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Distorsion percentage, power or voltage?
On 17 Jan 2004 02:02:57 -0800, (Svante)
wrote:
However, this would actually speak against
using dB as a measure of distorsion, since dB is fundamentally
intended to measure a POWER ratio.
The dB was originally a measure of sound pressure level, and the
logarithmic scale is used simply becuause our ears respond to sound in
a logarithmic fashion.
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?
Not at all, since a voltage ratio of 2:1 is approximately 6dB,
regardless of current flow. This is why voltage, not power, is used
as a standard measure of speaker sensitivity, since it is independent
of the load impedance.
I mean, the fundaments of dB
assumes that we measure a power ratio.
No, it doesn't. It is simply a useful logarithmic 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.
It assumes no such thing.
--
Stewart Pinkerton | Music is Art - Audio is Engineering
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