markv@uoregon.uoregon.edu (Mark VandeWettering) (01/04/89)
From uoregon!uw-beaver!mit-eddie!ll-xn!ames!amdcad!sun!pitstop!sundc!seismo!uunet!mcvax!hafro!krafla!raunvis!kjartan Fri Dec 30 00:44:13 PST 1988 Article 3903 of comp.graphics: Path: uoregon!uw-beaver!mit-eddie!ll-xn!ames!amdcad!sun!pitstop!sundc!seismo!uunet!mcvax!hafro!krafla!raunvis!kjartan >From: kjartan@raunvis.UUCP (Kjartan Pierre Emilsson Jardedlisfraedi) Newsgroups: comp.graphics Subject: Re: Sound tracing Keywords: Sound tracing, accoustics Message-ID: <240@raunvis.UUCP> Date: 29 Dec 88 16:02:17 GMT Organization: Science Inst. Univ. of Iceland Lines: 73 Status: OR I would like to present some preliminary ideas about sound tracing, and critique (hopefully profitably) the simple model presented by Kjartan Pierre Emilsson Jardedlisfraedi. (Whew! and I thought my name was bad, I will abbreviate it to KPEJ) CAVEAT READER: I have no expertise in acoustics or sound engineering. Part of the reason I am writing this is to test some basic assumptions that I have made during the course of thinking about sound tracing. I have done little/no research, and these ideas are my own. KJEP had a model related below: > We have some volume with an arbitrary geometry (usually simple such > as a concert hall or something like that). Squares would work just > fine as primitives. Each primitive has definite reflection > properties in addition to some absorbtion filter which possibly > filters out some frequencies and attenuates the signal. One interesting form of sound reflector might be the totally diffuse reflector (Lambertian reflection). It seems that if this is the assumption, then the appropriate algorithm to use might be radiosity, as opposed to raytracing. Several problems immediately arise: 1. how to handle diffraction and interference? 2. how to handle "relativistic effects" (caused by the relatively slow speed of sound) The common solution to 1 in computer graphics is to ignore it. Is this satisfactory in the audio case? Under what circumstances or applications is 1 okay? Point 2 is not often considered in computer graphics, but in computerized sound generation, it seems critical to accurate formation of echo and reverbaration effects. To properly handle time delay in radiosity would seem to require a more difficult treatment, because the influx of "energy" at any given time from a given patch could depend on the outgoing energy at a number of previous times. This seems pretty difficult, any immediate ideas? > Now for the actual sound tracing we do the following: > > For each patch of the two half-spheres, we cast a ray > radially from the center, and calculate an intersection > point with the enclosing volume. From that point we > determine which patch of the emitter this corresponds to, > giving us the emitted power. We then pass the corresponding > time series through the filter appropriate to the given > primitives, calculate the reflected fraction, attenuate the > signal by the square of the distance, and eventually > determine the delay of the signal. > > When all patches have been traced, we sum up all the time > series and output the whole lot through some stereo device. One open question: how much directional information is captured by your ears? Since you can discern forward/backward sounds as well as left/right, it would seem that ordinary stereo headphones are incapable of reproducing sounds as complex as one would like. Can the ears be fooled in clever ways? The only thing I think this model lacks is secondary "rays" or echo/reverb effects. Depending on how important they are, radiosity algorithms may be more appropriate. Feel free to comment on any of this, it is an ongoing "thought experiment", and has made a couple of luncheon conversations quite interesting. Mark VandeWettering
kiy@pte.UUCP (Kevin Young) (01/05/89)
In article <3458@uoregon.uoregon.edu> markv@drizzle.UUCP (Mark VandeWettering) writes: >I would like to present some preliminary ideas about sound tracing, and >critique (hopefully profitably) the simple model presented by Kjartan >Pierre Emilsson Jardedlisfraedi... > : : : : : : : : >Mark VandeWettering This is a fascinating discussion. I have one thought that I haven't seen mentioned (or obviously missed :-). This discussion seems to assume that light and sound are identical in property. From what I remember from high school physics, light is made up of discrete particles. Sound is the propogation of wave energy through a medium. I know light particles may be thought of as both waves AND particles but it seems as though ray tracing is particle oriented. The difference to me (and I certainly would like to hear arguments) is that as a sound wave propogates, each molecule transfers a part of its energy to severeral other molecules. The original molecule never actually makes it to the destination, only its energy. This seems to constrast the ray approach in which the ray continues, uninhibited, until it is absorbed or leaves the frame. Comments? kiy -- >|< Kevin Young uunet!{edsews,umix}!rphroy!pte!kiy Precise Technology & Electronics, Inc. (The opinions expressed here are true, the flames have been changed to infect the innocuous.)
ksbooth@watcgl.waterloo.edu (Kelly Booth) (01/05/89)
In article <3458@uoregon.uoregon.edu> markv@drizzle.UUCP (Mark VandeWettering) writes: > > 1. how to handle diffraction and interference? > 2. how to handle "relativistic effects" (caused by > the relatively slow speed of sound) > > The common solution to 1 in computer graphics is to ignore it. Hans P. Moravec, "3D Graphics and Wave Theory" Computer Graphics 15:3 (August, 1981) pp. 289-296. (SIGGRAPH '81 Proceedings) [Trivia Question: Why does the index for the proceedings list this as starting on page 269?] Also, something akin to 2 has been tackled in some ray tracers where dispersion is taken into account (this is caused by the refractive index depending on the frequency, which is basically a differential speed of light).
lee@uhccux.uhcc.hawaii.edu (Greg Lee) (01/07/89)
From article <364@pte.UUCP>, by kiy@pte.UUCP (Kevin Young): " ... I know light " particles may be thought of as both waves AND particles but it seems as " though ray tracing is particle oriented.... Then we should consider sound to involve the propagation of sonons. I think this approach is exploited in the field of ultra-sonic imaging. I ran across an interesting article on an uncertainty principle of sound waves by Dennis _____ (name escapes me), the discoverer of holography. (The article was in a collection titled Speech Compression.) I share the suspicion that this discussion is walking where others have run before. Greg, lee@uhccux.uhcc.hawaii.edu