outwards until they reach the edges of the pool in perfect concentric circles.

But if you drop two pebbles into the pool, so that they land at the same time but hit the water different at points, then you get two sets of ripples. Instead of the perfect circular ripples overlapping and passing through one another, these ripples collide and interfere with one another. This interference causes some waves to be cancelled out and others to be amplified.

Diagram from Wikipedia.

Exactly the same thing happens when two sound waves meet.

In or Out – of Phase

Sound is a wave of back and forth motion. Where two waves meet they can either add to one another or subtract from one another. In the following image two identical waves are in perfect alignment. Therefore the waves will add together, making their peaks and troughs twice as high. We describe these waves as being in phase.

But, if two identical waves are alligned in such a way that where one has a peak the other has a trough we can describe them as being out of phase. In this situation the waves will cancel each other out.

Activity

We can experience this cancellation effect by pointing two loudspeakers directly at one another and playing the same tone through both of them. Where the identical waves meet in the centre the sound will be cancelled out.

When the loudspeakers are set up as in the diagram, play this sound and place your head between the loudspeakers.
If you stand beside the speakers you will be able to hear the sound, but if you place your head directly in-between the loudspeakers the sound will drop in loudness. This is due to the sound waves cancelling each other out.

Fact

The frequency of 343Hz has a wacelength of one metre.

So one wave fits exactly in between the loudspeakers as set up. If the frequency of the tone were different then the sound waves would not cancel each other out perfectly.

Phase Cancellation as a Compositional Tool

The Phaser, Flanger and Chorus, all use the property of phase cancellation to create their effects. A copy of the original sound is made and delayed by a period of time. This is then mixed in with the original signal ‘out of phase’ creating the resultant sound. These manipulations are a great way to apply phase cancellation effects to your sounds.

Chorus

Flanger

Activity

Try experimenting with these tools in a composition. Does phase cancellation work for all types of sounds? What new timbres can you create using phase cancellation.

Phase Cancellation and Noise Reduction

Phase can be used to eliminate unwanted sounds. Modern smartphones have a microphone both on the top and the bottom, even though the user only speaks into the bottom. These two microphones are wired in so that they are out of phase. This means that any sound entering both microphones (for example: background noise) will be cancelled out, leaving only the spoken voice. Shotgun microphones are used to record interviews and these work in a similar way.

Phase Cancellation on the Beach

Listen to this recording and hear phase cancellation in action

Beating

We’ve described the phase interaction for identical sounds. But far more complex happenings occur when different sounds are combined (see also, Complex Soundwaves. Where two tones are very close to one another (less than about 50Hz apart) they will begin to create a pulsating, or beat, effect.

You can see the pulsating beats in the oscilloscope. These are created because the soundwaves are only slightly out of alignment, and one keeps overtaking the other. Think about two cars on a circular racetrack, one is travelling faster than the other and so this fast car keeps overtaking the second.

Exactly the same thing happens with beating sounds. When the sounds are in alignment (in phase) they add to one another, and when the sounds are out of alignment (out of phase) they cancel each other out. The time that it takes for one beat to occur is equal to the time it takes for the first sound to overtake the second.

Extra

The rate of beating is equal to the distance between tones, for example: Sine tones at 460Hz and 440Hz will create a beat tone with a 20Hz pulse. ( 460 – 440 = 20 )

Composition Tip

Try to use beating sounds within a composition. They can be very useful tools, especially in Ambient or Minimalist pieces, because they allow you to make subtle but very effective changes to the sounds. Take two copies of the same sounds and place them side by side (static tones work best). Transpose one very slightly, and they will begin to beat. The closer the sounds are in pitch, the faster the beating.