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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