--- In 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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