Audio amplifier power delivery capability (in watts) has a relationship to cost that is roughly linear, hence £ or $ per watt. But amplifier output capability has a highly non-linear, slack relationship with perceived loudness. Thus the particular power rating can matter surprisingly little (unless you are adept at thinking geometrically or logarithmically), so long as it is in the right area (numerically) and plays loud enough in practice.
The order of power required depends on a host of factors. These include:
i) the sensitivity (or efficiency) of the speaker system. Sensitivity is usually cited for 1 watt of drive, at 1 metre distance. The 1 watt is alternatively (and preferably) specified as 2.38v rms (the voltage for 1w⇒8Ω) into any impedance. Sound level drops off at between 3dB and 6dB per each doubling in this distance, depending on speaker directivity and the acoustic conditions.
ii) the maximum SPL required. For some kinds of music and situation, 105dB will do; for others, 140dBSPL will be required.
iii) the room size. How many metres distance are the above SPLs required at? If the room is at all big, then unless the listening position is near to the speakers relative to the room size, room acoustic data, such as reverberation time, will be involved. iv) the headroom required. Like a car with a larger than normal engine, an oversized amplifier won’t easily be stressed, and sonics can benefit accordingly. Pro audio systems routinely employ amplifiers rated at five to even ten times the longterm ‘AES/EIA’ rating of the speakers they are used with.
For domestic hi-fi, home cinema, and recording studio control rooms, there are usually few listeners, often 1 or 2 and perhaps 10 at most. Here, room size is the key factor. In small rooms, listeners are typically only 2 to 4m distant. In large rooms, distance ranges 4m to 12m at most. Domestic speaker sensitivities average 87dB @ 1w @ 1m but range from below 80 up to 97dB @ 1w @ 1m.
Maximum SPLs are usually satisfactory if between 100 to 120dB (C-wtd, peak) at 1m. There are already a range of variables, without mentioning the many other factors. In practice, power requirements are never calculated from first principles by users. Instead, rules of thumb and experience give figures which are near enough most of the time.
For domestic Hi-Fi:
-with ordinary speakers and commonly acceptable sound levels – 30 watts to 300w. Go higher for bigger rooms and/or higher levels.
-with horn-loaded speakers, 0.3w to 10 watts, and upwards to 1kW if the neighbours can handle it, else as above.
For studio nearfield monitoring 50 to 500 watts overall; or 30 to 300w per band if active. For main studio monitoring 250w up to 2kW per frequency band.
For live sound and 30+ people:
- in any indoor space, allow between 1 watt and 15 watts per person, with more watts per person for fewer people, and less for more.
- in any outdoor space, allow 2 to 15 watts per person for a large sound system.
The increase in the minimum wattage mainly affects large gatherings, and insures against the effects of wind and humidity on sound propagation. When taking a domestic system outdoors, Martin Colloms suggests that with the aid of a substantial wall, four times the indoor power would be needed, and considerably more in a fully open space. Naturally, the power decided upon has to be close to what’s available from the power amplifier, into the nominal impedance of the speakers it is to be used with.
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