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Grex Science Item 82: Acoustics
Entered by russ on Fri Oct 25 02:13:31 UTC 2002:

Acoustics looks like it rates a discussion of its own.
Ergo, here it is.  I'll start:

I think tsty is right when he says that the radiation pattern
of speakers is responsible for at least part of the perceived
loudness of mid-range frequencies over low frequencies.  The
wavelength of very high frequencies is so small relative to
speaker drivers that the "cones" of tweeters are often spherical
to get an even distribution.  Woofers, OTOH, are almost never
large enough to be considered more than a "point source" in
the far field; they are inherently omnidirectional.  Thus a
listener will hear a much louder sound from a watt of mid-range
frequencies than from a watt of bass, even if they did not have
hearing biased toward the former.  The situation would change
if they were standing behind the speaker rather than in front.

8 responses total.

#1 of 8 by tsty on Sat Oct 26 23:44:56 2002:

russ is precisely correct here. 
  
when teh measurements are done with instruments rather than ears
it has been shown that lower frquencies, actually, carry better
than high frequencies
  
air is not a high pass filter.
  
there are conversin efficiencies (inefficiencies) also that disallow
against electrical watts as teh source power.
  
when those are dialed into the situation, low frequency radiators
are muchmore efficient than hi frequency radiators. the misperception
is due to directionality coefficients and ear insensitivity.

#2 of 8 by rcurl on Sun Oct 27 05:52:26 2002:

What does "carry" mean? Quantitatively.

#3 of 8 by tsty on Sun Oct 27 10:20:55 2002:

good question - colloquialism for distance traveled while measureable.
  
that's teh best i can think of at the moment. does that work for you?

#4 of 8 by rcurl on Sun Oct 27 18:34:03 2002:

All sound frequencies travel at the same velocity and spread similarly
from a point or planar source. However the attenuation of sound waves by
absorption and viscosity does vary approximately by the 3/2 power of
frequency (Cremer's equ.). For 1000 Hz it is about 0.0056 db/m at 30% RH.
This isn't much in small spaces (though it does affect reverberation).

There are lots of other factors, which have been mentioned: speaker
efficiency, sound pattern (focusing), aural perception, etc. These
together would have more to do with the spectral perception of "loudness" 
than would attenuation (in small spaces).

#5 of 8 by tsty on Tue Oct 29 16:02:27 2002:

so at 1000 meters, spl loss by 30% humidity is 5.6 db. 

interesting, and measurable (calculable anyway) but not too mych
effect on reverberation or other more efficient losses.

#6 of 8 by rcurl on Tue Oct 29 19:05:07 2002:

Reverberation (or "echoing") has a lot to do with the sound in a concert
hall, and is important for how fast a sound "dies away". A few db *per
echo* is significant as reverberation involves many echoes.  Of course
furniture - and people - are also important factors.

#7 of 8 by tsty on Fri Nov 1 09:11:14 2002:

the specific measure is    rt60   expressed in seconds adn is the 
amount of time that any single sound drops by 60 dB fro the original level.
  
absorbtion, btw, is expressed in     sabins  and is the equivlant of
the maount of sound 'lost' due to a 1 square foot hole in a wall of
a room wth perfect reflection.
  
a human expresses about 4.5 sabins of absorbtion. so does a padded theatre
seat & seat back <g> .... and pew cushions, too.

#8 of 8 by tsty on Fri Nov 8 22:51:59 2002:

an interesting conflict in acoustic spaces is that speech and music
require conflicting reverb times.
  
iirc, somwhere between .2->.8 rt60 is best for speech and  for 
music, rt60s of 1.0->1.7 seem to be best.
  
speech, however , in an rt6 1.0 is smeared and inarticulate.
  
the great cathedrals of europe run rt60s in between 2.0->3+.

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