Standing waves - AVS Forum
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post #1 of 4 Old 06-17-2012, 01:53 PM - Thread Starter
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Would someone tell me what a standing wave is? I know they are unwanted, but I don't understand the concept.

Floyd

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post #2 of 4 Old 06-17-2012, 02:26 PM
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http://en.wikipedia.org/wiki/Standing_wave

In speakers standing waves can arise when the distances between panels are 1/4 wavelength and longer. With the shorter wavelengths of midrange and high frequencies damping is used on the panels to absorb reflections, preventing standing waves in those frequencies. Moderate thicknesses of damping have little effect on low frequencies, but their wavelengths are generally too long to be problematic, though tall towers may require six inches or so of damping at the top and bottom to stop resonances.

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post #3 of 4 Old 06-17-2012, 04:39 PM
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a guitar string, when plucked, creates a standing wave. it is a wave that doesn't appear to go anywhere. it just keeps reflecting back through itself. this is different from a wave that you might find in an ocean. that wave appears to moves over a distance.

a column of air in a pipe in a pipe organ does the same thing. all the energy builds up in a single frequency depending on the length of the pipe. the wave is reflecting back and forth through the pipe building up on itself.

the air in your room and the air inside a speaker are the same way. they have natural "resonances" where when energy goes into them at certain frequencies, they build up very quickly and/or cancel out very quickly. this creates peaks and dips in the frequency response.

there are a variety of ways of minimizing the effect, but that is essentially what standing waves are.

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post #4 of 4 Old 06-17-2012, 09:10 PM
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Quote:
Originally Posted by Floydster View Post

I don't understand the concept.Floyd

Have you ever been to an irregularly shaped "wave" pool?

We have one where I live and the irregular shape of the pool doesn't allow all of the wave to always flow in one direction away from the source.
Some portion of the waves hit the steep edge of the pool early on and cause them to get reflected back out into the pool area again.
In that area there are some spots where the water doesn't seem to travel in any direction at all but instead the water just alternates between being a really tall peak or a really deep depression.
There are other peaks next to it that do the same thing, but they are always at the opposite state. (eg. when one is a peak, the other is at a depression)

Being curious about these standing waves, I looked at them closer and noticed that the water located right at the center of these peaks would never move horizontally at all, only vertically.
Likewise, I also noticed in the sections of water located between neighboring peaks, some of the water never moved vertically at all and only moved horizontally back and forth.

You can think of standing waves occurring in audio as being very similar, but with minor differences:
- were audio reflections interfere constructively, the air doesn't actually move vertically like the water did, but instead it is just more compressed or rarefied than what a regular traveling wave would normally compress or rarefy the air.
- were audio reflections interfere destructively, the air is not compressed or rarefied at all, but instead just moves back and forth in a certain direction. (note: it could be any direction, not just horizontally like the water in the wave pool did)

If you walk around in a typical room while a steady sine wave tone is being played by a speaker, you will alternately walk though areas where the sound will be very loud and other areas where the tone is very quiet and almost silent. This is analogous to being in that wave pool where you're sometimes in areas where the water goes up and down tremendously and other areas where the water doesn't go up or down at all.

Now wave pools only make waves at one frequency. If they could change the frequency of the wave you would find that the position of those peaks and valleys would also change.
- If the frequency of the waves is faster, the individual peak/valley areas would be closer to each other.
- If the frequency of the waves is slower, then the distance between neighboring peaks/valleys would be further apart.

This happens in audio as well. If you walk around a room while a low frequency is being played you'll find the interference patterns are quite far apart from each other, whereas with high frequencies they can be quite close to each other. (Above 10kHz you can stand still and just rotate your head slowly to hear your left and right ears go through MANY loud and quiet zones)

What's unwanted with standing waves is that they end up emphasizing certain frequencies and nulling out others.
You may have a speaker with a perfectly flat frequency response, but if you listen to it in a very reflective room you end up with a very uneven frequency response, and that response will also change when you move to a different area in the room.
Equalizers can help flatten the response in one area in a room, but they usually make the response worse elsewhere. This is why controlling reflections in a room is more effective at obtaining a smooth frequency response, not just at one location, but everywhere else in the room as well.
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