Stereo Systems and Interference Patterns

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velocitous
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Hi,

Why is it that when you have a stereo system, with two speakers set at a distance from each other, their sound waves do not produce an interference pattern?

Thanks.
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Stonebridge
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(Original post by velocitous)
Hi,

Why is it that when you have a stereo system, with two speakers set at a distance from each other, their sound waves do not produce an interference pattern?

Thanks.
Because in order to produce a fixed interference pattern you need both speakers to be producing a continuous note of a single frequency. That is, an identical tone. It would be a very boring piece of music that consisted of a single note of a single frequency coming continuously from both speakers.

When you play music this doesn't happen. Music consists of sounds of many simultaneous frequencies which are constantly changing.

It is possible in the lab to create an interference pattern from two loudspeakers fed from the same signal generator. Reflections from the ceiling and walls tend to obliterate the patterns but it is possible to hear them.
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uberteknik
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Certain types of music can produce brief interference patterns: for instance a piece of modern synthesiser produced music with a sustained pure note will create an interference pattern for the duration of the note only. Stonebridge is also correct in saying it will be difficult to notice it because other reflections and harmonics together with the short duration of the notes will make them almost impossible to hear.

But you can most definitely hear the effects of phase cancellation within a room where a stereo system is set up. The phase differences are created by the sound bouncing off walls and ceilings and then the two paths arriving at the listeners ear at slightly different times. This will create both destructive and constructive interference i.e. 'dead' zones for low bass notes in some areas, whilst augmenting them in others. This is heard from any given seating position as an uneven frequency response especially in the bass region.

It's also the reason why a good stereo sound system is very room dependent and why adjusting the position of the speakers in relation to the listener is always a compromise to achieve the best possible sound.
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