> This suggests the 6db rule would only be accurate at one specific
> frequency, and any general rule would need to include an approximate
> adjustment for frequency.
> Does that make any sense,if so do you know of a common use equation
> that includes a frequency adjustment?
Here's a tech answer. Birds and bats. I've done quite a bit of reading
up on ultrasonic absorption in air. I found a graph published in the
Journal of the Acoustics Society of America vol 88 no 4 Oct 1990,
which also includes audible frequencies. I haven't got copyright
clearance to reproduce it but the humidity lines converge at common
humidities in the 3kHz to 20kHz region. Figures for "excess
attenuation" (after the 6dB per double the distance loss) are as
follows:
Frequency 400 1K 2K 4K 10K 20K 50K
Att dB/100 met 0.5 0.7 1 2 10 40 100
At the upper end, humidities below 50% increase the absorption.
At 10K the inverse square law attenuation between 10 met and 100 met
is -20dB and the excess absorption is another 9dB. (think about it)
:-) At lower frequencies excess attenuation soon becomes a minor
effect.
Mist fog and other precipitation greatly increase the excess
attenuation at all frequencies and the graph is for clear air only.
Thunder is often very treble attenuated due to water particles.
Note that at bat echolocation frequencies the excess attenuation is
around 1dB per metre, and many bat experts forget how limited bat
echolocation is, commonly 15 metre max. (120dB SPL at 100mm; -50 at
30met total echo path length; absorption -30dB =3D 40dB SPL less
reflection coefficient. For small targets the inverse square law
become the inverse quad law.
Lesser horseshoe bats (110KHz)are difficult to pick up at over 5
metres as I have confirmed at bat emergence counts. If you can't hear
it emerge, it's probably a "stealth" Long-eared or a LHS.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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