naturerecordists
[Top] [All Lists]

Re: the nature of parabolic reflectors

Subject: Re: the nature of parabolic reflectors
From: Walter Knapp <>
Date: Sat, 21 Feb 2004 17:46:33 -0500
From: Bret <>

> I have demonstrated above that the the ratio of dish diameter to
> wavelength in common audio dishes are equal or greater than this ratio
> for common rf dishes.
> 
> Please show me another more complicated formula for parabolic gain.  I
> am all ears 

One of the major differences is that rf is moving at the speed of light 
essentially massless movement. But, sound is moving particles of 
significant mass that are all but standing still by comparison. Treating 
it as a transverse wave is what gets things not agreeing with actual 
field experience. Sound is movement of particles of significant mass 
along the line of travel, which is a type of wave but has other 
characteristics. Which is why a parabolic much smaller than the 
wavelength still reflects the sound. With all that goes along with that.

Read Sten's paper for starters lots of formula to play with.
http://naturerecordist.home.mindspring.com/Parabolic.pdf

  Parabolics start to have gain at a point where their size is 1/64th of 
the wavelength according to Sten's paper. Let's see, for a 22" Telinga 
that's a wavelength of about 117 feet, my that's a low frequency. Now, 
that's the point where the gain starts to increase, it's still 1:1 at 
that point. But from there on up you do get rapidly increasing gain. 
Especially with something like the Telinga, with it's depth/focal length 
greater than 1 it avoids most of the standing wave problem. What I get 
in the field agrees quite well with Sten.

The 1/1 wavelength to diameter point is just the starting point where 
the already quite significant gain goes to being straight 6dB/octave. 
It's not the start of gain by any means. For the Telinga theoretically 
at the 1/1 point the gain is about 20dB. It passes 6dB gain at a 
wavelength of about 15'.

Note within the limitations of the mic mounted in the parabolic there is 
no frequency point where the sound is suddenly cut off. Below the 1/64th 
point it's just as if the reflector was not there, ie the mic picks up 
the sound like it was a bare mic (though the reflector may act as a 
sound barrier from behind). No sudden transition.

Walt






________________________________________________________________________
________________________________________________________________________

<Prev in Thread] Current Thread [Next in Thread>
Admin

The University of NSW School of Computer and Engineering takes no responsibility for the contents of this archive. It is purely a compilation of material sent by many people to the naturerecordists mailing list. It has not been checked for accuracy nor its content verified in any way. If you wish to get material removed from the archive or have other queries about the archive e-mail Andrew Taylor at this address: andrewt@cse.unsw.EDU.AU