Hi, Ciao,
Thank you so much for your quite valuable comments.
The idea of the below code that you shared is from the
Benjamin Bernfeld, "Absorption of sound by air : A personal calculator
program," J. AES, Vol. 28, No. 7/8, pp. 521-524, 1980.
I believe this implementation introduces the air absorption to sound in
air. I definitely agree your comments on this issue.
While what I am consider what the transfer function / Green function should
be when considering air absorption. Specifically, as you know, in free
field, the Green function / transfer function is
P(w, X, X') = exp[jw(d/c-t)] /
(4*pi*d) Eq.(1)
or
P(t, X, X'） = \delta(t-d/c) /
(4*pi*d) Eq.(2)
where w is the frequency, X and X' is the positions of source and receiver,
d is the distance between them, \delta is the room impulse response
function (time domain Green's function)
When considering absorption of wall surfaces, the Eq. (2) will be changed to
P(t, X, X') = \beta_{x1}^|n-q| * \beta_{x2}^|n| * \beta_{y1}^|n-j| *
\beta_{y_2}^|j| * \beta_{z1}^|m-k| *
\beta_{z2}^|m|
Eq. (3)
where \beta_{...} are the absorption coefficients of surfaces of room.
Likely, my question is whether the similar equations as Eq. (3) exist when
considering air absorption by extending Eq.(1) or Eq. (2). If it exists,
what this extended equation should be?
Thanks a lot again.
Best regards,
Junfeng
On Wed, Nov 23, 2011 at 5:16 PM, Fons Adriaensen <[email protected]>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.
>
> _______________________________________________
> Sursound mailing list
> [email protected]
> https://mail.music.vt.edu/mailman/listinfo/sursound
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<https://mail.music.vt.edu/mailman/private/sursound/attachments/20111123/db387950/attachment.html>
_______________________________________________
Sursound mailing list
[email protected]
https://mail.music.vt.edu/mailman/listinfo/sursound
