RMS vs PMPO (or Peak Power)

RMS Power refers to Root-Means-Square power. To get a rough figure for RMS power, you can multiply the Peak Power Rating x 0.6 EG: if an amplifier is rated at 100 watts peak power, the approximate RMS power would be 60 watts. The RMS rating is a measure of continuous power in watts and the higher the RMS power means the device is either able to provide or accept the specified power over a significant length of time.

Some manufacturers use the more impressive PEAK POWER ratings in their specifications. Using Peak Power (PMPO) ratings when assessing a music product is a bad idea as Peak Power does not reflect a a systems capabilities in normal use. PMPO is a measure of the maximum power that can be handled for a very short period of time. Speakers and amplifiers should always be compared via the RMS power ratings as these are far more reliable and far more likely to have a genuine impact on the sonic results you get..

If you have a system with a speaker that an RMS rating of 100 watts and the amp is similarly rated at 100 watts RMS, the speaker and amp should theoretically be perfectly matched and so result in an ideal performance from both products.

Real World Power and loudness

Perceived “loudness” varies logarithmically with output power a change in output power produces a much smaller change in perceived loudness. This is not to say that bigger speakers can produce more sound with less power but instead that larger speakers can generally take and output more initial power and thus need less amplification to produce the same perceived power output level. This is one reason why the speakers in a system are so important to overall sound quality. The best measure of a systems power is maximum loudness before clipping, in decibels (dB).

Remember as a rule of thumb: more speaker+less amp. = same “loudness” + less distortion

Dodgy Industry Practices to be Aware of

It is not unknown for some manufacturers quote maximum power at a higher than acceptable Total harmonic Distortion level (Usually < 1%) E.G. like 10%, making their equipment appear to be far more powerful than it actually is at audio levels where sound quality is not compromised.

Remember Peak Power (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.

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In audio reproduction between the input stage and output stage of a hi fi system there will be a certain amount of noise and distortion introduced to a signal. Some of this distortion is created by the circuits and physical elements introducing signal harmonics at double and triple the frequencies of the original signal. These added signals are technically known as Harmonic Distortion. THD is a measure of the differential between the input stages of the system and the audible extra signal at each harmonic. The THD calculation doesn’t consider the relative level of each harmonic. EG: One amplifiers output at the 2nd harmonic could be higher than the output at the 3rd. Another could create equal levels at the 3rd and 5th. The two amps would sound very different yet could have the same measured overall THD.

In most hi fi circuits such as amplifiers and pre amps one should expect a value of less than < 1% THD as it is relatively easy to remove a large amount of THD from amplifier circuits using various techniques. Unfortunately it is far harder to stop significant levels of THD from being produced by the physical elements of a setup such as the speakers. Much higher levels of THD are common in cheaper end speakers especially at higher volumes. Speakers with high levels THD tend to cause “smearing” of audio, and result in poor imaging and a “muddy” sound. In reality one should test a component ideally in the room it is to be used in and at a volume level it is likely to commonly be heard at to ascertain if the system is acceptable for purpose.

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