John Denker wrote > > >>> Just to clarify on the reference-dist, is it that this value is a > >>> diminishing effect, that is for reference-dist of 1 after distance 1 > >>> the volume is half original, after distance 2 the volume is 1/4 > >>> original (half of a half), distance 3 it's an 1/8th (half of a > >>> quarter). > >> yes, exactly. > >> > > not exactly, it's 1/8th at distance 4 (doubled distance result in half > > volume). > > Sorry, no, it's not any of those. In the present situation, the > levels go like this: > > r power power > / r0 / (w/m^2) / dB > 0.5 4.00000 +6.0 > 0.7 2.04082 +3.1 > 1 1.00000 0.0 <<< reference > 1.5 0.44444 -3.5 > 2 0.25000 -6.0 > 3 0.11111 -9.5 > 5 0.04000 -14.0 > 7 0.02041 -16.9 > 10 0.01000 -20.0 > 15 0.00444 -23.5 > 20 0.00250 -26.0 > 30 0.00111 -29.5 > 50 0.00040 -34.0 > 70 0.00020 -36.9 > 100 0.00010 -40.0 > > We see that at the reference distance (r0), the signal is not > attenuated at all. That's the defining property of the reference > point. At twice that distance, the signal is down by a factor of 4. > At three times the distance, the signal is down by a factor of 9. > It is the famous "one over r squared" law. It is a corollary of > conservation of energy. > > *) At larger distances sound energy is not (by itself) conserved, i.e. > dissipation becomes dominant, and we see a crossover to exponential > attenuation, but ... > > *) At the distances we see in flyby view, dissipation is negligible. > The 1/r^2 attenuation is the whole story. > > If you know the sound level at any one distance, you can calculate > it at any other distance. > > On 01/21/2009 05:46 AM, James Sleeman wrote: > > >> ... if we switch to tower view, it seems you can always hear the > >> aircraft no matter how far away you get, for example, I was 100 miles > >> from the tower and yet I had no trouble hearing the aircraft at all. > > That's a bug. The tower cab has lots of sound insulation, so the > tower guys are not going to hear the aircraft at all unless it is > very close. If it's not close, 1/r^2 attenuation predicts that > the sound level will be inaudibly low. And dissipation makes it > even lower. > > On 01/21/2009 05:14 PM, James Sleeman wrote: > > > It seems that a great many aircraft do not define > > these values at all. Is there a default definition for these somewhere, > > is one calculated by openal maybe in the absence of these specific > settings? > > IMHO it would be a step in the wrong direction to ask aircraft designers > to specify the reference distance. There's already a length-scale > built into the flyby view, namely the expected distance of closest > approach. > > There needs to be some headroom in the sound level, because the aircraft > might maneuver so as to come closer than expected. > > > > On 01/22/2009 03:17 AM, Vivian Meazza wrote: > > >> I would think that the attenuation of sound in air is amenable to > >> mathematical calculation. > > It is. In the near field it goes like 1/r^2. In the far field it > is exponential; see FAR A36.7 if you want the lurid details, or > see http://www.sfu.ca/sonic-studio/handbook/Sound_Propagation.html > if you want something more explanatory. > > >> Surely we shouldn't be guessing at some arbitrary > >> "reference distance"? > > There should be no guessing involved ... but there does need to be > a reference of some kind. There needs to be something to set the > scale. This is the premise of the statement above: > _if you know the sound level at any one distance_ > you can calculate it at any other point. > > > On 01/22/2009 06:05 AM, Melchior FRANZ wrote: > > > But it depends on the frequency pattern, no? So we'd need to > > analyze the spectrum ... time to use libfftw3. > > No, the 1/r^2 attenuation is independent of frequency. No FFT > required. > > The long-range exponential dissipation would be another story, but > we don't need to go there, not for the applications presently > contemplated. >
Looks good to me. Thanks for the explanation. I suppose we don't allow for humidity and pressure? I get the impression that sound in the flyby view doesn't vary with height? I don't think aircraft designers should be asked to specify the reference distance either, but the ability to do so should remain - for in-cockpit sounds for example where the attenuation isn't standard. I'd even settle for a simple <internal> tag so that such sounds were not audible in external views. Actually, I thought we had one. Guess I must have dreamt that one up! Vivian ------------------------------------------------------------------------------ This SF.net email is sponsored by: SourcForge Community SourceForge wants to tell your story. http://p.sf.net/sfu/sf-spreadtheword _______________________________________________ Flightgear-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/flightgear-devel
