Quiet-List 1997

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Re: Measuring noise



On Mon, 15 Dec 1997 09:37:29 +0100, 
PENTIUM 133  <aspa.strasbourg@wanadoo.fr> wrote:

>David Staudacher wrote:
>
>>It would be convenient to have a unit of measure in which distance 
>>was already factored in, like the "foot-candle"... unit for measuring
>>light intensity.
>
>Sound is a variation of pressure. Thus it is defined (amplitude) as a force
>on a surface like the velocity is defined in terms of distance and time... 
>About having a unit of measure in which distance is already factored in, I
>don't see what you mean...

   As Peter pointed out, trying to say how loud something is using dB 
values alone is meaningless unless you know the distance at which the 
measurement was taken.  90dB at 1 meter is quite different from 90dB 
at 10 meters.  Thus it is not unlike trying to make a statement about 
velocity in terms of time or distance alone, which is obviously absurd.
To overcome this, it would be convenient if there were some unit for 
sound in which distance was already factored in.  The "footcandle" (U.S.)
[or"lux" (metric)] measure of illumination is such a unit.  Being based 
on a standard distance, 90 footcandles is still 90 footcandles, whether 
the distance is 1, 10, or 100 meters. So I propose something like a 
"foot-bel" [or "meter-bel" (metric)] unit for sound intensity - also 
based on a standard distance.  Not that it would actually be necessary 
to _make_ the measurement at that distance.  Since intensity is 
inversely proportional to the square of the distance, simply knowing 
the distance to the source would do.  Stating the quantity in terms of 
the standard distance would be a matter of calculation.  

David Staudacher - quiet@igc.org

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