## Acoustical Question of Squeaky Claps

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Patren
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### Acoustical Question of Squeaky Claps

I went to school at James Madison University and around our quad area there were these circles made up of multiple bricks. When a person clapped, stomped, smacked their legs, pretty much anything similar in the center a squeaking sound would be made. Well, the sound wasn't actually made, but rather the person heard a squeak. If a person wasn't standing close enough to the center they would just hear regular clapping sounds. Which was rather humorous because people didn't always understand this, and would try to get people to hear squeaks, but the people listening weren't close enough, and just thought the person was crazy. My question is: Does anyone know how this works?

scarecrovv
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### Re: Acoustical Question of Squeaky Claps

When you clap, you generate a single large pressure wave in the air. As it travels out, it strikes everything, and is reflected. Now most things are not at right angles to your body, so they reflect the sound away from you, and you don't hear an echo. If you stand in front of a brick wall however, you will hear a distinct echo.

The concentric rings of bricks are special, because when you're standing at the middle, the entire ring is at right angles to you, and is at a uniform distance. That means that it reflects your pressure wave back at you, all at once. This in itself isn't terribly remarkable, but there are many rings of bricks, each at a slightly different distance from you, in uniform steps. That means that you get a series of reflections of your pressure wave coming back at you, at uniform intervals of time. This makes a reasonably pure tone, for a time dependant on the radius of the circle.

These sorts of setups can get even more elaborate. For example, if you walk into a large elliptical room, and stand at one focus of the elipse, and whisper, someone standing at the other focus can hear you just fine.

Patren
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### Re: Acoustical Question of Squeaky Claps

Thank you, that has been driving me nuts for years, and isn't exactly the easiest thing to google. Do you happen to know if there is a name for the effect because I'd like to read more about it, but even with more knowledge on it, I still can't get a search hit.

henre
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### Re: Acoustical Question of Squeaky Claps

The name of the effect is "sound". I'm not trying to be an arse or anything: any regular disturbance in air pressure (well, between 40Hz and 18kHz) will cause you to hear a sound. When you're standing in the middle of the rings, you're getting the clap reflected to you at regular intervals. It's kind of strange to imagine but even a regular disturbance in a sound will cause a sound.

You can witness this in another form if you have access to a synthesizer keyboard or know someone who does. Create a simple note that has a low-frequency oscillator applied to its amplitude. If you turn up the frequency of the oscillator until it reaches around 40Hz, you'll start hearing a magical second tone even though you're only playing one note!
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mrbaggins
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### Re: Acoustical Question of Squeaky Claps

henre wrote:You can witness this in another form if you have access to a synthesizer keyboard or know someone who does. Create a simple note that has a low-frequency oscillator applied to its amplitude. If you turn up the frequency of the oscillator until it reaches around 40Hz, you'll start hearing a magical second tone even though you're only playing one note!

Is this something like harmonics in music, specifically on guitar at least? If you pluck a string, you get a note. You can then mute it at key points (notable fractions along the string) removing some of the bigger vibrations, but leaving the smaller ones. EG: Pluck the fattest string (E) then, very lightly touch directly above the 12th fret. The string no longer moves at the 12th fret, but is still moving, with maximum amplitude at the 6th and 24th frets. (12 frets down the neck = half the length. This is why frets get closer together near the bridge. So 6th and 24th are both halfway between the 12th fret and the end)

Once you've muted the 12th fret, you end up with a much higher, cleaner note, called a harmonic. Nearly sounds like a harp in comparison to the guitar.
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Charlie!
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### Re: Acoustical Question of Squeaky Claps

I think there's another possible reason for squeaky clapping, and that's if only the higher frequencies of your clap are getting reflected back. If it's reflections off of lots of small clumps of bricks, it could even be that only things of wavelength smaller than the clumps will interact.

Approximate spacing of bricks needed in order to get even 440 hz (middle A): 1 every 0.77 meters, which is why I think there might be nother explaination.

Approximate size of clumps needed to reflect mostly 440 hz and up: 0.77 meters . That seems a bit more reasonable, and smaller clumps mean higher-pitched reflections.

Unless, of course, I'm horribly wrong.
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Rhubarb
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### Re: Acoustical Question of Squeaky Claps

With the guitar harmonics, what's happening is you're just muting the fundamental frequency, letting the others ring. With the squeaky claps you're basically layering the same sound on itself with a slight phase difference- you're hearing echoes that are all nearly but not quite on top of each other, which creates a high tone because the sound is repeating at a high rate. Unless it's just the high freqs being reflected as Charlie! says...

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### Re: Acoustical Question of Squeaky Claps

There are pyramids in Central America with steps designed specifically with this phenomenon in mind.
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thewhitehouse
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### Re: Acoustical Question of Squeaky Claps

Patren wrote:Thank you, that has been driving me nuts for years, and isn't exactly the easiest thing to google. Do you happen to know if there is a name for the effect because I'd like to read more about it, but even with more knowledge on it, I still can't get a search hit.

I believe there is a technical name for this phenomenon. It's called interference. Specifically, when you hear a different sound from the one that's emitted it's an example of constructive interference. When two sound waves are emitted from different locations D1 and D2 and arrive at a the same point a simple rule determines what you hear. If the distance is equal to an integer multiple of the wavelength of the sound emitted then the sound should be maximally amplified at that point. If the distance is a half-integer multiple then you hear nothing. And if it's neither then you hear a sound that is either amplified only slightly or slightly 'muted'. The way these clapping circles are constructed the points of reflection (i.e. the edges) are all equidistant from you when you stand at the center. Therefore the sound always travels at roughly an equal distance from the point of reflection to your position.

For the case of an elliptical room with two people standing at the foci the phenomena of constructive interference again plays a role. This follows from the geometry of an ellipse. All points on the ellipse maintain a constant distance from the sum of the two distances from each foci to the point. Therefore a sound wave emitted from one foci will rebound off of all points of the ellipse and will constructively interfere at the other foci where all the rebounding sound waves will have traveled an equal distance to get there.

f5r5e5d
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### Re: Acoustical Question of Squeaky Claps

diffraction can be interesting - I used to work next to a large warehouse with a vertical running corrugated iron siding - 100+ long

you could hear a chirp frequency in the backscatter echo when clapping, slamming our building's back door

I would look at Bragg diffraction for explaining that