PA Power Ratings

Understanding Output Power Terminology

One of the values stated - and alas frequently misrepresented - in PA system specifications is the system's rated power.

While we don't encourage you to compare systems on the basis of power alone (see below^[1], and have a look at the System Design page for other important factors), the description of systems according to their output power is endemic in the live sound reinforcement business. For that reason, we have followed the same protocol in describing some of our own systems.

However, there are several different descriptions of loudspeaker and amplifier power^[2] in common use (AES, average, continuous, EIA, FTC, peak, PMPO, programme, RMS...), so comparing systems from different manufacturers or rental companies may not be straightforward.

If you would like to look at it in greater detail there is further information about loudspeaker specifications and amplifier specifications in our Equipment Guide. However, in using measurements to compare equipment - whatever measure you apply - you should ensure that:

• The measurement you use is meaningful and appropriate (PMPO is neither);
• The measurement you use applies an accredited and replicable standard^[3] (FTC amplifier ratings apply a defined standard of measurement, but ‘Watts RMS’ does not);
• Your comparisons all use the same measure (don't compare one system rated in ‘Watts, continuous average’ with another rated in ‘Watts, peak’).

Where we describe a system by power-rating (e.g. 1.6 kW), we base this on the continuous average capacity (AES^[4]) of the Front-of-House speakers in that system. This is the most conservative power-rating we could use, and approximately corresponds with the ‘RMS’ value more often - mistakenly - given. Other suppliers may use different ratings: we have seen systems with less acoustic output than our 1.6 kW system advertised as ‘10k’!

Some of the ratings in common use are described below, and the equivalent ratings of our own 1.6 kW and High Output systems (including SPL capability, for reference) are given for comparison.

 

Each of our (nominally 300W) d&b audiotechnik E12 loudspeakers can produce a maximum sound-pressure of 134 dB @ 1 metre, equivalent to the output of six Martin Audio F12s or eight Martin Audio F10s. Comparing these with other systems on the basis of power handling alone is therefore inappropriate (it is a bit like using engine capacity to compare a Formula 1 car with a family hatchback), although the same standards of measurement should nevertheless apply.

All the amplifiers and speakers we supply will produce high levels of undistorted sound at their rated power over the system's whole frequency range, and will distribute the sound evenly. They will keep this up all night (i.e. indefinitely). Also, they sound great! If you think you have found something cheaper that is just as good or just as powerful, please call us to arrange a demonstration.

1. We approach the question of how much output volume (dB SPL) is required by looking at the dimensions of the auditorium, and calculating the total acoustic output (dB SPL) of a system that will produce the required volume (dB SPL) in a given audience position. We then work out how many speakers of a given type will produce that overall output. As long as speakers can be well-positioned and correctly arrayed, this works well enough. However, where very high sound pressure levels are required (either because the auditorium is very large, or because the style of music is very loud), using different types of speaker and array may be a better solution than adding more boxes.

   We base this calculation on the measured acoustic output of the speakers we use in our systems. Because they are designed by professionals for professional use, other specifications that are useful for calculating system requirements are given by the manufacturers of the speakers (d&b audiotechnik and Martin Audio) and amplifiers (d&b audiotechnik and QSC) we use.

2. The fact that a description is in common use does not make it valid or useful. ‘Nervous Breakdown’ is not a clinical or medical term (so a tabloid newspaper might use it, but a health professional wouldn't). ‘Watts RMS’ isn't a valid technical term (so a high-street salesman might use it, but a qualified audio professional shouldn't).

   You may well come across any of the following power descriptions:

   • AES. Generally seen in loudspeaker specifications, this means that the test used to establish a loudspeaker's power-handling capacity meets Audio Engineering Society standards.
   • Average. A loudspeaker's average power-handling capacity is calculated by squaring the applied RMS voltage and dividing the result by the loudspeaker's impedance. Commonly this is described as ‘RMS power’, which it is not.
   • Continuous. A loudspeaker's continuous power-handling capacity is derived from applying continous signal voltage to it. Generally you will also need to know either its peak capacity or the signal's Crest Factor to get an idea of its true performance.
   • EIA. Generally seen in amplifier specifications, this mean that the test used to establish an amplifier's output capacity meets Electronic Industries Alliance standards.
   • FTC. Generally seen in amplifier specifications, this mean that the test used to establish an amplifier's output capacity meets Federal Trade Commission standards.
   • Peak. Generally seen in loudspeaker specifications, peak power is calculated from the maximum instantaneous voltage a loudspeaker can withstand. This is not indicative of long-term capacity, and is generally only useful to give an idea of how much headroom there is between continuous capacity and likely failure: most loudspeakers subjected to peak voltage will fail in a matter of milliseconds.
   • PMPO. Peak Music Power Output. A wildy exaggerated statement of power-handling that bears no relationship at all to any standard of measurement. PMPO is the Instant Failure rating, and a loudspeaker that will fail instantly at 100W will also fail instantly at 200W or 300W, so we might as well call it 500W to be on the safe side.
   • Programme or Program. There is no recognised test of ‘programme power’, but the value stated will typically be double the (measurable, and hopefully measured) continuous value.
   • RMS. Root Mean Squared. If it is used to describe signal voltage, this has a useful meaning. If it is used to describe power, it does not. However, it is quite often used to mean Average (i.e. power calculated from RMS voltage).
3. A standard in measurement is one that has been agreed by consensus to provide the optimum replicability, consistency, and suitability. In sound equipment, consensus on measurement is found among nationally and internationally recognised industry-wide professional organisations, as well as - in some cases - government legislation or official advisory bodies.

   Some of the principal relevant organisations are:

   • AES. Audio Engineering Society (USA/international).
   • ANSI. American National Standards Institution (USA).
   • BSI. British Standards Institution (UK).
   • EIA. Electronic Industries Alliance (USA).
   • FTC. Federal Trade Commission (USA).
   • IEC. International Electrotechnical Commission.
   • IEEE. Institute of Electrical and Electronic Engineers (international).
   • IET (formerly IEE). Institution of Engineering and Technology (UK).
   • ISO. International Standards Organisation.
4. The AES continuous power-rating describes the average amount of power a speaker can handle over a long period (typical tests last two hours or more).
5. Nominal (Continuous and Peak) SPL is calculated from measured SPL per cabinet (pink noise, crest factor 4) at 1 metre and referred to summed output at 10 metres. This provides an effective assessment of system capability.

   The word ‘nominal’ should always be treated with suspicion, but we have used it here because there isn't a measured value for our complete systems. Although it is in fact a calculated value, it is not strictly replicable.

6. ‘Programme power’ doesn't have any reliable definition, and our own advice is to view any description of equipment based upon it with that fact in mind. Typically, the value given for ‘programme power’ is double the average continuous power (and we have given this value here for comparison, although there is no published ‘programme power’ rating for d&b audiotechnik or Martin Audio loudspeakers).
7. Peak power is not representative of overall speaker performance. As part of a valid technical test it describes the maximum (peak) part of a waveform whose average will be at least 6dB less. In a practical sense, it is the maximum power the speaker can handle for a few milliseconds at a time. Try it for a few milliseconds longer and Recones-Я-Us.

[Top of Page]