Measure actual watts with a multimeter


My question is simple but I have not found a solution on the internet.

How do you measure the watts that a speaker consumes when it is in use? It is connected to an amplifier.

The speaker spat because the amplifier not getting enough power.
See the answers for more details.

Is there a steady single frequency sound coming from the speaker? If so, you can use the AC part of the DVM to measure the voltage and then measure the AC current. Multiply them to get the watts.
If not a single frequency, then you can't measure the watts, because it changes from instant to instant.

Thank you

I did not quite understand how to do it.
I am using a 5VDC power supply.

I would put a continuous frequency ok.

Use as low a frequency as possible.
Set your multimeter to AC volts and measure the voltage across the speaker terminals.
Turn all power off and set you multimeter to AC current. Open one wire going to the speaker and connect the multimeter leads to where you opened the speaker circuit. Now apply power to the amplifier and measure the AC current going to the speaker.
Multiply those two multimeter readings to get the "power" going to the speaker.

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Thank you for your reply.

I tried to measure the voltage across the speaker but it doesn't work.
Yet I can measure the voltage across the amplifier, and the current.
Is the problem with my multimeter?

Then, I guess you need a different multimeter. What is the lowest AC voltage you can measure with your multimeter? Can your multimeter measure AC current?

I'm sorry I confused the symbol, rookie mistake.
Now it works.
I measure about 5v and 400ma (100hz) at max volume without spitting from the speaker.
If we multiply 5x0.4 that makes 2W, my speaker is indicated with a power of 10w. It's not normal.

The 10 watts for your speaker is just before it sends out smoke. 2 watts seems about right. That is REALLY loud from a speaker.

When you drive speakers with 10W of "rated power", and it's actually 2W, that's a little disappointing.

Why??? You are measuring AVERAGE power. Your speaker is rated at PEAK, INSTANTANEOUS power and at 1,000 Hz. You need to use a recording oscilloscope to measure that.

The speaker power rating is it's maximum... i.e. You can connect a 100W speaker to a 1W amplifier.

You can also calculate power as Voltage-squared/Resistance or Current-squared X Resistance. Typically, you measure voltage and use the speaker's impedance for the calculation.

Technically, the actual impedance of a speaker varies over the frequency range (so we say the speaker has a 4 or 8_Ohm nominal impedance.) And since it's not a pure resistance there can be a phase shift between voltage & current which can give you an inaccurate power calculation.

But audio amplifiers and speakers are measured/specified with "constant voltage", treating impedance as a constant resistance and we rarely worry about the true power dissipated by the speaker.

It's probably easier to estimate maximum power based on the amplifier's power supply voltage -

With a 5V power supply and a regular single-ended amplifier (where one speaker terminal is grounded) you can get 5V peak-to-peak (maximum). That's 1.77V RMS* which gives you 0.78W into 4 Ohms. (Half that into 8-Ohms.)

With a bridge amplifier you double the voltage to the speaker for 4 times the power. (When you double the voltage you also double the current, so 4 times the power.)

In reality, there is some voltage loss across the amplifier so you can't get the full supply voltage peak-to-peak.

A couple of notes - Regular music/program material has a rather high peak-to-average ratio (maybe 5:1 or 10:1). An amplifier hitting 100W on the peaks might be running 10 or 20W average. An analog meter usually does a pretty-good job of measuring the average voltage (or current) but digital meters are unpredictable.

You have to be careful testing high-power amplifiers with speakers connected... Speakers are rated for regular program material so a "100W" speaker is designed to be used with an amplifier occasionally hitting 100W on the peaks. You can fry it with constant 100W test tones. The 100W rating also assumes most of the energy is going to the woofer so it's even easier to fry a tweeter with test tones.

  • With a sine wave the peak is 1.4 times the RMS, and peak-to-peak is double that.

So why does my speaker stop working at less than 40% of the amp's maximum volume? I do not really understand.

I looked at the description of another product from the same seller. The nominal power is quite distinct from the maximum power: Maximum power: 15W Rated power: 10W

So the 10W announced would not be a maximum value.

It's still not clear if the 'speaker' stops working or if the amplifier just puts out a maximum of 2W into this speaker. I suspect the latter. It's perfectly possible your speaker may survive a higher power level. It's not sure for how long, if your ears will last and how quickly the neighbors will the cops. But the speaker may still work.

And real RMS power probably something like 2-4W.
Welcome to the world of superficial marketing.

I apologize for expressing myself badly. In fact at this volume the speaker spits, sizzles continuously and I therefore cut it off immediately.

It is therefore difficult to orientate oneself in purchasing.

The speaker used to be called a "reproducer" because it only reproduces what the amplifier puts out. The noise is coming from the amplifier, likely feedback. which may be electronic or may be mechanical caused.

Not necessarily. It can also be the speaker itself. When driven to the edge of the envelope, some pretty nasty sputtering and sizzling can occur as the membrane doesn't keep up with the voice coil. But you know this, I'm sure.

Only if the number of watts is a relevant criterion. It never is.

Speaker power specification and amplifier output specification are kind of like statistics, they can be made to say anything based on what assumptions you make. Statistically the average person has one tit and one ball.

What it is NOT is single frequency RMS power.

You say your amplifier is powered by 5V. That means the most you can get across the speaker is 5/2 = 2.5 Volts peak = 1.75 Volts delete(watts of RMS power) (yes I am ignoring amplifier headroom). See post #22 for the remaining calculations.

Now if you overdrive the amplifier and it starts to put out a flattened sine wave which will start approaching a square wave you are creating a lot of high frequency energy (even if you input a sine wave). This high frequency energy will sound grossly distorted and could sound like a buzz.

So the sad news is you get what you get. Input music into you amplifier and slowly increase the gain (volume) until the sound starts to distort and that is you limit (producing music). If you are generating tones, do the same thing.

Perhaps a way to look at is to compare it to rope you might buy in Home Depot. Each rope has an ultimate strength, but you wouldn't go testing rope at that amount of stress. Maybe a bad simile but the best I can think of.

Is your speaker in a proper enclosure?

No, 1.75V RMS. If my early morning pre-coffee math is right, that makes something like 400mW into 8R assuming nominal impedance happens to be real impedance at the tested frequency.

In practice noone measures the power, they estimate it assuming the speaker impedance is as printed on the speaker (which is a very rough approximation as impedance varies with frequency considerably).

Thus if you measured 10V rms from the amplifier into an 8-ohm speaker you'd calculate 10*10/8 = 12.5W for the estimate.

In practice an 8 ohm speaker might be 6 ohms at DC, more like 20 ohms at high frequencies, and have a resonant frequency where the impedance strongly depends on the enclosure acoustics and can be quite reactive (inductive or capacitive).

Under high power dissipation the speaker's voice coil heats up and its resistance increases, further complicating the situation - its basically messy and not useful to measure the true power, so you measure the voltage and make a simplifying assumption about the load being constant impedance.

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