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.
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.
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.
Walter? Klas? Anyone?
Syd Curtis in Brisbane, Australia.
PS. The above has only just occurred to me. It's now virtually the end of
the cicada season , and I don't know if any are still singing - it's wet
today. If there are any, I'll try to get a recording to demonstrate what
I've been writing about.
S.
The quote from the book:
"A problem in early radio astronomy - and one reason it drew little
interest at first from optical astronomers - was that radio telescopes like
Reber's couldn't measure a source's position in the sky accurately enough to
find out which visible object was emitting the radio waves. In order to
accomplish that there needed to be a hundred-fold improvement in resolution,
and that meant a telescope about one kilometre in diameter. ... A radio
telescope doesn't give good resolution unless it is quite a bit larger than
the length of the radio waves it's receiving. ... The much shorter waves
in the visible part of the spectrum allow optical telescopes to achieve
resolution with relative ease. Radio astronomers finally solved the problem
in 1949 by using networks of small telescopes linked to a receiving station
which combines the signals. Such a network or array is a 'radio
interferometer'.
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