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Re: DIY Parabolic Dishes.

Subject: Re: DIY Parabolic Dishes.
From: "Randolph S. Little" rsl54442
Date: Fri Oct 6, 2006 10:08 am (PDT)
--- In  Walter Knapp <> 
wrote (in part):

> What wavelength are we talking about here? Transverse or 
> It sure sounds like you are using transverse wave patterns in your 
> predictions, not the longitudinal pressure waves that's going to 
> sound. I really have a hard time coming up with a way of 
> that variations in the longitudinal wavelength will change the 
focus size.
> (For those that don't know what we are talking about, here's a nice 
> of graphics that illustrate the difference:

Good question Walt, and a helpful reference website.  It is indeed 
the longitudinal wave motion that is transduced by the microphone, 
and it is this wavelength to which I refer.  In air at standard 
atmospheric pressure, temperature and humidity the velocity of these 
longitudinal sound pressure waves is about 1000 feet per second.  The 
wavelength of a 1 KHz sound is about 1 foot, that of a 10 KHz sound 
about 0.1 foot, etc.  Thus, it is in the upper octave of human 
hearing (10 to 20 KHz) that the wavelength of the longitudinal wave 
and the diameter of the microphone diaphragm are the same order of 
At all practical distances from the sound source, the sound wave is 
planar by the time it reaches the diaphragm, and the whole diaphragm 
moves longitudinally as a unit, not unlike any one of the dots in the 
referenced animation of a longitudinal wave.  However, in the focal 
region of a parabolic reflector, many identical longitudinal waves 
are converging from many directions, having been reflected from 
different facets of the reflector.  Most of these components are 
impinging obliquely on the diaphragm, and therefore tend to sweep 
across the diaphragm instead of impacting it all at once.  At lower 
frequencies (longer wavelengths) these differences are negligible, 
but as the acoustic wavelength becomes shorter these differences 
result in less net axial movement of the diaphragm, hence less 
> I'd think the size of the focal point was related to a lot more. 
> how perfect a parabola you had, it's surface roughness and so on. 
> of course, I assume you are talking about a sound source that's 
> on line with the central axis of the parabola. As soon as that 
> source is off axis, even slightly, the focus point shape changes or 
> shifts position. That's why it's better to think of the focus of a 
> parabolic as a zone rather than a point.

This is another good point for discussion.  It turns out that minor 
deviations from true parabolic curvature are not terribly important 
to the acoustic performance of the reflector.  At very, very short 
wavelengths this becomes important (as for light or radio waves), but 
at our acoustic wavelengths a dent or dimple here and there is of no 

Good recording,

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