Driving a speaker ???

[KM4FER]

A simple beginner's question.

I have a speaker from an old transistor radio.  It is marked 8ohm, 0.3W.

Using power formulas and Ohm's law I calculate that to get 0.3W from 8ohms I need about 1.5V and 193mA.
OK, no problem.......so far.......

But would that be Peak voltage/current or RMS?
My thinking is that the heating effects of 0.3W would be averaged over many cycles and therefore RMS is the correct answer.
But then there is this niggling thought that with 0.3W RMS the peaks are going to exceed that and since there is probably very fine wire in the speaker which could heat up and melt quickly I should be thinking in terms of the peaks of the cycle.

Or, I guess I could just use the engineering practice of derating for safety's sake and limit input to about 0.15W?

Any guidance?

Thanks...
earl...

[Zero999]

Speakers are normally specified at the RMS power which is half the peak power for a sinewave.

You need a peak voltage of 2.2V to get 0.3WRMS out.

Unless the amplifier is bridged the supply voltage needs to be double that, before losses are taken into account so that's 4.4V but in the real world it needs to be higher than that.

[IanB]

If you power your amplifier circuit from a small 9 V battery it is unlikely to have enough power to damage the speaker. This is possibly the kind of battery the old transistor radio originally used.

[KM4FER]

RMS seemed the most likely to me so I'll go with that.

This is purely a learning experience for me so now I'll try to design an amplifier with 0.3W RMS output as one of the criteria.

Thanks again.

[MacAttak]

Speakers are normally specified at the RMS power which is half the peak power for a sinewave.

You need a peak voltage of 2.2V to get 0.3WRMS out.

Unless the amplifier is bridged the supply voltage needs to be double that, before losses are taken into account so that's 4.4V but in the real world it needs to be higher than that.

Is that correct? I thought RMS power for a clean sine wave was square root of two over two? (roughly 0.707)

[lutkeveld]

P=U^2/R 
Vrms=Vpeak/sqrt(2)

sqrt(2) squared is 2. So Vrms is indeed half Vrms

[lutkeveld]

The ratings of speakers are often thermal ratings. With average music power being at least 10-12db below peak, the excursion limit is more a problem. I would use a PAM8403 or similar powered by a single li-ion cell.

[Zero999]

Speakers are normally specified at the RMS power which is half the peak power for a sinewave.

You need a peak voltage of 2.2V to get 0.3WRMS out.

Unless the amplifier is bridged the supply voltage needs to be double that, before losses are taken into account so that's 4.4V but in the real world it needs to be higher than that.

Is that correct? I thought RMS power for a clean sine wave was square root of two over two? (roughly 0.707)

That's the voltage.

RMS power is half the peak power for a sinewave.