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Old 2nd May 2007, 07:10 PM   #1
kdw3 is offline kdw3  United Kingdom
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Default Ratio between wavelength and reflector size

Hi,

I've been trying to design an omni-directional sound source as part of my final year engineering project. Needless to say, it wasnt particularly effective.

The rig was set up like this. A 6" alpine speaker was hung from the roof in a spherical enclosure about 20cm off the floor. A reflector made of wood with the tip about 12cm from the speaker cone and the base on the floor. The base of the reflector is circular with diameter of 254mm and a height of 152. The shape was similar to parabola.

My question is this, the only two frequencies that were within the tolerancy were 4kHz and 8kHz. I believe that the main reason the other frequencies didnt work was because the reflector was either too big or too small. I have read a few articles on sound reflectors and they mention that the size of the reflector in relation to the wavelenth is important, but gives no mention as to what the ratio is.

I realise this is a difficult question, but any suggestions would be much appreciated

Regards
kdw3
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Old 3rd May 2007, 02:14 AM   #2
Ron E is offline Ron E  United States
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Have you played around with the java ripple simulator?
http://www.falstad.com/mathphysics.html
It might give you an intuitive grasp. You can draw reflectors, enclosures, etc... and see what effect they have on the wave propagation.
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Old 4th May 2007, 12:39 AM   #3
MCPete is offline MCPete  United States
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Default reflector sizing

For a given aperture of a reflector, sound of a wavelength greater than the aperture and projected towards the reflector isn't reflected and "sees" the reflector as an obstruction that it passes around. Conversely, where the wavelength of the sound is less than the aperture or roughly the width of the reflector, then the sound is reflected.

The above is an oversimplification as I think that the transition between reflecting and not reflecting is gradual. That is, if the wavelength of a sound projected toward the reflector is exactly equal to the aperture, then some sound is reflected and some passes around the reflector.

This should be applicable to what you are doing, but I have to say that I couldn't visualize your setup from your description.

-Hope that this is helpful.
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