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YEROC.US
Searching for Order in this World of Entropy

What is PMPO? It's An advertising gimmick!

PMPO is a measure of audio amplifier power output. It is deceiving because it considers an amplifier's power output under perfect conditions and only for a short instant.

PMPO

Have you ever wondered how that pair of speakers they are selling at the store for $10 can put out 120 watts of power? Well, depending on your definition of output power...they can...and it has fooled many people! I know people have told me that their home stereo unit has 2600 watts of power output...even though it is connected to a standard 120-volt power supply that is only allowed to draw 2000 watts before tripping the circuit breakers!

The answer to this issue lies in how they measure the output power of the device. Usually if you look at these huge numbers, you will see the letters "PMPO" underneath. PMPO stands for "Peak Music Power Output" or "Peak Momentary Power Output". Notice the word peak. The vendor of the product calculates PMPO based on the maximum power output of the device under perfect conditions. These conditions are impossible to obtain, and no device could sustain the PMPO power for any significant length of time without being destroyed, blowing a fuse/circuit breaker, or both. The PMPO power relies on the fact that amplifiers can provide short bursts (basically on the order of milliseconds) of very high power.

RMS vs. Peak

In electronics, the root mean square (RMS) of an alternating current signal such as music is an average with respect to time of the voltage, current, or power. A Peak value carries no relation to time - it is more of a "snapshot" taken at a specific point.

For a constant AC signal (such as the 60 or 50 Hz sine waves from the power grid), the Peak voltage is the voltage measured from the crest of the sine wave to the middle (zero). As the AC cycle progresses, the voltage will drop off and will fall and rise until the next crest is reached. Since the signal is sinusoidal (a sine wave) and in theory not fluctuating in frequency and amplitude, the peak voltage remains constant and the RMS voltage is 1/sqrt(2) times the peak voltage.

In music, however, this peak will be very different at different points in the song. The drums and guitar will impart large varying pulses that will have constantly changing peak voltages. One company could measure the highest peak produced by a song, while another could measure the mean or median of peaks produced by a song. There is no standard method of measuring PMPO power in the audio indsutry.

The almost random nature of music makes it impossible to express amplifiers by their "Peak" output powers. Only RMS values (which carry a relationship with time) are acceptable to engineers working' with such devices. Peak is meaningless, unless the goal is to see how hard the percussionist smacked the bass drum.

The power delivered to a speaker by an amplifier is most accurately found by measuring the current and voltage and then averaging them over time. This is the same thing the electric company does when they bill you for electric usage. They don't see every little spike from when you turned on a light or vacuum cleaner, they see the average power use for the month (in the form of energy). That's what matters, and a similar idea applies to audio amplifiers.

The Real Power

Vendors seem to use PMPO power to hide underpowered power supplies and amplifiers. When you see something with a PMPO measurement on it, turn it around and look at the specifications. Often, the input power from the AC mains is 5 or more times LESS than that of the PMPO power. You also may see the more accurate measurements - RMS (Root Means Square) power. Don't be fooled by cheap products which appear to put out big power.

Another PMPO vs. RMS Package

So, how do you find the RMS Power from the PMPO Power?

I have been asked this, and there is no single answer or mathematical formula to convert these values. The peak output of the amplifier is going to differ based upon the amplifier's design, and the type of sound that is being amplified. Often each electronics manufacturer will create their own proprietary formula for figuring PMPO power. Usually, the PMPO power is about five to ten times higher than the RMS Power.

There is NO mathematical formula that will convert PMPO to RMS values or vice-versa.

 

Last Modified: 05/25/2008
Created: 07/12/2004