I agree. Air absorption is substantive in concert halls
because the rooms are so large, so that the sound travels
a long way before finally dying out. This is a major effect.
Even a "bright" concert hall(comparatively bright) has
a considerable roll off of the top octave in its reverberant
field frequency response. (This is one reason why audio often
sounds odd compared to concert music--there is too much high frequency
content in the diffuse field). But in smaller rooms, of the domestic
listening room kind, the effect of air absorption is small.
The acoustical nature of the room in the high frequencies
is dominated typicallyby the absorbing of sound by the room and the
objects in it, not by air abssorption.
You can see some(interesting, I hope) material on this(including humidity
dependence and so on)
here
http://www.regonaudio.com/Records%20and%20Reality.html
Robert
On Wed, 23 Nov 2011, Fons Adriaensen wrote:
On Wed, Nov 23, 2011 at 09:52:22AM +0800, Junfeng Li wrote:
Now I am considering to improve the traditional image-source method to
generate more natural room impulse response with the consideration of air
absorption. I noticed that some open source codes (e.g., roomsim) have
already realized this idea. To reformulate the image-source method from
theory, however, my main concern is what kind of formulation the transfer
function or Green function (exp(jwd/(4*pi*d)) in the free field should be
when the air absorption effect is taken into account?
Could anyone direct me to some references on this issue?
Does any have any idea? or references on this issue?
?Though I believe it is a easy question, I cannot find the answer, ....)
I don't think you need to reformulate the theory. Air absorbtion
can be taken into account numerically just in the same way as
the frequency response of the reflecting surfaces.
Also, unless the room is large and the walls have low absorption
at HF the effect will not contribute much and can probably be
modelled using a simple lowpass filter.
The code reproduced below is a cleaned up version of an algorithm
I found somewhere (and don't remember where). As you will note
when experimenting with it, air absorption is very dependent on
temperature and humidity, so any room resonse computed from standard
conditions will be invalid if these change - assuming air absorption
contributes much at all.
// 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, frO, frN;
p /= 101325.0f;
t += 273.15f;
C = 4.6151f - 6.8346f * powf ((273.15f / t), 1.261f);
h = r * powf (10.0f, C) * p;
tr = t / 293.15f;
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 * f * f * (1.84e-11f * sqrt (tr) / p
+ powf (tr, -2.5f) * ( 0.01275f * (expf (-2239.1f / t) / (frO + f *
f / frO))
+ 0.10680f * (expf (-3352.0f / t) / (frN + f *
f / frN))));
}
Ciao,
--
FA
Vor uns liegt ein weites Tal, die Sonne scheint - ein Glitzerstrahl.
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