Audiopedia

Loudspeaker efficiency is defined as the sound power output divided by the electrical power input. Most loudspeakers are actually very inefficient transducers; about 1% of the electrical energy sent by an amplifier to a typical home loudspeaker is converted to the acoustic energy we can hear. 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.

Driver ratings based on the SPL for a given input are called sensitivity ratings and are notionally similar to efficiency. Sensitivity is usually defined as so many decibels at 1 W electrical input, measured at 1 meter, often at a single frequency. The voltage used is often 2.83 VRMS, which is 1 watt into an 8 Ω (nominal) speaker impedance (approximately true for many speaker systems). Measurements taken with this reference are quoted as dB with 2.83 V @ 1 m.

The sound pressure output is measured at (or mathematically scaled to be equivalent to a measurement taken at) one meter from the loudspeaker and on-axis or directly in front of it under the condition that the loudspeaker is radiating into an infinitely large space and mounted on an infinite baffle. Clearly then, sensitivity does not correlate precisely with efficiency, as it also depends on the directivity of the driver being tested and the acoustic environment in front of the actual loudspeaker. For example, a cheerleader’s horn produces more sound output in the direction it is pointed, by concentrating sound waves from the cheerleader in one direction, and thus “focusing” them. The horn also improves the impedance matching between voice and the air, which produces more acoustic power for a given speaker power. In some cases, impedance matching (via careful enclosure design) will allow the speaker to produce more power.

* Typical home loudspeakers have sensitivities of about 85 to 95 dB for 1 W @ 1 m - an efficiency of 0.5-4%.
* Sound reinforcement and public address loudspeakers have sensitivities of perhaps 95 to 102 dB for 1 W @ 1 m - an efficiency of 4-10%.
* Rock concert, stadium PA, marine hailing, etc speakers generally have higher sensitivities of 103 to 110 dB for 1 W @ 1 m - an efficiency of 10-20%.

A driver with a higher maximum power rating cannot necessarily be driven to louder levels than a lower rated one, since sensitivity and power handling are largely independent properties. In the examples that follow, assume for simplicity that the drivers being compared have the same electrical impedance, are operated at the same frequency which is within both driver’s respective pass bands, and that power compression and distortion are low. For the first example, a speaker 3 dB more sensitive than another will produce double the sound pressure level (or be 3 dB louder) for the same power input. Thus a 100 W driver (“A”) rated at 92 dB for 1 W @ 1 m sensitivity will output twice as much acoustic power as a 200 W driver (“B”) rated at 89 dB for 1 W @ 1 m when both are driven with 100 W of input power. For this particular example, when driven at 100 W, speaker A will produce the same SPL, or loudness, speaker B would produce with 200 W input. Thus a 3 dB increase in sensitivity of the speaker means that it will need half the amplifier power to achieve a given SPL. This translates into a smaller, less complex power amplifier and often to reduced overall cost.

It is not possible to combine high efficiency, especially at low frequencies, with compact enclosure size, and adequate low frequency response. One can, more or less, only choose two of the three parameters when designing a speaker system. So, for example, if extended low frequency performance and a small box size are important, one must accept low efficiency.[5] This rule of thumb is sometimes called Hoffman’s Iron Law (after J. A. Hoffman, the H in KLH).[6]

From Wikipedia article loudspeakers