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Re: [Phys-L] sound intensity problem



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

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