Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-L] sound intensity problem



There are a number of considerations here that have not been taken into
account:
1. A 100W amplifier does not put out 100W of sound. It depends on the
loudspeaker efficiency. Louspeakers can have very low efficiency.
2. A megaphone directs the sound and does not radiate uniformly into 4 pi.
So if you are off axis the sound level will be much lower. Indeed few
speakers of any kind radiate into 4pi. There was a Bose speaker which
simulated radiation into 4pi by being in a 3 wall corner. Of course it
really only radiated into 1/2 pi, using the walls as baffles. But the
audible effect was like a pulsating sphere in free space.
3. When talking into the megaphone the maximum power output may not be
achieved. You don't want to drive it into total clipping as that would
reduce intelligability.
4. As I recall high frequency sounds are absorbed so that they attenuate
with distance. Low frequencies do not attenuate as much. There are some
web sites that show calculations and graphs.
5. The zero dB level is just a nominal level. Most people have a
considerably higher threshold for hearing sound. With loud music and road
noise this problem is getting worse.

You need to look at the manufacturors rated sound level output at a
specified distance to do the initial simple calculation rather than the 100W
figure. Then you should look at the attenuation in air. Finally get your
threshold level checked.

John M. Clement
Houston, TX


I was considering this sound problem.. For a 100-W megaphone,
how far would you have to be so that it is barely audible (0 dB)?

I = P/A

Io = P/4pir2

With Io = 10-12 W/m2, the result is 2821 km = 1750 mi

This seems unreasonably far! I would think that the sound
level would drop off much closer, as we typically experience.
Or am I missing/miscalculating something?

_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l