Intereference waves in Music

zabachi

I understand that when constructive interference occurs, music is produced, destructive interference can also happen right? Then it is possible that sometimes no sound is produced at all? Thanks!

Simon Bridge

Um - what makes you think that constructive interference creates music?

When you have a number of different sounds, they will interfere constructively and destructively. It is the interplay of different harmonics and timings that gets interpreted by a human being as music or discord or just plain noise.

zabachi

This web site sit said this:"Another simple example of two sound waves with a clear mathematical relationship between frequencies is shown below. Note that the red wave has three-halves the frequency of the blue wave. In the music world, such waves are said to be a fifth apart and represent a popular musical interval. Observe once more that the interference of these two waves produces a resultant (in green) that has a periodic and repeating pattern. It should be said again: two sound waves that have a clear whole number ratio between their frequencies interfere to produce a wave with a regular and repeating pattern; the result is music." http://www.physicsclassroom.com/class/sound/u11l3a.cfm
So its possible that while playing music sometimes we wont hear anything?(theoretically)

Simon Bridge

Well yes - eg, when playing beats, there may be a moment of silence between them.
If you set up a standing sound-wave in a room, then there will be quiet spots and loud spots.

You can also get an interference effect - for instance, if two instruments play a perfect tone then there will be whole lines of people in the room who hear nothing and whole lines who will hear it very loudly ... the lines fan out from the musicians. You can get more complicated patterns.

In practice the acoustics will mess up the effects. (Sound from a loud zone can reflect off the back wall into a quiet zone for eg.) Some modern mall try to exploit the effect to minimize the shrill-noise you get in most malls from all the musack and people talking all at once.

In actual music, the wavelengths are changing all the time so you'd be hard-pressed to make an entire piece silent for some people in the room.

DrGreg

We usually only use the terms "constructive interference" and "destructive interference" when both of the waves are exactly the same shape. The "music" example is when you have different waves whose frequencies are related to each other in a simple ratio (3/2 in the example given), so you get a repeating pattern that is a pleasant-sounding musical chord.

Destructive interference needs two signals of exactly the same frequency and exactly "out of phase" with each other. This is very difficult to achieve in practice, but not impossible (as Simon Bridge points out).

If two signals are almost the same frequency, you will get a "beat frequency", getting louder and quieter, as explained later on the same webpage you quoted.

Simon Bridge

Oh it's a chord - I had this feeling I was missing something... :(

CWatters

Correct. Thats how noise cancelling headphones work. They have a microphone to measure the unwanted noise, they phase shift it 180 degrees and play the resulting "anti noise" out of the speaker. Destructive interference between the noise and the anti noise occurs somewhere near your ear drum...and the result is silence, the noise dissapears.