physics
posted by rick .
The average power dissipated in a stereo speaker is 54 W. Assuming that the speaker can be treated as a 4.0 resistance, find the peak value of the ac voltage applied to the speaker.

Is 4.0 supposed to represent 4.0 ohms?
If so,
Average power = Vrms^2/R = 54 W
= 1/2 * peak power
Vrms^2 = 54*4 = 216 volt^2
Vrms = 14.7 volt
Vrms is called the "root mean square" voltage, the usual value cited for alternating currents.
The peak voltage is sqrt2 times higher than Vrms, or 20.8 V 
still don't understand where the 20.8V comes into it how u get that?

I multipl1ed 14.7 (the root mean squere voltage, Vrms) by the square root of 2. That results in the peak voltage, for a sine wave variation. A singlepitch sound is a sine wave, and that is what a speaker would receive.