chung wrote in message ervers.com...
Svante wrote:

Harmonic distorsion is expressed as the ratio between the distorsion
components and the fundamental. What surprises me is that it is the
VOLTAGES that are compared (in the electrical case) not the POWERS. So
if we have a second harmonic 40 dB down, the second harmonic
distorsion is 1 %, not 0.01 %.
(In this case the voltage of the harmonic is 1% of the fundamental,
and its power is 0.01% of the fundamental)

What is the reason for this convention? I'd think that power would be
more logical.

Several reasons:

1. 40 dB down is 40 dB down, whether you're talking about voltage or
power, assuming constant load impedance. If the 2nd harmonic is 40 dB
down, it means the voltage ratio is 1%, and the ratio of delivered power
is 0.01%. A dB in voltage is a dB in power!

Yes, I am aware of this, and perhaps that is why I like expressing
distorsion in dB rather than as a percentage. "A dB is a dB". Or is
it? Hmm, se below!

2. Audio amplifiers are voltage devices. The actual power delivered to
the load depends on the load impedance. For example, let's say an
amplifer has 1% 2nd harmonic distortion in voltage. How much power is
delivered to the load at that 2nd harmonic frequency? The answer depends
on the load impedance at that frequency. It is not unusual for a
speaker's impedance to change substantially over an octave. So in this
case, the power ratio may not be 0.01%.

That is the best explanation so far, and the only one that doesn't
rely on a convention, but rather a physical fact (a frequency
dependent load resistance). However, this would actually speak against
using dB as a measure of distorsion, since dB is fundamentally
intended to measure a POWER ratio.

3. The measuring equipment measures ratios of voltages. It does not
measure power delivered to the load.

So... A spectral display based on voltage measurement should not
really be allowed to display "dB" on the y axis, unless we know that
we have a constant, resistive load? I mean, the fundaments of dB
assumes that we measure a power ratio. The equation for "voltage dBs"
(20*log(u/uref)) is a derivation based on that p~u^2 neglecting the
effects of varying load resistance. It simply assumes that the load
resistenace is constant.

Hmmm... Interesting.