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?

  • 1
    \$\begingroup\$ 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. \$\endgroup\$
    – Justme
    Nov 3, 2020 at 18:41
  • \$\begingroup\$ 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. \$\endgroup\$ Nov 3, 2020 at 18:49
  • \$\begingroup\$ 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. \$\endgroup\$ Nov 3, 2020 at 18:57
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    \$\begingroup\$ There is no sensible case for using the term RMS power. Power is power i.e. volts x amps (instantaneous and averaged). End of. \$\endgroup\$
    – Andy aka
    Nov 3, 2020 at 19:01
  • \$\begingroup\$ 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? \$\endgroup\$
    – Justme
    Nov 3, 2020 at 20:55

3 Answers 3


I think you are going about this the wrong way. It's not clear whether or not you have control of the signal source. The approach might be different for a fixed CD player and quite different if the users can use their phones, for example.

My approach would be to choose a piece of music with a reasonable volume level, set the output on the device to max and then adjust the amplifier gain to a suitable level and maybe a little more to accommodate quiet pieces. While adjusting listen to the output for signs of distortion.

Of course, before you start you would check that you have matched the speaker and amplifier impedances before you start.

  • \$\begingroup\$ I've edited my question to give a bit more context. I do have control of signal source volume. I don't always get to choose the speakers and amps I work with. I often work with transducers (bass shakers) that don't necessarily have audible distortion even when operating out of spec, so using my ears isn't always enough. Thanks \$\endgroup\$ Nov 4, 2020 at 17:48

If the volume control is turned up too high so that the amplifier produces clipping distortion then the fragile tweeter will soon burn out even if the speaker power rating is higher than the amplifier power rating. Frequently speaker power ratings are lies. My Radio Shack Minimus 7 speakers were rated at 40W but when the woofers burned out when driven from a 10W per channel amplifier I looked inside the speakers and saw that the woofers were stamped 5W not 40W.


One quick way of doing that, without blowing the amplifier and/or speakers, is to use a lot of high power resistors or even incandescent lights in series/parallel matching the expected amplifier output impedance and expected output power. And if any of those blows, turn off the amplifier to prevent that all the power is sent back to it, even if it has open/short circuit protection. Then, you can connect a computer and generate a sine wave with some open-source software. Then, measure the output voltage and use the formula P = U^2/R. But this will give you an idea, since is not the most common case use, unless you want to do morse code on a speaker or something similar.

If you want accuracy, you need to use a RMS power meter on the bandwidth intent to be used and to generate brown noise instead of a sine wave.

Keep in mind that high power resistors or incandescent lights have inductance, despite very low, so if you do THD measurements, you need to consider that you have a inductive load and not a resistive load.


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