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Re: sound wavelength and parabola size

Subject: Re: sound wavelength and parabola size
From: Marty Michener <>
Date: Mon, 24 Feb 2003 11:54:01 -0500
At 11:30 PM 2/23/2003 -0500, you wrote:
>Precisely. That is why I forwarded the sites. What arrives at the face
>of a parabolic dish is a logitudinal wave front. not a transverse wave.
>I believe if one thinks in terms of wave fronts which are essentially
>spherical surfaces  (at least spherical segments), then one can more
>easily see why even a small diameter dish can pick up lower frequencies
>with longer wavelengths. Of course, the more distant the source, the
>more closely the wave front resembles a flat wall when it reaches the
>parabola.  Likewise, small microphone elements can detect low
>frequencies. The nature of the frequency responses in either case is
>dependent on much more than the relative size of the receiving elements
>be they parabolic dish or mic diaphram.
>
>Jim

Very excellent points.  The difference between sound coming from left and 
right, looked at this way, is the angle of the wave front compared to that 
of the parabola face.

Of course the sphericity of the wave front is another way to look at the 
proper focusing point.  A very distant (near-plane) wave will focus 
directly at the focal length of the parabola, say 30 cm.  A closer sound 
source, producing a more convex wave front at the parabola, will focus 
further away from the reflector, as given by the equation for Object 
distance, image distance and focal length.  In English:

reciprocal focal length      equals      the sum of reciprocal image 
distance     plus     reciprocal object distance.

1/f          =    1/I      +        1/O

(The problem with typing "ohs" and "eyes" is the letters look a darn-sight 
like zeros and ones! ;^)

Intuitively, the closer the sound gets, the further the reflected focus 
globe of sound at which the mic should be placed for best sound gain.

The practical limit is at 2f, or twice the focal length, where object 
distance now equals image distance and you would clearly be better off 
leaving the parabola at home, since you are direct-micing the 
beast.  ;^0)   (Think about it.)

my best regards,

Marty Michener
MIST Software Associates PO Box 269, Hollis, NH 03049

EnjoyBirds.com  - Software that migrates with you.    http://www.EnjoyBirds.com



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