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-   -   Parabolic Microphone Reflector (http://www.diyaudio.com/forums/everything-else/6982-parabolic-microphone-reflector.html)

mrfeedback 22nd October 2002 12:10 AM

Parabolic Microphone Reflector
 
Has anybody tried useing a Satellite Tv dish (60 or 80 cm) as a parabolic microphone reflector?, and results ?.

Eric.

pinkmouse 22nd October 2002 11:24 PM

Never used a satellite dish, but in school we once made reflectors using paper mache over half a large (18") balloon and they worked pretty well, we could pick up what the teacher was saying in the classroom across the quad:)

P.Lacombe 23rd October 2002 12:03 AM

I have used satellite TV dish to record bird songs at very high distance (100 m) Works very well, but unusable at low frequencies. Some equalization can be implemented in order to correct the increasing efficiency with the frequency. The position of the microphone must be carefully adjusted by extensive tests, because it's not possible to simply put the microphone instead of the LNA, which phase center is generally not well known.

Regards, Pierre Lacombe

mrfeedback 23rd October 2002 12:29 AM

Pierre, thanks for your reply.
"the increasing efficiency with the frequency" - is this a normal characteristic of parabolic reflectors at audio frequencies ?.

Eric.

Circlotron 23rd October 2002 03:37 AM

Mary Poppins was really a spy, you know...
 
An umbrella with the inside surface coated with a more sound-reflective material e.g. varnish etc might be suitable. You also have a central pole to attach the microphone to, though it gets in the way a bit.

GP.

P.Lacombe 23rd October 2002 10:52 PM

Eric,

The increasing efficiency with frequency of parabolic reflector is normal at audio frequencies, as well as radio frequencies. But this is true only for a near strictly parabolic reflector (as SATV dish) not for umbrella or such approximate reflector, which exhibit a maximum efficiency for a particular wavelenght, depending of the actual incurvated form.

SATV dishes are computed for max efficiency at 4 or 5 GHz (in the USA) i.e. about 3 inches wavelenght. I suppose that the max efficiency occurs in the range 2...6 kHz for audio frequencies. European SATV dishes are computed for max efficiency at 12 GHz, requiring a more precise production, and the max efficiency occurs at about 6...15 kHz, giving a noticeable presence effect with a good electrostatic microphone (I use B&K 4133).

Regards, Pierre Lacombe.

mrfeedback 24th October 2002 12:45 AM

Thanks Pierre,

I guess that the lifting response makes it better for eavesdropping on neighbours then. :D
I thought that a parabola is a parabola and effective at all frequencies, and the low frequency gain dependant on the area of the reflector.
I mostly posed the question for interest sake, but if I can get hold of a disused dish I am keen to try the experiment.

Eric.

Circlotron 24th October 2002 04:07 AM

I'm on to you!
 
You just want to listen in to me dreaming up nutty ideas over this side of the country. I know...

GP.

mrfeedback 24th October 2002 04:34 AM

Can you send me some of that water that you are drinking ?.... :D

Eric.


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