Walt,
I agree, that pressure and reflection are related to the same thing.
Of course, if you look at single molecules, there will be no
differences at low and high frequencies. But there will be
differences when looking at the larger scale...=20=20=20
> The only real difference between the microphone diaphragm and a
perfect
> reflector is that the diaphragm absorbs energy to move it. A
perfect
> reflector returns all the energy. I'm sure that any real world
parabolic
> reflector only approaches a perfect reflector.
Unfortunately, the real-world reflector will only reflect the energy
at higher frequencies (because of its limited size). As you say, the
parabola is not an ideal reflector nor it is an ideal high-pass
filter. To some degree, it is similar to an electronic high-pass
filter. It is impossible to design a filter with an extremely steep
transition between the stopband and passband. Therefore you will get
some gain at freqncies below the corner frequency. Also, the
transition from stopband (at low frequencies) to passband (at high
frequencies) to is not very smooth (as it would be in an electronic
filter). There are many mountains and valleys in the frequency
response at that range. This is one drawback of the parabola
compared to a shotgun (as long as your subject is located exactly on-
axis).
> Don't believe me? Go read some of the accounts of designing
microphones.
> You will find quite a few comments on controlling reflections from
the
> parts of the microphone. All of which are smaller than the
wavelengths
> involved. If there are reflections there, they are no different
from
> those from the parabolic reflector.
You are right, there are reflections on the surface of the
microphone diaphragm too (especially at higher frequencies).
But these reflections are not essential to the operation at low
frequencies. The air just moves the diaphram (a special kind of
barometer). As mentioned earlier by others, this principle will also
work at infrasound. At wavelengths in the range of the of diaphram
size, reflections will come into play too. But these reflectections
are not desired, because they attenuate the output signal (loss of
flatness). This is the reason, why microphones (I mean the classical
diaphram designs) for high frequencies must have smaller diameters.
> Or take a look at a PZM boundary
> microphone, where only reflected sound reaches the mic. The
boundaries
> are much smaller than the wavelengths involved.
As I know, those microphones must be placed on a large surface (e.g.
a wall). Then you will then get a larger boundary.
If you use such a microphoen in a room it will also pick up low-
frequency sounds reflected by the walls and the ceiling (but these
are large).=20
Raimund
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