Playing with Loudspeaker Sensitivity
September 7, 2011
The loudspeaker sensitivity specification is the amount of sound pressure level at a distance of 1 meter (3.28 feet) from the loudspeaker, with a 1 watt input to the loudspeaker. The measurement is usually taken over the range of frequencies that the loudspeaker is designed to operate, or over a specified range of frequencies.
Using this spec to compare loudspeakers can be tricky. First, make sure that all are at 1 watt and at 1 meter (1W–1m). If the spec indicates otherwise, you’ll need to convert to 1W–1m.
Next compare the frequency range and kind of test stimulus used (pink noise, speech-shaped noise, frequency sweeps, for example). Some loudspeaker manufacturers are better than others in clearly indicating this. If these are different, you can’t really directly compare one loudspeaker’s sensitivity to another.
You will also need to consider the loudspeakers’ frequency responses over this frequency range. With the same stimulus over the same frequency range, but with a different frequency response, the sensitivity values could be different.
Even with these considerations, loudspeaker sensitivity is useful to indicate the sound pressure level that could be available at a listener’s position and how much power you’d need to obtain it.
The lower the sensitivity, the more power you’ll need to get a certain sound pressure level (SPL) at a given distance (making sure you don’t exceed the maximum power or SPL of the loudspeaker).
Doubling the power input to a loudspeaker will add 3 dB to the SPL at a given distance. Keeping the power input constant, doubling the distance from the loudspeaker will reduce the SPL at that distance by 6 dB.
When you play with these numbers to obtain needed amplifier power for a required sound pressure level at a specific distance, remember to add in some amp headroom to the power applied to the loudspeaker. This is to allow for the difference between average and peak levels. Depending on the application, 10dB to 15 dB of headroom are good minimums to start with.