A speaker's efficiency is measured in decibels (dB), with the reading made at a distance 1 meter away from the front of the speaker while the speaker is being driven at 1 watt. The measurement is further qualified by stating the impedance of the speaker in question, measured in Ohms. For example, a speaker would be quoted as having an efficiency of 87 dB at 8 ohms.

It is important to know a speaker's efficiency in order to match a given speaker to the amplifier driving it and the other speakers used in a system. It is also useful when comparing speakers for sheer loudness. If you know the efficiency of any given speaker, you can calculate how loud it will be at a given power input, measured in watts.

In order to increase sound output by 3 dB, one must double the output power. this means that a speaker with an efficency of 90 dB will be able to use half the power of a speaker with an efficiency of 87 dB to achieve the same volume (provided the two speakers have similar power ratings.):

Power -- Speaker 1 -- Speaker 2 -- Speaker 3
01 Watt -- 87 dB ---- 90 dB ------ 93 dB
02 Watt -- 90 dB ---- 93 dB ------ 96 dB
04 Watt -- 93 dB ---- 96 dB ------ 99 dB
08 Watt -- 96 dB ---- 99 dB ------ 102 dB
16 Watt -- 99 dB ---- 102 dB ------ 105 dB
32 Watt -- 102 dB ---- 105 dB ------ 108 dB
64 Watt -- 105 dB ---- 108 dB ------ 111 dB
128 Watt -- 108 dB ---- 111 dB ------ 114 dB
256 Watt -- 111 dB ---- 114 dB ------ 117 dB

The 96-dB-efficient speaker is as loud at 16 watts as the 90-dB speaker is at 64 watts, a significant power savings. A three-decibel change is sound is considered a benchmark value for "perceivably louder". Most home theater amps will only go to 100 watts RMS (continuous) per channel, which is also the per-channel spec to meet THX requirements.

A speaker's efficency is often related to its sound quality. An infinite baffle (sealed) speaker will tend to sound more accurate, but will be relatively inefficient, and a ported speaker will be more efficient, but unless very carefully designed, will sound less "tight".

Changing a speaker's impedance also affects efficiency, but one must be careful that the amplifier used is compatible with the impedance. An amplifier is able to drive more power into a lower impedance, but will run hotter and may fail if it is not designed with low-ohm loads in mind. This is why many car installs wire the speakers in such a way that the impedance is lowered, to get the maximum power from the amp. One- or two-ohm car systems are not unheard of. Most home speakers are 6- or 8-ohm.