Quiet-List 1997

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Re: Measuring noise



David,

> I think we should strive to make distance a routinely expected part of 
> any decibel-based statement, just as we routinely expect velocity to be 
> stated in terms of time and distance.  No one should accept one without 
> the other.

There are different ways of expressing the strength of sound. Note I used 
the rather ambiguous term "strength" in order to convey the intuitive idea
without sticking to any specific physical descriptor. The differences
arise as regards to the applications.

When you refer to the source, that is, to noise *emision*, you may use the
sound power level, SWL (and not SPL, to avoid confusion with the sound
pressure level--W as in Watt), which remains the same regardless of
distance (except if you take into account the attenuation by the air
itself, which is significant only several tens of meters away (a hundred
feet or more). SWL is measured in decibels referred to 1 picowatt (1
picowatt = one millionth of a millionth of 1 Watt). Alternatively, you may
use the sound pressure level (SPL), which depends strongly on distance,
but not only on distance but also on direction, so it wouldn't be
complete enough to specify only the SPL with mention of the distance. You
should specify the angle with respect to the principal axis as well.
Finally, you could also use the Sound Level (SL), which includes the
frequency and time weighting, that is, A, C,... and fast, slow, ...
I don't need to mention that you should specify also the distance at which
the SL is measured and the angle with respect to the axis.

When you refer to the receiver (usually a human being) you are talking
about *immision* noise, and in this case the distance to the source is
irrelevant. I don't mean that it wouldn't affect the measure, but that
the measurement should be made with the source and the receiver at their 
normal positions. In this case you use, depending on the application, SPL 
or SL, in this case with an appropriate weighting.

When you say that normal speech is about 55 dBA - 65 dBA, you are
implicitly taking into account that the source and the receiver are at
normal-speech-distance, that is, from about 50 cm to 2 m apart.

When you say that noise at the street is about 70 dBA - 80 dBA, it's an
implicit assumption that you mean that the source is traffic (actually,
the vehicles) and is "located" on the street, as usual, and the source is
a person standing at the sidewalk.
 
>    Part of why I originally questioned Stephen's figures is that I like
> to think of whispering and normal conversation as being definitely on 
> the low end of any noise scale. When a whisper is rated at 30 dB, 
> what is left to rate at 20 dB and 10 dB??

This doesn't seem necessary nor useful. Acoustically there are relevant
sounds much below the whispering level, including many audible ones.

> I thought the whole point of such scales was to compensate for the way 
> in which humans perceive wave-like phenomena and therefore allow a greater 
> sense of equivalence between equal units.

While originally it was that the intention, it has been demonstrated that 
it is not the case.

Best regards and wishes. Merry Christmas!

Federico Miyara

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