[LAD] sound dissipation in air - formulas and filters?

[Fons Adriaensen]

On Fri, Jan 30, 2009 at 09:15:10PM +0100, Jörn Nettingsmeier wrote:

> so far, all i've found is an online calculator at
> http://www.sengpielaudio.com/calculator-air.htm, but the javascript in
> there is not exactly fit for citation :)
> plus i know there is a standard (ISO 9613-1:1993) that deals with sound
> attenuation, but i don't have access to a standards library and don't
> feel like buying it...
> 
> a rough calculation at 18°C and 85% relative humidity (using sengpiel's
> tool) yields:
> 
> f[Hz]	dp[dBSPL/m]
> ----------------------
> 20	0
> 40	0
> 80	0
> 160	0
> 320	0.001
> 640	0.003
> 1280	0.006
> 2560	0.011
> 5120	0.032
> 10240	0.112
> 20480	0.404
> 
> which indicates that for my usecase (close-miked sounds that need to be
> placed at distances between 10 and 45 m), the air-damping non-linearity
> is really important.

Put the lines (the nunbers) above in a separate
text file, and plot with gnuplot using a log x
scale, e.g. to see the effect at 20 meters:

plot 'thefile' u 1:($2)*20 w l 3

This will show you that a simple 2nd order
lowpass will do the job - it's not a perfect
match but good enough. It's not critical at
all - for small distances you can even use
a standard shelf or parametric.

Ciao,

-- 
FA

Laboratorio di Acustica ed Elettroacustica
Parma, Italia

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