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Re: Measuring noise
STEPHEN O. FRAZIER <SFNABQ@compuserve.com> wrote:
>The pamphlet "Noise, Ears and Hearing Protection" dated 8/88 from the
>American Academy of Otolaryngolgy lists a whisper at 30 dB, normal
>conversation at 60, a lawnmower at 90.
And Peter Donnelly <firstname.lastname@example.org> wrote:
>...figures are rather meaningless anyway unless you know the distance at
>which the measurement is taken.
Good point! I think most everyone understands that sound levels
diminish with distance, yet we seldom find distance included in
statements about sound levels - even authoritative references like my
Britannica article and Stephen's AAO pamphlet. 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.
It would be convenient to have a unit of measure in which distance
was already factored in, like the "foot-candle" (or lumen) unit for
measuring light intensity. I think there may already be such a unit.
I came across something called "Sound Power Level" (SPL) which I think
attempts to do this, but I don't know much about it. Keeping in mind
that the "decibel" scale we now use was originally conceived of as the
"bel" scale (by Alexander Graham Bell) for measuring sound in units
from 1 thru 10, I would propose something like the "foot-bel" as a
standard unit. On such a scale, everything would be stated in terms
of a standard distance.
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?? If doubling this level to
60 dB brings us to the level of normal conversation, it just doesn't
seem reasonable that another 30 dB increase should bring us to the
level of a power mower. I know that logarithmic scales can be
misleading as, for example, with the Richter scale where a difference
of 1 Richter in the high end can be tremendous, but 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.
On the Right to Quiet web page ("www.islandnet.com/~skookum/quiet"),
there is an excellent article called "The Physics, Physiology and
Psycology of Noise", taken from a talk given by Jeremy Tatum of the
University of Victoria to a meeting of the Right to Quiet Society
which makes some very good points regarding problems with using the
decibel scale as a measure of noise. He states, "Sixty decibels is
sometimes described as being about the level of conversation. Well,
we all know people who do talk that loud. I would describe 60 dB as
more like shouting; 55 dB is more like civilized discourse... Let me
put it this way, however. If there was a noise in this room at 58 dB,
you would not be able to hear what I am saying. [So] whenever you
are asked to say what is meant by a noise level of 58 dB, you should
not say that it is about the level of conversation; rather, it is the
level of noise that *seriously interferes* with normal conversation."
(This whole article is really quite excellent and I highly recommend
reading the complete text. It's only a few pages long.)
David Staudacher - email@example.com
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