Sound Wave Interference

Wave interference is the interaction of two sound waves traveling in the same medium. The interference of waves causes the shape of the pressure wave to combine in the medium into a complex waveform.

If the compressions of two similar sounds meet, the compression will combine and have twice the amplitude. If two rarefaction sections intersect, they will combine in the medium and a rarefaction of twice the amplitude will result. If a compression wave of one source meets the rarefaction wave of another source, the two waves will cancel each other and leave the medium in its equilibrium state.

Remember, sound is a compression and rarefaction cycle passing through a medium such as air. Thus, a compression is a location where the air molecules have compressed together into a high pressure zone. As rarefaction passes through, the molecules separate creating a low pressure zone. Interference occurs when the compression and rarefaction zones of two separate sound sources intersect. The result will be the net addition of the two separate sources. This ability of the medium is known as superposition.

Superposition

Air is amazing in its ability to transmit sounds with almost perfect superposition. This means that waveforms are able to intersect and not be affected by the intersection. Water waves are a great example of a medium with a limited superposition ability. When two waves meet and create a strong peak in water, the water will separate and turn into a splash of white water that steals energy from the wave.

When two sound waves interact to create a louder or higher amplitude sound, it is called constructive interference. If, however, two waves cancel each other out, it is known as destructive interference.

In short:

Compression + Compression = Large Compression or Constructive Interference

Rarefaction + Rarefaction = Large Rarefaction or Constructive Interference

Compression + Rarefaction = Equilibrium, or Destructive Interference

Let Consider the Implications

You can have high amplitude sound signals interfering with each other and at the point of interference the air particles stay in equilibrium as though they were not being exposed to a sonic pressure disturbance passing through them. Yet, the sound energy continues to travel away from the source and becomes audible again.

When the compression of one wave meets the rarefaction of another, destructive interference results.

The tendency of a compression wave to push particles together is cancelled by the rarefaction's tendency to pull the particles apart, leaving the particles in their orginal original equilibrium.

Nodes and Anti-Nodes

If, because of speaker placement or environment, a particular location experiences destructive interference, it is given a special name. These zero quiet spots are known as nodes. Anti-nodes are the opposite spots where you consistently have constructive interference and the resulting high volume. Nodes and Anti-nodes are the bane of sound engineers at concerts who work hard in placing the speakers to avoid such unpleasant artifacts.

Auditoriums and concert halls are specifically designed to reduce destructive interference. The interference is normally created by the reflections of sounds off the walls and ceilings. Auditoriums are therefore normally designed with lots of baffling and rough textures on the walls and ceilings to absorb the sound and avoid creating clear reflections that can cause the destructive interference.

If you've ever been in a small, two engine plane, you might have noticed these nodes and anti-nodes. At times, you wouldn't hear the engines, at other times, you would suddenly hear the engines very loudly. You were in a quite node point when you couldn't hear the engines and in an anti-nodal point when the engines were especially loud.

The Positive Side

Interference has a positive side, however. It is commonly used in the tuning of musical instruments as you'll see in the section on Beat Frequencies.