Re: [Phys-L] sound intensity problem

[Bill Nettles <bnettles@uu.edu>]

To get an idea of loudspeaker efficiency you can consult the spec sheets at any number of speaker manufacturer websites.  Look at Peavey, Tannoy, Klipsch, Bose, Electrovoice, etc. They all publish the specs and give you sound pressure levels at 1 kHz at 1 m on axis with a specified power input.  That would be a decent problem for your students.  

Thunder is definitely "lensed" because of temperature gradients.  Railway and highway noise is, too.  At one house I lived in, there was a railroad about 2 km away.  Some days you could barely hear the trains coming through.  Other days (and nights) it sounded like there were in the back yard because of the refraction of the sound based on which way the thermal gradient was stacked.

Acoustical scattering as well as absorption?

> -----Original Message-----
> From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of John Clement
> Sent: Friday, April 05, 2013 2:49 PM
> To: Phys-L@Phys-L.org
> Subject: 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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