From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: wattage meter exists??
Date: Mon, 09 Sep 2002 00:55:51 GMT
NNTP-Posting-Date: Sun, 08 Sep 2002 17:55:51 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
John Woodgate wrote:
> I read in sci.electronics.design that Robert Baer
> wrote (in <3D7AF3C1.B3A42BAD@earthlink.net>)
> about 'wattage meter exists??', on Sun, 8 Sep 2002:
> > Assume 6V supply and the speaker drivers go rail to rail (a number of
> >modern analog ICs now do this).
> > Also assume a full H-bridge driver so the speaker can be driven from
> >+6V to -6V (a number of modern analog ICs now do this also).
> > That is 12V peak to peak. Now P=(V*V)/R, so assuming 60 watts, R=2.4
> >ohms (unlikely as the lowest Z i have seen is 4 ohms).
> No way. Power is not peak-to-peak voltage squared divided by resistance.
> > Furthermore, power level is nominally specified as RMS, which is 0.707
> >of peak if sine wave, which is the other standard for specification.
> >This gives P=(0.707*V*0.707*V/R or R=4.8 ohms, which is more like a
> >nominal rating.
> Now you have V = 24 V, which seems a mite improbable, no?
> 12 V peak-to-peak is 4.24 V r.m.s. The 60 W is fictitious; the actual
> amplifier power is either 4.24^2/8 = 2.25 W or 4/2/4^2/4 = 5.5 W,
> depending on whether the loudspeaker is nominally 8 ohms or 4 ohms. But
> that applies only if indeed the amplifier is H-bridge (or BTL - Bridge-
> Tied Load, as it's called in audio circles). It probably isn't: in which
> case the powers are 1.125 W and 2.25 W.
> > You sure as heck will *not* get more power (100W) with less voltage (2
> >D cells = 3V supply).
> I think we can agree on that.
> Actually, all this 'power' business is thoroughly misleading; a
> loudspeaker is a voltage-operated device, not a power-operated device,
> in the sense that you get the frequency response you want (or are
> willing to accept) with constant voltage applied, not with constant
> power absorbed. The absorbed power mostly just heats up the voice-coil.
> We should rate amplifiers in terms of voltage and minimum load impedance
> and loudspeakers in the same terms. It would be far less confusing and
> far less open to abuse.
> Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
> Interested in professional sound reinforcement and distribution? Then go to
> PLEASE do NOT copy news posts to me by E-MAIL!
Actually, speakers are *current* operates devices: the current thru
the coil creates a magnetic field which interacts with the permenant
magnet's field, the resulting force moves the voice coil (and thus the
Note the measured impedance of a speaker (in a given environment) not
only varies greatly as a function of frequency, but is a complex vector
(real and imaginary components).