On Oct 8, 12:39=A0pm, Randy Yates <ya...@[EMAIL PROTECTED]
> wrote:
> c...@[EMAIL PROTECTED]
writes:
> > On Oct 7, 9:14=A0pm, Randy Yates <ya...@[EMAIL PROTECTED]
> wrote:
> >> glen herrmannsfeldt <g...@[EMAIL PROTECTED]
> writes:
> >> > Richard Owlett wrote:
>
> >> >> c...@[EMAIL PROTECTED]
in *ERRROOOORRRR* wrote:
>
> >> >>> Even in acoustics, we have 100 dB is 0.946 watts per square
meter.
>
> >> >> I would agree IF *and only* IF you stated dbV, dbm, or db.... with
> >> >> stated/implied load/impedance/****
>
> >> > I presume that is for a propagating wave in air. =A0The actual
power
> >> > that goes into some detector (or other medium) depends on the
> >> > impedance in that medium.
>
> >> But it's just like the electrical situation - if we assume the
> >> same impedance for the reference as for the measured signal,
> >> the two cancel and don't really matter - we can compute it from
> >> the amplitudes (or equivalently, sound intensities).
>
> >> I think Richard is right. I think what Clay meant was 100 dB SPL.
> >> --
> >> % =A0Randy Yates =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0% "Watching all
th=
e days go by... =A0 =A0
> >> %% Fuquay-Varina, NC =A0 =A0 =A0 =A0 =A0 =A0% =A0Who are you and who
a=
m I?"
> >> %%% 919-577-9882 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0% 'Mission (A World
Re=
cord)',
> >> %%%% <ya...@[EMAIL PROTECTED]
> =A0 =A0 =A0 =A0 =A0 % *A New World Record*,
ELOh=
ttp://www.digitalsignallabs.com
>
> Hi Clay,
>
> > Well when one talks about sound, db specifically refers to sound
> > pressure level -
>
> I agree with that ...
>
> > that is why one doesn't need the "m" or something else to designate
> > the reference power.
>
> Well, if you don't, then the reference power is unknown.
>
> That's why stating (even within the context of sound power) "100 dB is
> 0.946 watts per square meter" isn't really correct, just as stating "100
> dB is 39.2 watts into the load" would not really be correct. It isn't
> absolute power unless the reference power is explicit, and with a simple
> "dB", the reference power isn't explicit.
>
In the case of sound, which is a perceptual thing involving humans,
the reference is found emperically. The standard way is to fill a room
with people around the age of 21 years and play a 1000 Hz sound
starting from almost absoulute quiet until about 1/2 of the group can
detect the sound. Guys in the 30 to 40 year old range need 2 to 3
times as much acoustic energy as the younger set to just make
threshold of detection, and it gets worse from there as one ages.
As I alluded to earlier 100 dB SPL being 0.946 watts per square meter
is the resulting experimental value that corresponds to 0 dB being the
threshold of human detection. So if I say a sound has a level of
100dB, I'm not talking about a signal in a wire or such. I'm talking
about the strength of the acoustic vibrations in the atmosphere at
STP. The reason I bring this up is this is the case where we don't
need to use a reference since sound is perceptual and therefore the
reference is humans.
Many units have built in to them a customary method of measurement.
This is highlighted by the old riddle of what weights more: an ounce
of gold or and ounce of lead? Well we know the trick's answer is an
ounce of gold. Precious metals are measured in troy ounces which are
1/14th of a pound. Other things are in avoirdupois ounces which are
1/16th of a pound. So the reference here is linked to what we are
talking about. Sound levels imply SPL.
IHTH,
Clay
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