Sensitivity vs Wattage

 

Many people think loudness is easily determined just by watts… more watts = louder.  Actually, other factors must be considered to determine the speaker loudness, and the biggest of them all is sensitivity. Sensitivity represents what SPL a loudspeaker can produce with a given input level.  It is always measured on axis of the loudspeaker.  A loudspeaker with a higher sensitivity will produce more SPL with less distortion for the same given input level.  This also means greater SPL with less power being pulled from the wall. Similar, but not the same, is efficiency. Efficiency is how well a loudspeaker converts an incoming signal to sound energy, radiated in all directions.  For this article, we will concentrate on sensitivity.

Let’s start by considering “dB” (decibel) differences.  3dB is considered the smallest change in level that most people can detect under normal listening conditions. For a 3dB increase in SPL, you need to double the watts.  Going from 1000 watts to 2000 watts, produces the same increase in SPL as going from 2 to 4 watts, as does 50 to 100 watts.  It is NOT the actual watts that are important, but rather the ratio of the watts.  This is why thinking in terms of dB is more important than actual watts.

When looking at loudspeaker specs, it is a combination of power handling and sensitivity that gives a better picture of how loud a particular loudspeaker will be.  Of course, this loudness says NOTHING about what it sounds like.  A piece of sheet metal through a saw is really loud, but doesn’t sound too nice to most people.  Consider two loudspeakers with roughly the same frequency response. Loudspeaker A has a 2000 watts capacity and sensitivity of 103dBSPL.  Loudspeaker B has a 3000 watts capacity with a sensitivity of 100dBSPL.  Assuming all other factors are the same, which one can get louder?  Of course, loudspeaker A will be louder with the same input level due to higher sensitivity, but, let’s do some math for maximum SPL.  First, convert the wattage capability to dB gain. The formula is: dB gain=10log watts X/watts Z . 

Loudspeaker A:   33dB=10log2000/1* (Assuming 1watt sensitivity).   So, add the sensitivity of 103dB + 33dB power gain to get 136dB max continuous SPL

Loudspeaker B:   34.8dB=10log3000/1* (Assuming 1watt sensitivity) So, add the sensitivity of 100dB + 34.8dB power gain to get 134.8dB max continuous SPL.

Note: We could also have used voltage capacity and drive voltage to come up with the same answers.  The voltage formula is: dB gain=20log Voltage X/Voltage Z.  We are just saving a step by not having to convert the wattage to voltage.

The moral of the story is You MUST look at the dB differences, NOT the simple watts number. Sensitivity often makes a greater impact than watts on the total SPL. Higher sensitivity can mean cost savings in terms of amplifier power, wall power, and cable size.