Many of you may have thought a lot of what is the power specifies in the certificates of acoustic systems and sound-amplifying equipment. Unfortunately, there are not many clear articles on this subject on the Net.

International standards

RMS - Root Mean Squared of the power value limited by the established harmonic distortions.

Power is measured by a 1 KHz sinusoidal signal at 10% THD. It is calculated as a product of root-mean-square values of voltage and current at the equivalent quantity of heat created by direct current. It means that power is numerically equal to a square root of a product of mean voltage and current squared.

For a sinusoidal signal a root-mean-square value is V2 times less than an amplitude value (x 0.707). It is, however, a virtual value. A term "root-mean-square" can be applied to voltage or current, but not to power. Its analog is an effective value.

Now I will try to explain why this concept is not important for audio characteristics description. Root-mean-square power makes sense in electrical engineering. And does not concern necessary a sinusoid. In case of music signals loud sound are heard better, than weak ones. And it is amplitude values that affect ears stronger than root-mean-square ones. It means that volume is not equivalent to power. That's why root-mean-square values make sense only in an electric meter, and amplitude ones in music. As for FS, I should note that falls in FS are noticeable less than peaks. It means that loud sounds are more informative than low sounds, and an average value will tell little.

So, RMS standard is not the best attempt to describe audio system parameters since it doesn't reflect volume as a value.

In amplifiers and acoustic systems this parameter finds narrow application as well - an amplifier which gives out 10% of distortions on not the maximum power is not widely spread. Transistor amplifier distortions, for example, do not exceed one hundredth of a percent before the max power is reached, and after that they sharply go up (irregular mode). Many acoustic systems working for a long time with a such distortion level can fall out.

PMPO (Peak Music Power Output) means max reachable peak signal's value irrespective of distortions at the min time period (usually at 10 ms).

The parameter shouldn't be, though, taken seriously.

DIN 45500 is a complex of IEEE generally accepted standards which describes different sound-amplifying characteristics of the equipment by a more reliable way.

DIN POWER is a value received on a real load (for an amplifier) or power input (to AC) limited by harmonic distortions.

It is measured with applying of a 1 KHz signal to the device input during 10 minutes. Power in measured at 1 % THD.

There are other types of measuring, e.g. DIN MUSIC POWER, which describes power of a music signal. Usually DIN music is higher than the DIN value.

General terminology

Pink noise is a group of random signals with a uniform spectral density of distribution to frequencies, it decreases with the frequency rise with 3 dB decrease per octave in the whole measuring range, with the dependence of a middle level on a frequency in the form of 1/f. Pink noise has a constant (in time) energy on any part of the frequency band.

White noise is a group of random signals with a uniform and constant spectral density of distribution to frequencies. White noise carries equal energy on any frequency part of the band.

Octave. A music frequency band, correlation of the extreme frequencies of which is equal to 2.

Electric power. Power dissipated on ohmic equivalent resistance numerically equal to a nominal electric resistance on AC, with the voltage equal to that on the AC clamps. I.e. on the resistance emulating a real load in the same conditions.

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