clay@[EMAIL PROTECTED]
writes:
> [...]
Hi Clay,
Thanks for this, and sorry I took so long to respond.
> As promised I dug deeper into this acoustic conundrum. Basically you
> can use either an intensity (power per area) or a pressure as a
> reference. The gotcha is their link:
>
> (assuming air - change for other media as needed)
>
> Intensity = (pressure^2)/(density of air)(velocity of sound in air)
I would be a little more explicit and write
Intensity = (pressure^2)/( (density of air)(velocity of sound in air) ).
I agree - after fiddling for a half an hour, the units work out!
W/m^2 = kg/s^3
p = N / m^2
N = (kg)(m)/s^2
d = kg / m^3
v = m / s
Cool.
> This relation is temperature and pressure dependent!
Sure: density of air depends on pressure and temperature. I
think velocity does too, but I can't see it right off.
> According to Klein's book "Science of Measurement", intensity was
> often used as the reference early, but now more often than not
> pressure is being used for the reference.
But they're the same, assuming a standard atmosphere and room
tempearture, right?
> So for a reference pressure of 20 micropascals (newtons per square
> meter) we find the equivalent intensity to range from 0.917 up to
> 0.998 picowatts per square meter over a temperature range of -10C up
> to +40C. Some use an intensity of 1.0 picowatt per square meter.
>
> This means then 1 watt per square meter is essentially 120 dB SPL, so
> I errored when I gave 0.946 w/m^2 as 100 dB SPL - for that I apologize
> for the error in the exponent (20 dB offset). Incorrect data is worse
> than no data at all.
If I had a dollar for every error I've made on comp.dsp...
> For SPL measurements, we may write
>
> dB = 10*log(Intensity/Intensity_reference)
>
> from the intensity to pressure relation, we then find
>
> dB = 20*log(pressure/pressure_reference)
>
> where the common but not universal references being:
>
> pressure: 20 micropascals
>
> intensity: 1 picowatt/meter^2
>
> I hope this clarifies any issues I created.
You've shown that an acoustic reference level can be in either pressure
or intensity. I agree, but that wasn't the point of contention.
Very simply, what I heard you saying is that, within the context of
acoustics, we can interpret a "****d" dB value as an absolute power
level. For example, we could say this or that sound was "x dB".
I disagree.
Without somehow indicating the reference level (whether that level is
intensity or pressure is largely irrelevent to my point), e.g., by
stating it explicitly in a sentence or using some standardized notation
like "dB SPL", "dB" is ambiguous.
The only way I could see your assertion being valid is if the reference
level in acoustics is so commonly known that it is understood if the
context is acoustics. Thus I also provided examples of two different
reference levels that have been used in the context of "human" acoustics
to sup****t my position that such a universal reference level (for
acoustics, and even for human acoustics) doesn't exist.
Does that make sense, Clay? Did I mistake what you were saying?
--
% Randy Yates % "...the answer lies within your soul
%% Fuquay-Varina, NC % 'cause no one knows which side
%%% 919-577-9882 % the coin will fall."
%%%% <yates@[EMAIL PROTECTED]
> % 'Big Wheels', *Out of the Blue*, ELO
http://www.digitalsignallabs.com
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