RZ,nI: Claude Shannon

From: Jerome Fine <jhfine_at_idirect.com>
Date: Wed Feb 28 12:41:25 2001

>r. 'bear' stricklin wrote:

> >On Wed, 28 Feb 2001, Gabaly, Stephen J wrote:
> > Take a 1 Vpp_at_40Hz Sinewave, highest-frequency spectral content is
> > 40Hz, sample it at twice this frequency, 80Hz, sample at 0 and 180
> > degrees (0Vpp Amplitude), the reconstructed sinewave will be 0Vpp at 0
> > Hz. Oh well, guess Shannon's theorem is incorrect... steve
> yeah, but who would be daft enough to sample a sine wave? sheesh.

Jerome Fine replies:

Actually, that is exactly what I helped an acoustical engineer do about
10 years ago (so this reply is on-topic). The goal was the analyse the
sound reduction properties of a box around a large compressor station
(pumping oil or natural gas). The requirement was to reduce the noise
level from the station to below the ambient outside noise level when the
measurement was taken over 100 yards away.

Since a sound meter to measure a noise level normally can't measure the
intensity of a sound which is below the background white noise, there
seem to be two solutions:

(a) Brute Force - bring in a train load of speakers and amplifiers to generate
a sound loud enough to be heard above the ambient noise level at 100 yards
from the station - a bit inconvenient since even if it could be done, they would
not fit inside the station.

(b) Intelligent Measurement with a computer using both A/D and D/A hardware
along with that (sheesh) sampling of the required sine waves.

If anyone wants further detail and needs to have this done for an actual application,
I can refer you to the proper source. The key point is that the intelligent use of
computer hardware and software was able to provide a solution that could
actually prove that the acoustical shell (noise suppressing material) around the
station reduced the noise produced by the compressor to a value which could
not be heard by the normal human ear above the background noise when
the person was over 100 yards away. And the only speaker that was used
did not generate a sound louder than the noise level already inside the station.

And that, my dear Virginia, is why sampling of sine waves is sometimes needed.

Sincerely yours,

Jerome Fine
Received on Wed Feb 28 2001 - 12:41:25 GMT

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