Audio power
From Wikipedia, the free encyclopedia
| Sound measurements |
|---|
| Sound pressure p |
| Sound pressure level (SPL) |
| Particle velocity v |
| Particle velocity level (SVL) |
| (Sound velocity level) |
| Particle displacement ξ |
| Sound intensity I |
| Sound intensity level (SIL) |
| Sound power Pac |
| Sound power level (SWL) |
| Sound energy density E |
| Sound energy flux q |
| Acoustic impedance Z |
| Speed of sound c |
The term sine 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.
Contents |
[edit] 'PMPO' and misleading specifications
Peak Music Power Output (PMPO) (also Peak momentary performance output) is a much more dubious measure, of interest more to advertising copy-writers than to consumers. The term PMPO has never been defined in any standard but it tends most often to be the peak power of every amplifier in a system added together. Peak power is twice the sine wave power, so, for example, a 5 channel system using 10 watt amplifiers might be specified as '100 watts PMPO'. Sometimes, an extra factor is applied to get an even higher figure. The term PMPO is greatly despised by audio professionals; it is considered misleading and without purpose. Most systems cannot sustain their PMPO for more than a few seconds, loudspeakers being particularly vulnerable. The voice coil might burn out, or the speaker may actually catch on fire, from the voice coil becoming hot enough to ignite the speaker cone. In other situations, the crossover in the speaker cabinet may fail, including (but not limited to) burnt-out choke coils, exploded electrolytic capacitors, burnt-out resistors, or blown fuses. Other dubious techniques of "measuring" PMPO include attaching extremely low and non-reactive loads to an amplifier (or even short-circuiting its output) and measuring the "peak" power output it can sustain for a small amount of time, usually microseconds, before individual components start to burn and fail. Such a "measurement" has no practical utility or significance regarding the amplifier's nominal usage. There have been genuine attempts to measure 'peak music power', as described below, but in general the term is not at all useful.
[edit] Power and loudness in the real world
While it is tempting to try to get a bit more power out of an amplifier, or to criticise certain amplifiers for not being able to sustain their rated power out when, for example, all channels are driven simultaneously, these details are really just academic in terms of what really matters — loudness. Because our ears respond logarithmically to intensity, we normally measure this in dB (decibels). A change of 1 dB corresponds to a 25.9% change in power level, and yet is considered the very smallest change in level that anyone can detect even under test conditions. 3 dB corresponds to a doubling of power, and yet represents a change in level that is just-noticeable. So anything less than a doubling of power is hardly worth bothering about! The sensitivity of loudspeakers is much more important. Many high quality domestic speakers have a sensitivity of 84 dB for 1 W at 1 metre, but professional speakers can have a figure of 90 dB for 1 W or even 100 dB (especially for some large-coned woofers). 6 dB represents four times the power, so it would take a 400-watt amplifier driving the '84 dB' speaker (assuming it didn't burn out) to produce the same loudness as a 100-watt amplifier driving a '90 dB' speaker. A '100 dB' speaker could produce the same loudness with just 10 watts (though in practice modern sub-woofers are often driven with high power amps to overcome the restriction of a small enclosure through the use of equalisation).
A better measure of the 'power' of a system is therefore a plot of maximum loudness before clipping, in dB SPL, at the listening position intended, over the audible frequency spectrum. A good system should be capable of generating higher sound levels below 100 Hz before clipping, as the human ear is less sensitive to low frequencies, as indicated by Equal-loudness contours.
[edit] '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 amp 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 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.
[edit] 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 handled 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 wideband percussion riding on top of low-frequency 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.
[edit] 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 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 slowly phased into the market by many manufacturers.
There are no similar laws in much of the rest of the world.
[edit] 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.
[edit] Power calculations
AC power, which includes audio power, is best measured as an average power. This is the accurate method. It is based on this formula[2]:
For a purely resistive load (not a speaker), a simpler equation can be used:
In the case of a steady sinusoidal tone (not music) into a purely resistive load, this can be calculated from the peak voltage and the resistance:
Though a speaker is not purely resistive, these equations can be used to approximate power measurements for such a system, as follows.
An ideal (100% efficient) class AB amplifier with a 12-volt peak-to-peak supply can drive a signal with a peak amplitude of 6 V. In an 8 ohm (see impedance) loudspeaker this would deliver:
- Ppeak = (6 V)2 / 8 Ω = 4.5 watts peak instantaneous.[3]
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 Ω gives a power of:
- Pavg = (4.242 V)2 / 8 Ω = 2.25 watts average [4]
Thus the output of an inexpensive car audio amplifier is limited by the voltage of the battery. In most actual car systems, the amplifiers are connected in a bridge-tied load configuration, and speakers are no higher than 4Ω. High-power car amplifiers use a DC-to-DC converter to generate a higher supply voltage.
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. A switch-mode amplifier (known as class D) can achieve much higher efficiency, sometimes as high as 95%. A linear car amplifier labeled "500 W PMPO" but fitted with a 5-amp fuse can, at most, deliver an average power of 5 A × 14.4 V × 60%, or about 43 watts.
[edit] See also
[edit] External links
- RMS values and measurement
- An explanation of why RMS is a misnomer when applied to power
- Audio Amplifier Projects
Categories: Sound | Power
