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Re: Power to speaker



I have a student wanting to measure the power input to a speaker. We have
an oscilloscope ( old, single trace, but could get a better one). We can
see the input voltage and the speaker is rated to be 4 -6 Ohms. I regret
that I cannot think of how to measure the power input from what we have.

Any guidance would be appreciated.

Get a dual trace oscilloscope and display input voltage and input
current (use a one ohm resistor in series with the speaker feed).
Be careful to use a common ground and to trigger both traces from
the same source. You should now be able to measure both amplitudes
Vo and Io, and the relative phase, A, of the sinusoids. The input
power, P, is given by

P = 1/2 Io Vo cos A

If one assumes that the speaker is a resistive load R then Vo
alone can be used to calculate P':
2
Vo
P' = -----
2 R

I'm betting that P and P' will be different.

Leigh