Loudspeaker efficiency refers to the efficiency with which the loudspeaker converts electrical energy into sound energy. Efficiency is usually expressed in decibels (dB) and indicates how much sound power is produced in decibels (dB) per watt of electrical power.
A loudspeaker with a higher efficiency produces more sound pressure for the same energy input than a loudspeaker with a lower efficiency. In other words, a loudspeaker with an efficiency of 90 dB/W/m produces a sound pressure level of 90 dB at a distance of 1 metre with an output of 1 watt.
A higher efficiency means that the loudspeaker can achieve a higher volume at lower amplifier powers. This is especially important for applications where little power is available, such as in hi-fi systems driven by a weaker amplifier.