# Measuring RMS power output from audio amplifier channel

I am looking for a simple way for getting the RMS power output of an amplifier channel set to some arbitrary gain. I want this in order to find the maximum level I can safely set the amplifier at for a speaker with a given continuous power rating.

Edit to add more context: I often don't get to choose the gear I work with, so there are often mismatches between amplifier power and speaker maximum power handling. I want to find the maximum I can set the amplifier gain at without blowing speakers.

The volume control will be at signal source. I want to know that even if the signal gain is set to 0dB, the amplifier's volume will ensure the speakers will not be blown. If it helps, think of it as a theoretical exercise.

I came across this youtube video that suggests that by measuring the resistance of the disconnected speaker, and AC voltage across speaker terminals (while outputting to speaker), you can calculate RMS power from the equation $$\𝑃=\frac{𝑉^2}{𝑅}\$$, but all the other articles and videos I found suggested using a clamp meter to measure current, plus a regular multimeter to measure V, and calculate P from: $$\P=IV\$$

My question: Are both methods equally reliable? Are there practical considerations besides the fact that method 1 requires disconnecting the speaker and method 2 requires an extra measurement device?

• Are you sure there is a problem? What make/model speaker you have, and what is it's power rating, sensitivity and impedance? What is the power rating of the amplifier? The speaker is not a resistance, so current and voltage do not go in phase, so power is not voltage*current except at very low frequencies. Clamp meter may not also have the 20kHz bandwidth for audio measurements. – Justme Nov 3 '20 at 18:41
• This isn't for a specific setup: I'm a tech for interactive art projects, and budget/resource concerns often leave us with power mismatches between speakers/transducers and amps. If I leave the amp maxed, the speaker and/or amp might have a shortened lifespan. The amps are often hidden away in tech spaces, so I want to be able to set the amp gain to a safe max level when I install it. – user2418372 Nov 3 '20 at 18:49
• Re: "So power is not voltage*current except at very low frequencies" Okay, that makes sense. Both tests are supposed to be done with a low frequency (~50hz) sine tone as input. I guess that's why? I'm definitely in over my head on the theory here. – user2418372 Nov 3 '20 at 18:57
• There is no sensible case for using the term RMS power. Power is power i.e. volts x amps (instantaneous and averaged). End of. – Andy aka Nov 3 '20 at 19:01
• Power mismatched amps and speakers? You don't need 200W speaker combined with 200W amplifier. Typical speaker is around 80 dB SPL at 1 meter. If you need 80 dB SPL, you need 1 watt. For 90 dB SPL at 1 meter, you needs 10 watts, and that is already known to damage hearing under prolonged exposure. How many dB SPL you need and at what distance? – Justme Nov 3 '20 at 20:55