Interesting. I would have interpreted those figures somewhat
differently, as noted below...
> kilopascal wrote:
>
> 2003-02-09
>
> What a weird mess!
>
> I too was confused by the awkward use of decibels until I went to
> Rowlett's page and found that there are many different decibels. I
> have highlighted the definition of the two decibel units seen on the
> package in red.
>
> I would interpret the "106 dB spl/mW +/-4 dB" to mean that for each
> milliwatt of power applied to the speaker, there is 4 Pa (2.5 - 6.3
> Pa) of pressure applied to the ear drum or produced by the speaker
> diaphragm. I found this by setting 106 = 100 + 20�log10(P/2) and
> solving for P. The decibels are positive because the pressure is
> greater then the 20 ?Pa reference.
Here, John, I would have taken the +/-4 dB as applying to the first
figure, thus in a slightly different format:
SPL(re 20 �Pa) = (106 4) dB at 1 mW input power.
It cannot be SPL/mW = [etc.] since doubling input power would raise the
SPL (sound pressure level) by 6 dB; it would *not* double it to 212 dB
of course!
Likewise, I would interpret the next figure for microphone sensitivity
as being
L_V(re 1 V) = (-39 � 5) dB at 1 Pa SPL.
Of course, that would be better written as
L_V(re 1 V) = (-19.5 �- 2.5) dNp at 1 Pa SPL
since voltage comparisons call for the use of napiers while power
comparisons call for the use of bels. (Read L_V as "L sub V".) There is
some information on this in IEEE std 260.1 which is about to be put to
ballot.
The above comments may not be the most elegant way to put things, but I
think that they are closer to what was meant. But, of course, I'm
guessing, so I could be wrong about that.
Jim
--
Metric Methods(SM) "Don't be late to metricate!"
James R. Frysinger, LCAMS http://www.metricmethods.com/
10 Captiva Row e-mail: [EMAIL PROTECTED]
Charleston, SC 29407 phone: 843.225.6789
