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Re: Making a parabola

Subject: Re: Making a parabola
From: "Avocet" madl74
Date: Fri Sep 21, 2012 6:41 am ((PDT))
> This formula is a general formula for waves, light, radio or sound
> in a parabola,


This is exactly the problem. Sound waves are fundamentally different
from light and radio waves. Firstly, they are longitudinal not lateral
waves and secondly, their wavelength is comparable to the dimensions
of the parabola. Thirdly we are using them over a 20:1 range of
frequencies, not a narrow band like light or radio waves. The good
news is that the coefficient of reflection of sound with a solid
surface is near 100%.

The bad news is that you won't get a clear focussed image from the
dish. The best you can hope for is the sound equivalent of a fuzzy
blob where most of the focussed energy will completely miss the mic.
That's the simplest explanation for the loss of low frequencies where
the blob gets larger. You can plot the focus pattern by moving around
a small mic.

A parabolic mic does its job well, which is bringing in kilohertz
frequencies where it it more directional than any other type of mic.
Just don't expect a good music balance from one. :-)

A general rule with location sound recording is to use what works in
practice. In the end our ears are the best judge rather than the


David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce

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