One of the many confusing issues in professional audio is that of loudspeaker power handling. On one side, manufacturers use a variety of terms such as peak, RMS, average or program power. On another side, there exist differing methods to determine the power handling of speaker system or component which yield different results. We will try to throw some light into this subject.
Power is energy per time. It is measured in watts. Power delivered by an amplifier to a load (speaker) is normally determined by dividing the voltage (V) squared by the impedance (Z) :
The resulting type of power will depend on what voltage we use. If peak voltages are used, then the result is peak power. If RMS voltages are used, then average power (often wrongly referred to as "RMS") is obtained. RMS (root-mean-square), is only a mathematical method to extract the mean value of an alternating signal (one with negative as well as positive values).
2. Power tests
To determine a speaker's power handling, one needs to subject it to a power test. This consists of feeding the speaker with a test signal, typically some kind of noise signal with controlled dynamics, for a given amount of time, normally ranging from 2 to 100 hours.
The test signal is commonly some form of pink noise. Pink noise is a random signal that shows the same energy in all frequency bands. This signal is not constant in time, but possesses some degree of dynamics. Pink noise allows us to perform tests were the speaker is stressed thermally as well as mechanically.
The dynamics of a signal are expressed in the form of the crest factor. This is determined by the ratio of the peaks (crests) to the average level of the signal. The graph below shows a 6 dB crest factor signal, i.e. one that has peaks which are 6 dB louder than the average level. This is equivalent to a 2:1 ratio between the peak and RMS voltages, which corresponds to a 4:1 ratio between peak and average ("RMS") power, since power calculations are based on voltage squared. These dynamics are commonly specified by international standards.
There exist several standards that specify power test procedures. The most relevant are :
2.a. The AES2-1984 standard
This is a standard for loudspeaker components by the Audio Engineering Society. It is very commonly used, and, although meant for components, it is also often used for individual ways of an active system. It specifies a 6 dB crest factor pink noise signal, with a bandwidth of one decade. For example, a bass loudspeaker could use a 50-500 Hz band, whereas a high frequency unit could use 1000-10000 Hz. The illustration shows the spectrum of both AES signal spectrum examples. The duration of the test is 2 hours, after which the component should not show appreciable damage.
Comparative spectra for different power test signals as seen by a constant percentage bandwitdh (RTA type) analyzer
(Pink noise would show a flat line)
2.c. The AES2-2012 standard
The AES2-1984 standard has now been superseded by AES2-2012. Crest factor for the pink noise signal has been increased to 12 dB (4:1) and nominal impedance is now used for the calculation of power. The latter means that the resulting power level for a given loudspeaker component will now typically be around 20% lower than with the 1984 version of the standard (that used minimum impedance for the calculation of power), which may cause confusion. Also, the band-limiting filtering has now been changed to 24 dB per octave (1984 version used 12 dB/octave, as seen on the graph above).
2.c. The IEC268-5 (1978) standard
This is a standard by the International Electrotechnical Commission from 1978 and reaffirmed in the eighties. It specifies a 6 dB crest factor pink noise signal over which an IEC programme filtering has been applied. This programme spectrum tries to approximate the frequency content of real music, and shows reduced lows and highs. The illustration compares this spectrum to the AES ones. The terms "Rated noise power" and "power handling capacity" are used.
Test duration is 100 hours, after which the speaker should not show appreciable damage.
NOTE: To make matters more confusing, there's another 268-5 standard from 1972 that specifies a different signal spectrum and time, but this is rarely used.
2.d. The EIA RS-426-A (1980) standard
This is a standard by the (USA) Electronic Industries Association. The duration of the test is 8 hours, after which the speaker should not show appreciable damage. The signal is also 6 dB crest factor pink noise signal, but with programme filtering that is different to the IEC standard and is also shown on the previous illustration.
2.e. The EIA RS-426-B (1998) standard
426-B means quite a deviation from 426-A. The result of this test is not a "power handling" specification anymore but an "optimum amplifier power", which is the maximum input power at which the product under test is rated for acceptability under all three limit categories: a power compression test with a fast variable rate 40-10 kHz sweep sine wave that gets played continuously in a loop, a distortion test and an 8-hour "accelerated life test" with 6 dB crest factor pink noise at half the rated optimum amplifier power and with the spectrum shown on the graph above. None of the tests should result in appreciable damage or change to the unit. The measurement procedure for this standard is rather complex, tedious and subjective at times; at the moment it has not been widely accepted by the sound reinforcement industry and I have serious doubts it will be, with the possible exception of the accelerated life test, which is not significantly different to 426-A except for the wider spectrum.
3. Types of speaker power specifications
3.a. Average power. Often wrongly referred to as "RMS" power, since it is derived from RMS voltage readings. RMS (root-mean-square) only makes sense on variables that have negative as well as positive values. Power is only positive (goes from the amplifier to the speaker, not the other way round), so does not need the "root" and the "square" of the RMS process (which just extract the sign out of a number), just the "mean" (average). Average power is therefore that which uses RMS voltage for its calculation.
3.b. Programme power. It is an archaic term that derives from old swept sinewave power tests. Nowadays, it does hold no real meaning. For most manufacturers, it is simply twice the average power, although other manufacturers may use ratios other than 2:1. It may be used as a guideline to the selection of amplifier power. For instance, a speaker with 300W average power and 600W programme power (2x300W) might use an amplifier with 600W output. This is for carefully controlled conditions; for more usual applications with some degree of abuse that amplifier would be too large.
3.c. Peak power. Corresponds to the calculation of power based upon peak voltages. For a 6 dB crest factor signal, peak power is four times the average power.
From all the above, it can be concluded that, for power test signals with 6 dB crest factor, the ratios of the three types of power would be as follows:
3.d. Continuous. Simply specifies that the power signal is applied all the time, since there are some standards that specify intermittent signals.
4. Causes of speaker failure
The causes for speaker failure can be either thermal or mechanical.
The causes for thermal failure are :
To prevent thermal failure, avoid amplifier clip, use LF and HF shelving EQ in moderation, and ensure that the speaker is only receiving frequencies within its bandpass, using high-pass and low-pass filters to limit the frequency content being fed to the speaker.
The causes for mechanical failure are always linked to excessive diaphragm (cone) movement. The speaker shows greater excursion (backward and forward movement) the lower the frequency. Hence a signal low enough in frequency and large enough in level may cause the voice coil to exit the gap, resulting in the coil rubbing, and possible ending up shorting or opening. The worst case scenario happens when the coil former hits the bottom pole piece ("bottoms out") and gets deformed. To prevent mechanical failure, avoid using signals below a speaker's bandpass, and use an amplifier of the correct power output.
5. Selecting amplifier power
In general, the amplifier power needs to be larger than the speaker's rated power. This is because an amplifier only delivers its rated output power with sinewave signal, and delivers much less with a real signal with dynamics. As general guideline, it is recommended to use an amplifier delivering 50% more power than the speaker's average ("RMS") power. For example, for a speaker with 450W average power, an amplifier with an output of 700W may be used. If a small amplifier is used, sufficient level will not be reached, nor the perception that it is attained, so the signal will tend to be clipped to compensate, thus endangering the integrity of the speaker.
NOTE: The recommendation above is for sound reinforcement. For different applications other ratios may be used and they may even be contrary; for instance, the power handling of the loudspeaker in a guitar combo needs to be significantly higher than the amplifier power, given that the latter is often badly overdriven.
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