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Car Audio Q&A

How to measure your own wattage

by Eddie Runner (NU5K)

how to measure your wattage

This question is asked a lot on rec.audio.car

For a pretty close guesstimate, it can be as simple as hooking a voltmeter to your speaker leads when you play your amplifier and using the formula voltage times voltage divided by impedance. For instance, if your voltmeter says 10 volts and you know your speaker to be 4 ohms then simply calculate 10 volts times 10 volts which is 100, then divide by the impedance 4.. 25watts… Very simple!

There are some problems with this method! And this method should not be used to see HOW MUCH power your amp has when turned all the way up! The speakers will play so loud they could be damaged and your ears may not like it much either.

Also it will not be accurate at all frequencies since the speakers impedance is not constant at all the frequencies, for instance the impedance might be way higher playing at 30Hz than 100Hz. The speaker manufacturers give us the nominal or approximate impedance to help us in putting together systems and this impedance can be used for estimates.

To get around the impedance problems and the blown speaker possibilities and to test the amplifiers MAX outputs we use what we call Dummy Loads, these are basically BIG resistors. Usually 4 or 8 ohm resistors capable of holding MORE then the wattage we will be testing. Dummy loads are hooked up INSTEAD of the speakers, so you will hear no sound and wont have to worry about blowing your speakers, but be carefull because the resistors can get quite hot. Now just measure the voltage on the speaker leads coming out of the amplifier under test. Again use the formula (V^2)/R

Real techs use more test equipment and get much more accurate measurements. Normally if you read the specs on your amplifier, or read the amplifier tests in the car audio magazines you might notice the power rating also includes a distortion measurement and frequency response… When you reach the upper limits of an amplifiers output power the frequency response and distortion can be VERY BAD. So the techs turn the volume down slightly until they see distortion and frequency figures they like and then they read the voltage and calculate the power output… An amp might have maximum of 20 volts into 4 ohms (100watts) but the distortion might be VERY BAD and the frequency response might be really lousy, so the tech might turn the volume down until the frequency response is near perfect and the distortion (or THD) gets down to much less than 1 percent, or might turn the volume down till the THD gets all the way down to .01 percent or there abouts! At this point he might read the voltage of only 15volts (56 watts)

To do these tests that real techs do, they use an audio generator to generate tones since music is to much of a complex waveform to get a stable reading.. They also use very sensitive volt meters, a distortion analyzer to see the THD while testing, and maybe an oscilloscope to actually watch the sound wave while testing it. And of course the previously mentioned Dummy load and also for car audio amplifier testing it is important to have a 12 or 14 volt power supply capable of running the amplifier..

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