A serious and reliable measure of the power output of a loudspeaker or audio amplifier is average power, commonly called watts RMS. RMS stands for root mean square and is actually a misnomer here but has nonetheless become the common term. The "RMS" power is found by averaging the instantaneous power output over a long period of time, so it is actually the average power or mean power. The term RMS is used due to the fact that the mean power is calculated from the RMS voltage and current (or one of them and the impedance).

Peak momentary performance output (PMPO) is a much more dubious measure of more interest to advertising copy-writers than to consumers. The PMPO is calculated by the vendor or tester of the speakers as the maximum power in watts available under perfect conditions that are impossible to achieve in practical use. No sound system can sustain its PMPO for more than a few milliseconds without being destroyed.

"Music Power" is a somewhat arbitrary multiplier based on the fact that amplifiers cannot provide full power for long, but can produce high power in short bursts. This allows them to reproduce normal audio, which has a high peak-to-average ratio. It is usually about twice the "RMS" value but like PMPO varies by manufacturer.

US market

In the US on May 3, 1974, the Amplifier Rule CFR 16 Part 432 (39 FR 15387) [1] was instated by the Federal Trade Commission (FTC) requiring audio power and distortion ratings for home entertainment equipment to be measured in a defined manner with power stated in RMS terms. This rule was amended in 1998 to cover self-powered speakers such as are commonly used with personal computers (see examples below). This regulation does not cover automobile entertainment systems, which consequently still suffer from power ratings confusion. Unfortunately there are no similar laws in much of the rest of the world.

Actual ratings compared

To get an idea of the relationship between PMPO watts and watts "RMS", consider the following numbers advertised for some current loudspeakers. These models have been selected at random, and inclusion in or exclusion from this list is neither a recommendation nor a criticism.

• Teac PM-100 3D surround-sound speakers: 16 W RMS, 180 W PMPO
• Kinyo "200 W" PC speakers: 3 W RMS, 200 W PMPO
• Philips Fun Power Plus MMS-102 PC speakers: 10 W RMS, 120 W PMPO (The Philips data sheet mentions only the "RMS" value; the PMPO value is claimed by retailers.)

This list shows that PMPO figures are hugely exaggerated compared with the "RMS" values used by professionals. It also shows that there is little consistency in how much the figures are exaggerated making them almost totally meaningless.

Power calculations

AC power, which includes audio power, is rated by the mean power in watts. This is equivalent in heating value to DC watts. This mean power is often referred to as "RMS" largely because it is derived from the RMS voltage or current (but being proportional to the square of them it is not in itself technically a RMS quantity; just a mean quantity). This is the honest method. A given direct-coupled amplifier (transformers are rare in non-distribution equipment) can obviously only swing its output from ground to the power supply voltage at best. This limits a simple car amplifier (excluding those that use DC-DC conversion) to something like a 12-volt swing, peak-to-peak.

For example, an ideal (100% efficient) amplifier with a 12-volt peak-to-peak supply can drive a signal with an amplitude of 6 V. Into an 8 ohm (see impedance) loudspeaker this would deliver

6 V × 6 V / 8 Ω = 4.5 watts peak instantaneous.

If this signal is sinusoidal, its RMS value is 6 V × 0.707 = 4.242 V(RMS). This voltage into a speaker load of 8 ohms gives a power of:

4.242 V × 4.242 V / 8 Ω = 2.25 watts("RMS")

With a 4-ohm speaker these figures double. You can see which the marketing person wants to use. You can also see why some car audio uses 2-ohm speaker loads.

With a differential (push-pull) output the swing is doubled so the power increases by a factor of four. Therefore, with a 14.4 V supply, which is the highest voltage usually available from a fresh car battery, the maximum possible output into a four-ohm speaker is about 14.4 × 0.707 × 14.4 × 0.707 / 4 Ω or about 26 watts.

To get more power, high-end car audio equipment uses a DC-to-DC converter to generate a higher supply voltage (at the expense of drawing more current from the battery).

The true power output of an amplifier can be estimated by examining the input current. Linear amplifiers tend to be about 60% efficient at best. An amplifier labeled "500 W PMPO" but fitted with a 5-amp fuse can therefore deliver an average power of 5 A × 14.4 V × 60%, or about 43 watts.

It is not uncommon to see two apparently incompatible claims in a list of technical specifications of sound equipment, viz. a "4,500 watt PMPO" delivered from a "fitted plug with 3 A fuse" (British plugs are fused).

External links

• RMS values and measurement
• An explanation of why RMS is a misnomer when applied to power