To calculate the theoretical power requirement for an amplifier you need to know 4 values:
1) The distance from the speaker. This is normally the distance from the speaker to the last row of seats in the room. Type in feet or metres (the metres is calculated from the feet – if feet is used). From this distance the Sound Pressure Level (SPL) loss over that distance is calculated.
2) The SPL required. This is how loud you want the sound to be at the distance specified. Some general levels are:
- 70-80dB for speech only
- 80-95dB for light music
- 95-110dB for heavy music
3) The speaker sensitivity. This is not an indicator of how sensitive the main (human) speaker is, but rather a measurement of the sensitivity of the loudspeaker. It should be available in the specifications for the loudspeaker. It is normally stated as the SPL measured 1 metre in front of the speaker with 1 watt of power driving the speaker. Hence the specification will read something like:
Sensitivity (1W/1m) = 91dB
4) Amplifier headroom. This is an allowance for the amplifier to cope with peaks without distortion. Note that for every 3dB allowance, the power requirement doubles.
Use the calculator below with different values to see the effect a change in any of these parameters makes to the amplifier power required.
Note: these calculation are for “open-air” where there are no reflections from walls, ceiling and/or floors. When reflections are present, the SPL losses can be reduced by up to 6dB.
For those who need to know, the formulas used for these calculations are: