<We are often reminded that sound attenuation with distance approximates a =
reduction of 6dB for each doubling of distance. However, as you and Klas po=
int out, the actual attenuation over distance is highly frequency dependent=
. This suggests the 6db rule would only be accurate at one specific frequen=
cy ...>
The 6 dB rule is the theoretic attenuation casued by the geometric spreadin=
g (divergence), which itself does not depend on the frequency.
<... and any general rule would need to include an approximate adjustment f=
or frequency. Does that make any sense,if so do you know of a common use eq=
uation that includes a frequency adjustment?>
Yes, there is an additional atmospheric attenuation that depends on the fre=
quency but also on thje temperature, pressure and humidity. The underlying =
equation is quite complicated, but there are websites that can estimate the=
effect:
http://resource.npl.co.uk/acoustics/techguides/absorption
In addition to that, there are a number of other mechanisms (such as reflec=
tion, diffraction, refraction) that can also influence propagation of sound=
. See http://encyclopedia2.thefreedictionary.com/Atmospheric+Acoustics
Regards,
Raimund
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