David
Thanks for all informations
I hope to do some test in this WE.
The main problem is that I have no anechoic chamber, however I have a
frequency generator up to many MHz and a 300 MHz oscilloscope ... and a
dehumidifier too!
I have a vacuum pump but nothing in which to put the array to test the vacuum!
I'll think up something
Ivano
---In <> wrote :
> regarding the amplification it's necessary to use a precision operational
> amplifier, the problem to keep the SNR at a good level. The distance is
> completely relative ... to so many factors!
Ivano,
My guess is that the thermal noise of the air could swamp the mic and amp
noise at higher frequencies. For the array, I calculate a 36dB forward gain
increase for the array over a single unit, and a corresponding air noise
increase of 18dB - an improvement of 18dB. Thermal air noise will rise at
10log(KHz)dB. Check this as I am in bed with a cold and muzzy-headed. :-(
I'd be interested how it works in practice. As for distance, there is a
calculator for air absorption on:
http://www.sengpielaudio.com/calculator-air.htm
http://www.sengpielaudio.com/calculator-air.htm
For the bats I am studying at present, Lesser horseshoes, This gives 4.5dB
per metre absorption at 110KHz on a damp evening. In practice, you lose them
on a bat detector at about 5 metres.
I can't find a figure for the thermal noise of the air, but it is going to
rise at about root(kilohertz). That would be a rise of 16dB from 1Khz
bandwidth to 40KHz.
> regarding the amplification it's necessary to use a precision operational
> amplifier, the problem to keep the SNR at a good level. The distance is
> completely relative ... to so many factors!
With your 64-array, it should be possible to derive thermal air noise
figures, especially If you have access to a vacuum for reference.
BTW I found measuring U/S mic responses indoors was a nightmare due to
standing waves. I went outside over soft ground and used a hose spray and an
airline as sources and looked at recordings on a spectrogram. Pointing the
mic upwards should give a good air noise indication. especially if you use
an absorbent baffle like a cushion in and out to indicate any HF background.
Passing aircraft won't be an U/S problem. Birds sound very different in
ultrasound. Insects, especially crickets, can be very noisy in U/S as bat
enthusiasts well know.
David Brinicombe
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