How to figure RMS wattage using a VOM?

[MichaelD]

If you don't know what a VOM is, please exit this thread now; thanks.

Good, at least you know what a VOM is! I'm trying to figure out the RMS wattage of my truck's head unit (Isuzu-branded radio). Assuming a I have CD with some pink noise on it/test tones, how can I use the VOM to get RMS wattage? I know the audio out of the deck is DC volts, but I'm no mathematician. How do you convert volts into RMS watts? Thanks.

 

[Viperoni]

Actually it's in AC volts
The only way I know of how to do it is to grab a resistor of known value (a 100watt 4ohm resistor would be ideal), and wire that in series with the speaker outs + and -.
Then just chcek your voltage rating, then plug it into: Watts = Voltage^2/Resistance
That'll give you a pretty decent reading.

BTW: Don't most VOM's measure the effective/RMS value of voltage anyhow?

 

[Jerboy]

<< If you don't know what a VOM is, please exit this thread now; thanks.

Good, at least you know what a VOM is! I'm trying to figure out the RMS wattage of my truck's head unit (Isuzu-branded radio). Assuming a I have CD with some pink noise on it/test tones, how can I use the VOM to get RMS wattage? I know the audio out of the deck is DC volts, but I'm no mathematician. How do you convert volts into RMS watts? Thanks. >>



You simply can't.

Audio signal varies constantly and you need a wattometer that automatically multiplies instantaneous voltage and amps together and display it in n seconds avg or plot it out on graph.


Or you can use 1KHz sinusoidal AC(1KHz is base freq for many audio equipment measurement) for general measurement or 60Hz-100Hz sinusoidal AC for bass measurement. You'll use a funciton generator to create the aforementioned signal and feed it into stereo input.

Place a 0.05ohm resistor between the speaker and head unit. Measure the voltage across the speaker and the resistor. For every amp, there will be 50mV potential difference across the resistor. So Vspeaker*Vresistor*20=RMS watts

 

[Harvey]

If you're dealing with A.C., either you have to know it is a sine wave, or you need a true RMS voltmeter. You will get the right answer if you use a sine wave test signal.

 

[MichaelD]

My goodness, that's way more complicated than I figured it would be! :Q I'm really racking my brains trying to figure what kind of wattage the stock head unit puts out. I know most stock radios are no more than 10w RMS (w/o any outboard amplification, ala a Bose system).

I pulled one of the speakers from the front doors. It's a Sanyo unit. Rated at 15w RMS/30w peak. Interesting for a stock speaker. I'm looking at getting a nice Kenwood receiver w/22watts RMS. Hopefully it makes a big diff. There is simply no place to mount an external amp (I have two, rotting away in my closet) in my truck without mounting it in the rear of the truck, which I don't want to do.

Thanks very much. You guys blow me away sometimes with all the knowledge you've got tucked away. Have a good night.