Speaker sensitivity is the level produced by the speaker at one meter, when supplied with one watt of power from an amplifier. It’s a way of describing how well the speaker produces acoustic energy, based on a given input. The level is measured in dB SPL – this shorthand for ‘sound pressure level’ – sound is basically fluctuations in air pressure, so any time you see SPL, this means ‘sound in the real world’. So, let’s say we have a speaker with an 87dB sensitivity, and a speaker with a 90dB sensitivity – the 90dB speaker is going to be considerably louder, as every 3dB added represents a doubling of power.