Hello all, actually, it is the thermal relaxation and rotational losses of molecules that augment/reduce sound abortion (among other processes). And this process is not linear with frequency/temperature/distance/humidity. You may consult http://www.kayelaby.npl.co.uk/general_physics/2_4/2_4_1.html for some insight in the theory, and this matter is in fact standardized in ISO 9613-1 and ISO 9613-2. Dave, in fact the sound is deadened with increase in humidity, but just for frequencies beyond 16kHz. I guess that from there, the water molecules just absorb too much, but this is just me thinking...
Isaac On Wed, Nov 23, 2011 at 4:16 PM, Dave Malham <[email protected]> wrote: > There's actually an online calculator here > http://www.csgnetwork.com/atmossndabsorbcalc.html and a good paper by Dennis > A. Bohn at www.rane.com/pdf/eespeed.pdf (from JAES 1988). Somehow, the fact > that absorption peaks at low humidities always seems somehow > counter-intuitive - we (or, at least, I) kind of expect sound to be deadened > in a high humidity situation like a fog. I _know_ the difference is that a > fog is water droplets, rather than just humidity, but it still doesn't > _feel_ right. > > Dave > > On 23/11/2011 16:02, Robert Greene wrote: >> >> 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. >>> >>> _______________________________________________ >>> Sursound mailing list >>> [email protected] >>> https://mail.music.vt.edu/mailman/listinfo/sursound >> >> _______________________________________________ >> Sursound mailing list >> [email protected] >> https://mail.music.vt.edu/mailman/listinfo/sursound > > -- > These are my own views and may or may not be shared by my employer > /*********************************************************************/ > /* Dave Malham http://music.york.ac.uk/staff/research/dave-malham/ */ > /* Music Research Centre */ > /* Department of Music "http://music.york.ac.uk/"; */ > /* The University of York Phone 01904 432448 */ > /* Heslington Fax 01904 432450 */ > /* York YO10 5DD */ > /* UK 'Ambisonics - Component Imaging for Audio' */ > /* "http://www.york.ac.uk/inst/mustech/3d_audio/"; */ > /*********************************************************************/ > > _______________________________________________ > Sursound mailing list > [email protected] > https://mail.music.vt.edu/mailman/listinfo/sursound > _______________________________________________ Sursound mailing list [email protected] https://mail.music.vt.edu/mailman/listinfo/sursound
