Layman's question-Graph for sound attenuation...

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JJ - 29 Jun 2005 21:41 GMT

Hello,
Does any one know where or how I could find a graph or table with the
attenuation of sound by air for different frequencies (and distances)?
I found this equation:
I*exp{-2E-13*(f^3)*r}
I = ------------------------------
4 * pi * r^2

But my math is pretty rusty...
Thanks,
JJ

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Don Pearce - 29 Jun 2005 21:48 GMT

>Hello,
>Does any one know where or how I could find a graph or table with the
[quoted text clipped - 7 lines]
>Thanks,
>JJ

Here is a table which gives a pretty good approximation of the
attenuation of sound by air at 20C and 70% RH. THis must be modified
for other temperatures and humidities, but I don't have that data.

Frequency    Attenuation
125 Hz        0,3 dB/km
250 Hz        1,1 dB/km
500 Hz        2,8 dB/km
1000 Hz        5,0 dB/km
2000 Hz        9,0 dB/km
8000 Hz        76,6 dB/km

d

Pearce Consulting
http://www.pearce.uk.com

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JJ - 29 Jun 2005 22:30 GMT

Thanks!
JJ

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Eberhard Sengpiel - 29 Jun 2005 23:20 GMT

> Thanks!

Try this calculator:
Calculation of the absorption of sound by the atmosphere
in dB per kilometre.
Enter air pressure in Pa, temperature in °C, humidity in %,
and frequency in Hz.
http://www.measure.demon.co.uk/Acoustics_Software/iso9613.html
The used formulas you find in the source code.

Cheers

Eberhard Sengpiel
German forum for microphone recordings
and sound studio techniques
http://www.sengpielaudio.com

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Noral Stewart - 29 Jun 2005 23:38 GMT

JJ should note that the information provided by Pearce and Sengpiel is for
the atmospheric absorption of the sound which is in addition to the the
normal 6 dB per doubling of distance or inverse square law attenuation.

Also, within the the first several hundred feet or meters from the source,
if the source and listener are near the ground and the ground is soft, you
will get additional attenuation from the ground interaction.

At longer distances, depending on wind direction and whether the temperature
is increasing or decreasing with altitude, the sound could be decreasing
much more rapidly.

>> Thanks!
>
[quoted text clipped - 12 lines]
> and sound studio techniques
> http://www.sengpielaudio.com

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The Ghost - 30 Jun 2005 01:32 GMT

> JJ should note that the information provided by Pearce and Sengpiel is
> for the atmospheric absorption of the sound which is in addition to
> the the normal 6 dB per doubling of distance or inverse square law
> attenuation.

JJ should also note that the 6dB decrease in sound pressure level per
doubling of distance applies only in the far field and only if the source
is a point source (one having dimensions that are small compared to a
wavelength). For a line source (eg a long straight-line stretch of
freeway), the decrease in the far field is 3dB per doubling of distance.

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Noral Stewart - 30 Jun 2005 12:21 GMT

True and these are basics that should have been the starting point.

>> JJ should note that the information provided by Pearce and Sengpiel is
>> for the atmospheric absorption of the sound which is in addition to
[quoted text clipped - 6 lines]
> wavelength). For a line source (eg a long straight-line stretch of
> freeway), the decrease in the far field is 3dB per doubling of distance.

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Natural Science Forum / Physics / Acoustics / June 2005

Layman's question-Graph for sound attenuation...