Testing for RMS with meter

I have heard that you can read the amp's output voltage at the terminals with a multimeter, then use Ohm's Law P = E squared / R to find the RMS.  I finally bought a Sears Craftsman DMM (after years of wanting a new one), and made this test today.  While playing a music CD, the readings were erratic on the meter. But I have a test tone CD, so then I used it.  First, this is what I'm working with:

The system is a Ford factory head unit, 2001 Taurus, using 2 (high-quality) LOC's to connect to a Profile CL 4 channel amp.  Amp is rated at 50W RMS X 4 @4 ohms; 4 @75 W RMS @2 ohms;150W bridged RMS X 2  @4 ohms.  There are 4 Pioneer coaxials for the front/rear, paralleled to the amp channels 1 and 2, at 2 ohms stereo.

The single sub is bridged at 4 ohms to channels 3 and 4.  A CD changer is installed.  I grabbed the speaker leads for the LOC's at the tuner, which is mounted in the trunk.  The amp gains are set properly to the head unit output.

I played a track from the test CD with a tone of about 35 Hz, with the head unit volume to the max position that I know I can go (just on the verge of clipping),  at just over half on the dial.  I read the AC volts on the bridged channels at 29.9 volts.  The number didn't waiver, it stayed a steady reading.  Applying that to Ohm's Law, I get 223 Watts.

The channels 1 and 2 each showed readings of 13.7 volts, playing a track at 200 Hz.  That amounts to 93.8 Watts for each of those channels.  Again I got a steady reading.  On each test, if I turned the volume down, the volts would decrease.  So these numbers are at full power.

These numbers exceed the manufacturer's RMS rating of the amp:  223 instead of 150;  and 93 instead of 75.  Question:  am I getting an RMS value with the DMM testing this way?   Am I getting more voltage  with the factory head, and is that added to the results I am getting from the amp?

Your theory is correct, but your application is slightly flawed.  The nominal impedance rating of a drivers is not always accurate.  Usually it is a percentage above the DCR, but many companies actually rate it at the DCR.  But when a speaker is playing, the impedance of the driver changes with frequency.  You would have to know what the impedance is at a certain frequency, then measure the voltage at the frequency to figure out how much power your amp is producing.

Steven Kephart

Adire Audio

On second thought, I think that wouldn't tell me anything.  I would still get the same voltage reading.  I have to know what the ohm resistance is for each frequency.  So, if I know what the ohms are for just one freq, and test it at that, I use that number for R in the formula?  Do speaker manufacturers give out that data?  The sub is Cerwin-Vega HED 12SVC.  The spec sheet shows a value for "Revc" at 3.6 ohms.