Speaker sensitivity, also sometimes referred to as efficiency, is a reflection of how many decibels of sound will be produced by a speaker with a set amount of power input from the perspective of someone standing a set distance away. Typically, speaker sensitivity is given in decibels per watt per meter (three feet), with the measurement being taken by a device placed three feet (one meter) away from the speaker in a chamber which has been specifically designed for speaker testing. Speaker sensitivity is one among many features which can be used to evaluate the quality of a speaker to determine whether or not it is appropriate for a setting.
Essentially, speaker sensitivity is a measurement of efficiency, demonstrating how efficiently it can generate sound with a given amount of power. The higher the sensitivity rating, the less energy will be needed to power a speaker. This can be critically important for people who like things loud, because a three decibel increase requires a doubling of power. This means that a speaker with a sensitivity of 95 can produce the same decibel level as a speaker with a sensitivity rating of 92, with half the power.
Speaker sensitivity does not necessarily have an impact on quality. A number of things influence the quality of the sound, and it's possible to have a very sensitive speaker which will sounds bad, just as it's possible to have a speaker with low sensitivity which sounds excellent. For someone with a low powered amplifier, a speaker with high sensitivity may be a good idea because it will be able to generate louder sound than a low sensitivity speaker connected to the same amplifier.
Size is not necessarily an indicator of speaker sensitivity. Very large speakers can be extremely sensitive, while very small speakers can have low sensitivity, contrary to the belief that large speakers are usually inefficient. People who are trying to achieve a particular type of sound may want to consult the staff at a speaker store to get recommendations, or talk to sound engineers and other music professionals about the various options available.
Manufacturers usually print the speaker efficiency rating on the packaging of the speaker. Speaker sensitivity may also be printed somewhere in or on the speaker, so that it will still be available in the event that the packaging is lost. Consumers should be aware that these ratings are arrived at with a speaker in optimum conditions which has been connected to an amplifier properly and with the best equipment, which means that their experiences may vary.