Re: [PSUBS-MAILIST] Passive sonar research

[andy goldstein <andy.goldstein@videoray.com>]

David,

The basic idea is that you can mix (multiply) the high frequency signal 
with another sinusoid.  This has the effect of creating two new 
signals.  This is shown by the trigonometric identity Sin(a)*Sin(b) = 
1/2 * Sin(a-b) - 1/2 * Cos(a+b).

So you now get two side bands at the difference and the sum of the two 
frequencies.  You can then filter out the sideband you dont want.  In 
this fashion you can move the desired frequency into audible range.  
Bandwidth considerations aside. 

The process is called Heterodyning and there are tons of web sites.

If you are doing it digitally you can play other  tricks like resampling 
at a different freq.  or other multirate techniques.

Quick google gave:
http://www.stanford.edu/~omeri/Detection%20and%20Conversion%20of%20Ultrasonic%20Frequencies.htm

-a


Jon Wallace wrote:
> David,
>
> Do an internet search for "bat detector" or "listening to bats" or use 
> similar wording.  Somebody out there has a schematic for a home-built 
> bat listening device.  I've seen it before but didn't keep a reference 
> to it.  It detects the ultrasonic "pings" of bats looking for food at 
> night.  This will save you from having to reverse engineer an existing 
> device.  As I recall, the circuit is minimal and quite simple.
>
> Jon
>
> >    I was able to locate a kids toy that could detect and allow the 
> > listening to bats. Perhaps if I took this simple device apart, I 
> > could modify it to detect slightly higher frequencies I so desire.
> >  
> >    
>
>
>
>
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