Re: Power to speaker

["Bernard G. Cleyet & Nancy Ann Seese" <georgeann@REDSHIFT.COM>]

Since you have only a cheap o'scope and the impedance of a spk. is quite variable (freq. - external conditions).

I suggest you measure the V drop across a resistor (ten Ohms is convenient) and the voltage across the spk.  Since you can't do this simultaneously you'll have to drive it with a sig. generator.  One
big (but I don't think bad) assumption is that the V and I are in phase, definitely not the case.  Furthermore, the phase angle will change markedly with freq. (and ext. conditions).  However, if the
speaker is heavily damped (in a sealed box with absorbent material) I suspect far from it's resonance the angle will be close to zero.


Spkng. of resonance, when you sweep the frequency its resonant behavior will be very evident.  This alone is instructive and a good project its self. Try different baffles, gluing weight to the cone,
removing part of the annulus to reduce the restoring force, adding glue to the annulus to stiffen it, etc.

bc

Note an advanced exercise is to K-K transform the absorption to phase etc.

SSHS KPHOX wrote:

> 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.
>
> Ken Fox
> AP/IB Physics Teacher
> Smoky Hill High School
> Aurora, CO