Interference

Waves of any sort can be combined by a process called interference. The way we can look at it is that interference is the final wave when two or more waves are superimposed on each other. If you need a refresher on waves, click here.

There are two types of interference: constructive and destructive interference. Constructive interference is when two waves are in phase and when combined, the final wave has a higher amplitude (the combined position of the crest and the trough of the wave - can be thought of as the total height of the wave) than the starting waves. Destructive interference is when the waves are combined in such a way that the amplitude of the final wave is smaller than the initial waves or even non-existent.

To look at constructive interference simply, let's take a look at just two waves. If the waves are in phase, the crests of both waves (and in turn, the troughs) will line up, and the final amplitude of the wave will be just the sum of the amplitudes of the original waves.

Vice versa, if the waves are out of phase by 180°, the crest of one wave will align with the trough of the other. The amplitudes cancel out. This is destructive interference.

In reality, most interference involves more than two waves that are necessarily in phase or 180° out of phase. The waves themselves might not have equal amplitudes either, so the combined wave may look a little more funky than what we see above.

 

You can actually experiment with interference in your own home. Surround-sound stereo systems work on the principle of interference for around waves. If you stand equidistant from either speaker, you hear the constructive interference of both speakers. Turn one speaker off, and the amplitude of the sound is lower. There are actually spots in your surround-sound system where there will be destructive interference, but these locations depend on the wavelength of the sound waves.