Seiche and Standing Waves

[WMcD]

This comes from the experience of finding something interesting while looking up something completely different. Bear with me. There is a nice graphic at the end. I've harped on standing waves before. Others have too. It is part of the mysterious world of wave mechanics. While the math is difficult, the concepts are not.

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The issue of standing waves in rooms often comes up. Generally this is where a bass signal (but higher also) can be very strong at a some points in the room and be almost silent in others. These are called anti-nodes and nodes.

I often suggest that people who want an ear opening experience try the effect of sine wave fed to one speaker in your room. In the old days you could only do that with a hardware signal generator But now there is freeware out there which will generate a tone from your computer. One is the digital recorder, Audacity.

It is quite astonishing to feed one speaker with, say 100 Hz and then walk around the room. When you are at a point of constructive interference of the two waves, the sound is strong. A few feet away there is destructive interfereence and the sound goes to almost zero, as if someone turned off the amp.

The most elementry standing wave is set up by two sources and we contemplate one dimensional transmission line. One source can be a generator (speaker) and the other is a reflection from a hard wall. We've got a lot of hard walls, six in every room when you include the floor and ceiling. That makes things more complicated.

We also have to realize that sine waves 180 degees apart, are out of phase, and add up to zero. That "out of phase" depends on the wavelength, room size and where you are standing. OTOH, at some point they are 360 degrees apart, or in phase.

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The beaches in Chicago were closed down a week ago. This was because officials thought there would be a seiche. Basically a storm surge reflected from the eastern, Michigan side.

I had never heard the term seiche!

Wikipedia has a good description. http://en.wikipedia.org/wiki/Seiche

Included there is a nice animated graphic about standing waves. As you can see, there really are travelling waves originating at each end of what I'm calling a transmission line (our room). But keep an eye on the red line which is the sum of the two waves. At some points it is a sine of high magnitude bouncing up and down, which is the sum of the two travelleing waves. At other points we still have a sum, but the sum is always zero.

The sums are the standing waves which seem to stand still.

This may give insights about the term of "standing waves".

Best,

Wm McD

[Don Richard]

Here's more:

http://www.kettering.edu/~drussell/Demos/superposition/superposition.html

[boom3]

This can be observed in bowls of water by tapping the rim. Concentric standing waves form and move to the focus, back to the rim, etc until the oscillation dies out.

My she-cat has learned to use this. She approaches the steel water bowl and her tags clink on the rim, setting up a seiche. She can see the waves and thus judge where the water surface is so she doesn't dunk her face. I was wondering why she didn't care for the white Corell water bowl nearby; now I know that the seiche in the steel bowl is much more visible to her.

[greg928gts]

I was excited at first, thinking you were going to continue on and explain how to fix standing wave problems. [*-)]

Greg