Speaker sensitivity—many times erroneously referred to as speaker efficiency—is used to determine the amount of power necessary to drive or operate a loudspeaker. It is a measurement of the amount of sound output derived from a speaker with one watt of power input from an amplifier. Sensitivity is usually measured with a microphone connected to a sound level meter placed one meter in front of the speaker. The resultant number is expressed in dB.
Most speakers are actually very inefficient; only about 1% of the electrical energy sent by an amplifier to a typical home loudspeaker is converted to acoustic energy. The remainder is converted to heat, mostly in the voice coil and magnet assembly. The main reason for this is the difficulty of achieving proper impedance matching between the acoustic impedance of the drive unit and that of the air into which it is radiating. The efficiency of loudspeaker drivers varies with frequency as well. For instance, the output of a woofer driver decreases as the input frequency decreases.
Horn loaded speakers—such as Klipsch products—can have a sensitivity approaching 110 dB at 2.83 volts (1 watt at 8 ohms) at 1 meter. This is a hundred times the output of a loudspeaker rated at 90 dB sensitivity, which would be excellent for a traditional radiating cone type.
The key advantage to efficient speakers is that they require less power to drive them; they also generate less heat and generally can boast a longer component life.
Note: For assistance on what to look for when choosing a receiver/amplifier, please see our article - Choosing the Right Receiver/Amplifier.