Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!mailrus!cornell!uw-beaver!uoregon!markv
From: markv@uoregon.uoregon.edu (Mark VandeWettering)
Newsgroups: comp.graphics
Subject: More Sound Tracing
Message-ID: <3458@uoregon.uoregon.edu>
Date: 4 Jan 89 08:23:06 GMT
Reply-To: markv@drizzle.UUCP (Mark VandeWettering)
Organization: University of Oregon, Computer Science, Eugene OR
Lines: 87

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