I wouldn't necessarily say that "decibels for acoustics are always positive
numbers." Whenever the bel or decibel scale is utilized, whatever is being
measured is compared to some reference level. If the quantity being
measured is larger than the reference level, then the value for the
measurement will be a positive dB value. If the thing being measured is
smaller than the reference level, then the value for the measurement will be
a negative dB value.
For "sound pressure level" the reference is 10^(-12) W/m^2 which is
supposedly the threshold of hearing. But this varies from person to person,
and also changes with frequency. There can certainly be sound waves for
which the SPL is less than the 0 dB reference level. In that case the SPL
would be expressed as a negative dB value. Perhaps no one could hear it,
but it would exist.
I didn't carefully read the full Wiki entry that was referenced by others,
but my first impression of it would be that it is okay.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270 (office)
edmiston@bluffton.edu
-----Original Message-----
From: Kyle Forinash
Sent: Sunday, August 28, 2011 3:59 PM
To: phys-l@carnot.physics.buffalo.edu
Subject: [Phys-l] decibels
Hi;
Decibels for acoustic applications are always positive numbers,
basically because the log of the ratio is intensity (W/m^2) to a very
small reference intensity (10^-12 W/m^2) and ambient sound is always
greater than this reference so the ln is positive.
I'm a little confused about what ratio of reference power or voltage is
used in electronics/engineering (where the dB scale is usually a set of
negative numbers). In particular, electrical engineers seem to come up
with this negative scale even when the circuit is measuring input from a
microphone. Can anyone point me to a reference that makes the connection
between the acoustic scale and the electrical engineering scale?
kyle
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