Root Means Square, RMS Means Real Power. AKA. Lets measure my amps REAL power output

Lets learn a few basics.. how to measure AC amps, AC voltage, ohms law, watts law, and how to use them to accurately calculate what your amps doing.

For measuring power you need to measure AC Volts and AC Amps. Volts with a DMM, Amps with an AC Clamp. You need to blow a single test tone for the test as well, as voltage & amperage will change depending on frequency. 60hz is the recognised frequency to test with..

1. Attach your woofer to your amp
2. Place the clamp meter over the +ve wire going from the amp to the sub
3. Place the two probes on the +ve & -ve wire from the amp to the sub
4. Set both (clamp & DMM) to Peak Hold
5. Blow a 60hz test tone at maximum volume

When you have your results:
Amps X Volts = Watts PMPO (peak measured power output)
Multiply the VAC by the AAC for PMPO power... this is what they call MAX power.
Multiple PMPO by 0.707 for the approx. RMS Wattage... this is your true power.

Or is it ?

I bet measuring using the method described doesnt quite work the way you'd think.. you think your 2ohm woofer is a 2ohm load, so your amp is making XXXX watts RMS @ 2ohm ohm. Right ? Wrong !

Here's some figures to work with.. A JBL W15GTi woofer, 3ohm configuration, is connected to a JBL BPX2200 amplifier and measured with a DMM & clamp with a 60hz tone. The amps rated for 2200WRMS @ 2ohm.

Static Impedence = 3ohm
VAC = 84V
AAC = 16A

84 x 16 = 1344 PMPO (Watts law, Amps X Volts = Watts)
1344 x 0.707 = 950WRMS (Calculated RMS from PMPO)

"But why is the RMS power so low" you ask ? "An amp rated at 2200WRMS @ 2ohm should make LOTS more than that at 3ohm". It would, IF it had a 3ohm load !

Ohms law states Volts / Amps = Resistance (impedence)

84V / 16A = 5.25 ohm

At 60hz this particular woofers actual impedence is 5.25ohm, almost 2x its Static Impedence !

Why ? Because impedence changes as the coil moves inside the motor (hook a DMM in ohmage range to a speakers terminals and push the cone for proof), it also changes with heat. Power applied to the coils causes them to heatup, changing the impedence. Impedence isnt static !


So what have we learnt ?

1. We can use OHMS law to calculate an accurate impedence when testing AC amps & volts
2. How to calculate Watts RMS from Watts PMPO

How is this useful ?

In SPL competition you can figure out what impedence load you're getting on your note and exploit it with lower impedence subs to increase power at your given note !