Interference

Interference of Waves
When two sound waves meet, they interfere with each other in some way. When two waves of the same frequency interfere, their interference is studied more closely. This is because, depending on when they interfere, can make the sound louder, or sometimes cancel each other out so that there is no sound at all.

CONSTRUCTIVE INTERFERENCE is when two waves move towards each other that are in phase. When this type of interference occurs, an amplitude greater than each of the originals because the amplitudes add together

Example: a wave is moving left to right with an amplitude of 3cm. A second wave is moving right to left with an amplitude of 2cm, and the two waves are in phase.

A1+A2= 2+3=5cm. When the waves meet they will create one wave with an amplitude of 5 centimeters.

DESTRUCTIVE INTERFERENCE The same theory applies for destructive interference, but the waves move toward each other out of phase, so a crest meets a trough and the waves create a smaller amplitude, possibly even zero.

The drawing below shows examples, along with some very simple equations, about how the amplitude is found during interference.

BEAT INTERFERENCE
A beat frequency occurs when two waves interfere, but they are neither in phase or exactly out of phase.

Two waves of different frequencies interfere, and they do in and out of phase with each other.

This graphic shows an example of a beat frequency. Say for example that the gray wave has a frequency of 155Hz. Also say the red wave has a frequency of 100Hz. By subtracting the frequencies you would find that the beat frequency has 55 beats per minute.

This sound file is of a very basic beat frequency. If you listen carefully, you can hear the sound get louder and softer repeatedly, similar to a pulse.

media type="file" key="beatfrequency.wav"