Hi, all,
According to the following paper, the h = r * powf (10.0f, C) / p should
be correct, where p = P_s/P_{s0}, the relative pressure.
Benjamin Bernfeld, "Absorption of sound by air: a personal calculator
program," J. AES, vo. 28, 1980.
If anyone could share the PDF of ISO 9613-1, it will be greatly
appreciated.
Cheers,
Junfeng
On Sun, Nov 27, 2011 at 3:07 AM, Fons Adriaensen <f...@linuxaudio.org>wrote:
> Hello all,
>
> A few days ago I posted a simple C function to calculate
> sound absorption in air (reproduced below), adding that
> the origin of this code was not clear.
>
> Joern reported that it could have been [1]. That is not
> the case, in fact it was the Javascript code at [2].
>
> But comparing the two, and also [3], I'm now convinced
> that both [2] and my code are wrong, and that the line
>
> h = r * powf (10.0f, C) * p;
>
> should be
>
> h = r * powf (10.0f, C) / p;
>
> Which is also logical as 'h' is a concentration (in %),
> so it should be the ratio of two pressures, not their
> product (the factor powf(...) is a pressure), r is
> a ratio in %).
>
> Both [1] and [3] use this form.
>
> [1] <http://www.sengpielaudio.com/AirdampingFormula.htm>
> [2] <http://www.csgnetwork.com/atmossndabsorbcalc.html>
> [3] <http://resource.npl.co.uk/acoustics/techguides/absorption/>
>
> If anyone has ISO 9613-1, please check !
>
> Ciao,
>
>
>
> ***** WARNING: this is probably wrong, see above *****
>
> // Compute air absorbtion in dB/m
> //
> // p pressure in Pa
> // t temperature in centigrades
> // r relative humidity in percent
> // f frequency in Hz
> //
> float airabs (float p, float t, float r, float f)
> {
> float C, h, tr, f2, frO, frN;
>
> p /= 101325.0f;
> t += 273.15f;
> C = 4.6151f - 6.8346f * powf ((273.16f / t), 1.261f);
> h = r * powf (10.0f, C) / p;
> tr = t / 293.15f;
> f2 = f * f;
> frO = p * (24 + 4.04e4f * h * (0.02f + h)/(0.391f + h));
> frN = p * powf (tr, -0.5f) * (9 + 280 * h * exp (-4.17f * (powf (tr,
> -1/3.0f) - 1)));
> return 8.686f * f2 * (1.84e-11f * sqrt (tr) / p
> + powf (tr, -2.5f) * ( 0.01275f * (expf (-2239.1f / t) / (frO +
> f2 / frO))
> + 0.10680f * (expf (-3352.0f / t) / (frN +
> f2 / frN))));
> }
>
> --
> FA
>
> Vor uns liegt ein weites Tal, die Sonne scheint - ein Glitzerstrahl.
>
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>
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