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## Re: DIY Parabolic Dishes.

 Subject: Re: DIY Parabolic Dishes. "Randolph S. Little" rsl54442 Fri Oct 6, 2006 10:08 am (PDT)
 ```--- In m("yahoogroups.com,","naturerecordists"); Walter Knapp wrote (in part): > What wavelength are we talking about here? Transverse or longitudinal? > It sure sounds like you are using transverse wave patterns in your > predictions, not the longitudinal pressure waves that's going to fit > sound. I really have a hard time coming up with a way of visualizing > 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 set > of graphics that illustrate the difference: > http://www.gmi.edu/~drussell/Demos/waves/wavemotion.html 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 magnitude. 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 sensitivity. > I'd think the size of the focal point was related to a lot more. Like > how perfect a parabola you had, it's surface roughness and so on. And, > of course, I assume you are talking about a sound source that's exactly > on line with the central axis of the parabola. As soon as that sound > 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 consequence. Good recording, Randy ```
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