Syd Curtis wrote:
> I have just been reading "Measuring the Universe - the Historical Quest to
> Quantify Space" (Kitty Ferguson, Headline Book Publishing, London, 1999).
> At page 194 the author points out that to achieve sufficient resolution to
> pin-point a radio source in the sky the telescope needs to wider than the
> wavelength - about a kilometre in diameter, a problem solved by linking a
> network of small telescopes. (I append the actual quote.)
>
> Walter pointed out that sound 'waves' are totally different to
> electromagnetic waves, but I wonder if something similar applies: that with
> a parabolic reflector, sound waves longer than the diameter of the dish are
> reflected but increasingly, as the wave length increases are not focussed.
> The pressure exerted on the microphone by such low pitched sounds is
> increased but not to the extent that applies with waves shorter than the the
> dish diameter that are focussed.
There is considerable difference. Sound moves by vibrating atoms or
molecules, electromagnetic radiation is off into the realm of sub atomic
stuff. Sound is, therefore vibrating something that has mass. Not so
with electromagnetic, (or the mass is extremely tiny, don't know the
current thinking right off). And the detection process for sound
continues the mechanical theme, the diaphragm is measuring kinetic
energy. Whereas a radio telescope is depending on the interaction of a
local oscillator or tuned circuit with the electromagnetic wave brought
to it from the reflector.
The application of electromagnetic wave theory directly to sound without
considering these differences is how we arrive at a situation where real
parabolics can often record far better than theory. When the first
nature recordist to think of using a parabolic asked the physics folks
about it, they claimed no audible frequency could be reflected by a
parabola and based that on wave theory. Lucky for us that nature
recordist and quite a few others tried it anyway. Physics was forced to
backpedal some, but there still is this difference between them and
actual experience in real field recording.
> If this is the case, it might explain something that has been puzzling me.
> A local cicada species that often 'sings' in my garden makes a continuous
> buzz (fascinatingly similar to my tinnitus!). I have yet to find the insect
> - small I suspect - and too high off the ground for me readily to see it.
> In trying to find it I've used a recorder and headphones with Klas's so
> excellent Telinga set-up. The general direction of the insect immediately
> becomes apparent (which it is not to my naked ears) but when I get the dish
> focussed exactly on the cicada, there is a marked change in the sound, and I
> wondered what was going on.
>
> Could it be explained this way: When not precisely focussed, the dish is
> reflecting all frequencies and amplifying them at the microphone to some
> extent - there is an obvious increase in volume, but no change in the sound,
> as I swing the dish towards the insect. But when I chance on the exact
> direction of the sound, the higher frequencies are then focussed precisely
> and greatly amplified while those longer than the dish diameter are not
> focussed and no more amplified than before, hence the change in the sound.
By the current take of the wave theory folks this would not be correct.
Off axis would not change the frequency response much if at all. At
least as far as due to straight wave theory vs reflector size as we have
been discussing.
There are a number of things going on in a parabolic as you go off axis.
For one, there are now phase differences from the differences in
distance from source from the near and far side of the reflector. So,
you will get some comb filtering that's less present in the on axis
situation. The comb filtering will effect the higher frequencies to a
greater extent than the lower ones. And that's probably most of what you
are hearing.
> Is that a likely explanation?
>
> Might it also explain the source of the apparent difference between the
> theory of parabolic reflectors for sound recording, and what actually
> happens in the field.
No, I believe that is a separate problem. The differences in question
are for on axis, where predicted performance by wave theory does not
have a good track record vs actual performance field recording on axis
with some parabolics. Or maybe we should say by observation as there is
very little real accurate testing under field conditions to give
performance numbers.
Even on axis there are differences in distance from source for various
points on the reflector. This is especially true of flatter parabolics.
And may account for the better performance of the deep dish Telinga over
similar sized flatter parabolics. Even over some larger parabolics.
That at least the DAT Stereo mic element is a multicapsule pickup on a
boundary surface may also be how it avoids some of this and gets better
gain characteristics. I've not taken the mic element apart on mine, but
the extra area of the capsules I assume is there coupled with the
boundary may tend to smooth out the phase problems as it's output would
be a integration of some variation in distance all the time. So testing
of a Telinga reflector with some other brand plain mic may not give a
good indication for how the entire Telinga system works as a unit.
And that's all probably a oversimplification. Being a mere biologist I
try to avoid messing in this stuff.
I am in the process, as I have time, of designing and machining a
similar stereo boundary system for the telinga reflector based on MKH
mics. Actually a mic module that could be used with other reflectors as
well. It will be interesting to see how that does with this. Just
satisfying some curiosity.
Walt
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