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# Re: [Phys-L] sound intensity problem

It would seem that 1750 miles is unreasonable. If a thunderstorm is that
far away you won't hear it. Indeed if it is only a few miles away you only
hear the boom and not the initial crackle. The sound from a thunderstorm is
much more powerful than a megaphone and it is in the air where objects won't
block the sound. So the attenuation of high frequencies is substantial. You
can get an order of magnitude change in sound level by just the
inefficiencies in the megaphone.

I really doubt the lensing is much of a factor, but backgound noise can be a
factor, especially in a stadium. I would believe a mile of so, but not that
large a figure.

John M. Clement
Houston, TX

The answers from the two Johns are useful, but the bottom
line is that in theory the distance is NOT unreasonable. A
way to think about it is to imagine outer space filled with
wind free air (OK--no longer outer
space....but) and now do the experiment. The key here is the
large ratio between the initial sound intensity and the
intensity at the
threshold of hearing. Even at inverse square fall-offs, it still
requires a very large spherical surface to reduce the
intensity by 10-12 orders of magnitude.

R.W.Tarara

On 4/5/2013 2:00 PM, Anthony Lapinski wrote:
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?

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--
Richard Tarara
Professor of Physics
Saint Mary's College

free Physics instructional software
www.saintmarys.edu/~rtarara/software.html

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