>FROM A PREVIOUS EPISODE OF "AS THE DB TURNS"
But in the end it doesn't matter, does it? A dB is a dB. An increase in
SPL
of 6 dB is the same as an increase in sound power of 6 dB. Just like a
6dB
change in voltage results in a 6dB change in power. Wonderful things,
those
dB's..
Careful...It does matter. They aren't as wonderful as you think!!!
Take an analogy from electronics
A 6dB increase in voltage will not necessarily result in a 6dB increase in
power. You would have to make a bunch of assumptions, such as a constant
resistive load.. What if the load was non-linear? Power is the product of
voltage and current. If you have a 6dB voltage increase with no current
increase, then you only have a 3dB power increase.
Going back to sound
An increase in sound pressure level may not necessarily translate to the same
increase in noise power. This problem isn't so bad because most sound
measurements are made assuming that air is the "resistance" or transmissions
medium. However; the reference level is different. If we assume air as the
transmission medium, a sound intensity of 0 dB (referenced to 1pW/m^^2) is not
produced by a sound pressure level of 0 dB (referenced to 1 x10^^-5 lb/in^^2).
The person who originally started this thread wanted to estimate the sound
increase from adding multiple cards to a shelf. If those cards had been
characterized to have x dBpW/m^^2 of noise; then adding 6dB for each factor of
two would result in an extremely high estimate. On the other hand; if those
cards had been characterized to have x dBlb/in^^2; then adding 3dB for each
factor of two would result in an extremely low estimate.
Even worse, if the standard assumes dB pW/in^^2; and he was working in dBa or
dB lb/in^2; his answer would be wrong before he even put pen to paper.
I know that I'm beating a dead horse; but how else would you know how it died
:-) I'm not participating in this thread to prove anybody right or wrong; I
just want to make sure that one of our colleagues doesn't add apples to
oranges and end up with fruit salad.
Chris Maxwell | Design Engineer - Optical Division
email [email protected] | dir +1 315 266 5128 | fax +1 315 797 8024
NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA
web www.nettest.com | tel +1 315 797 4449 |
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