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Audio Facts and Fallacies

True vs 'PMPO' Power

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1 Introduction

The term '''audio power''' is used in the specification or measurement of audio amplifiers or loudspeakers. A meaningful and reliable measure of the power output of an audio amplifier, or the power handling of a loudspeaker is ''continuous sine wave power'', or more strictly 'continuous average sine wave power'. Such a figure will often be found in advertising literature referred to as "true RMS power", but this is quite incorrect. Although there is such a thing as RMS (root mean square) power, it is neither useful as a measurement nor what is intended by those who use the term. The sine wave power is found by averaging the instantaneous power output over a long period of time (or one complete cycle), so it is actually the ''average power'' or ''mean power''. The term RMS is used mistakenly due to the fact that the mean power is calculated from the RMS voltage and current (or one of them and the impedance); power being proportional to the square of voltage or current.

2 ‘Music Power' - the Real Issues

The term ''"Music Power"'' has been used in relation to both amplifiers and loudspeakers with some validity. When live music is recorded without amplitude compression or limiting, the resulting signal contains brief peaks of very much higher amplitude (20 dB or more) than the mean, and since power is proportional to the square of signal voltage their reproduction would require an amplifier capable of providing brief peaks of power around a hundred times greater than the average level. Thus the ideal 100-watt audio system would be capable of handling brief peaks of 10,000 watts in order to avoid clipping (see Programme levels). Most loudspeakers are in fact capable of handling peaks of several times their continuous rating (though not a hundred times!), since thermal inertia prevents the voice coils from burning out on short bursts. It is therefore acceptable, and desirable, to drive a loudspeaker from a power amplifier with a higher continuous rating several times that of the speaker, but only if care is taken not to overheat it, which is difficult, especially on modern recordings which tend to be heavily compressed and so can be played at high levels without the obvious distortion that would result from a 'real' recording when the amplifier started clipping.

Music power is a less valid term when applied to most amplifiers. Most power amplifiers can give more output on brief bursts than their continuous rated output, but not usually to an extent that is relevant in the context of the above. There are three reasons for the enhanced short-burst power.

Most amplifiers do not have regulated power supplies but rely on a full-wave rectifier and large smoothing capacitor to provide a reasonably steady supply voltage. This charges to its peak voltage on quiet passages where little current is being drawn, but 'sags' to around 10% less under heavy current demand. Since 10% voltage drop corresponds to 20% power drop, the steady-state power output of the amplifier, which has to be quoted is always some 20% lower than the brief power capability. A 100-watt amplifier is therefore likely to handle brief peaks of up to 120 W without clipping. This might sound good in a specification, but it should be noted that it is only 1 dB, which is a change in level not usually even detectable by the human hearing system! It is also usually only available for some 10 milliseconds, which is too short to be of much benefit in real programme material. The term peak music power, in this context, is of no real significance.

It is possible to take a cost-effective approach to power amp design by reducing the size of the heat sinks on the output devices below that needed to avoid overheating on continuous sine wave drive at maximum output. Such an approach was once valid, as it recognised that fact that on 'real' recordings there is no need to provide for continuous full output as the gross distortion caused by clipping on brief peaks will result in the user turning down the volume before damage is done. On modern amplifiers it is possible to take such an approach without risk of damage, using integrated amplifier chips, which tend to incorporate 'thermal protection'. However, the trend towards heavy compression and limiting on commercial recordings in recent years means that people expect to play these at high volume without clipping, and so the validity of the 'peak music power' approach to amplifier design has mostly been removed.

While the above is true for most 'domestic' amplifiers, it need not be so, especially in relation to monitoring, and uncompressed reproduction. Some professional amplifiers, and 'active' speakers, incorporate sophisticated electronic thermal protection circuits which integrate the power delivered to the speaker and take account of its thermal capacity properly. This enables them to handle peak power levels safely while limiting the continuous power that can be applied in a way that makes sense.

3 Power Handling in 'Active' Speakers

Active speakers often use two or three power amplifiers, each handling only part of the audio frequency spectrum. The main benefit of this approach is that it enables complicated crossover filters to be used on the low level signal, and eliminates the bulky and awkward inductors and capacitors normally used in crossover networks. There is, however, another big advantage that is not usually recognised. When two tones are reproduced simultaneously, a single amplifier normally has to handle the peak power that results when both are at their crest. Because of the square-law relationship, this means that two tones each generating 10 watts result in a power handling requirement of 40 watts. With multiple amplifiers, the two tones can be handles separately, by 10 watt amps. Thus a 'bi-amped' system can handle peaks of up to twice the combined rating of its amplifiers, and a 'tri-amped' system, on three tones, gains even more! This is of course because the signal has a high 'crest factor'. In practice, music peaks often consist of percussion riding on top of bass notes, and so the benefit is very real, as these are each always handled separately. This is a benefit that would cost a lot to realise if the single amplifier approach were taken, making 'bi-amping' a very cost-effective approach.

4 US Market Regulations

In the US on May 3, 1974, the Amplifier Rule CFR 16 Part 432 (39 FR 15387) 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 did not cover automobile entertainment systems, which consequently still suffer from power ratings confusion. However, a new regulation called CEA 2006 includes car electronics, and is being phased into the market as slowly as possible by many manufacturers.

Unfortunately there are no similar laws in much of the rest of the world.


By Pete Skirrow

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