From: "Randolph S. Little" <>
> Rich refers to my "Acoustic Properties of Parabolic Reflectors" thesis at
> Cornell's School of Electrical Engineering. Unfortunately, it is even harder
> to get than Sten's paper (Sten cites it), so perhaps I could be coerced into
> transcribing it and posting it on my website some day. In the meantime let
> me offer one all-important point about all of these theoretical analyses, at
> least all that I know of to date. And this I have discussed with Sten who
> also agrees.
If it provides additional info above Sten's paper, I'd love to read it.
Perhaps a pdf?
> Key point:
> Whereas the mathematical analyses attempt to predict the acoustic
> pressure at the focal POINT, the actual microphones that we use to transduce
> this acoustic pressure into an electrical signal are sensing the mean
> acoustic pressure over some finite area approximating that of the diaphragm.
>
> Thus, the net acoustic pressure over a certain focal area (I have previously
> called this "focal volume") is what counts, and this does NOT increase
> monotonically with frequency. In fact, it levels off for wavelengths shorter
> than the diameter of the microphone.
This, I assume is a good part of the reason for the graphs of real
parabolics (of older varieties now, shown in Sten's paper) to have such
a fall off compared to theory. I expect a lot of the older parabolics
were using mics with pretty big diaphragms.
With the much smaller diaphragms now used this is probably not as much a
factor?
Perhaps should make it clear that we are still talking considerable gain
even if it levels off.
Of course the Telinga is different here too. With it's boundary mic,
multicapsule approach. Wonder how the math looks for that?
Walt
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