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Efficiency of loudspeakers

Efficiency

The sound pressure level (SPL) that a loudspeaker produces is measured in decibels (dB). The efficiency is measured as dB/(Wm) - decibels output for an input of one nominal Watt measured at one metre from the loudspeaker usually on the axis of the speaker. This is called the "sensitivity" rating. Loudspeakers are very inefficient transducers. Only about 1% of the electrical energy put into the speaker is converted to acoustic energy. The remainder is converted to heat. The main reason for this low efficiency is the difficulty of achieving proper matching between the acoustic impedance of the drive unit and that of the air. This is especially difficult at lower frequencies. The better the matching, the higher the efficiency. Large horn loudspeakers that used to be used in cinemas, were very efficient by todays hi-fi speaker standards. From a technical standpoint "sensitivity" is not the absolute reference of efficiency. As an example, a simple cheerleader's horn makes more sound output than the cheerleader does by herself, but technically the horn did not "improve" or increase the cheerleader's "efficiency". True or absolute efficiency is the ratio of "desired" output power divided by total input power.

Current state-of-the-art loudspeakers can approach efficiencies of 70% or higher. This is partly due to a very high magnetic field and partly to a high amplitude displacement (speaker cone pumping in and out). The ratio of the sound output to the mass of the cone/coil combination grows significantly at high sound pressure levels i.e. above 140 decibels. In closed or small environments (such as cars or bedrooms) it is far more important to have a speaker with a high Xmax (cone eXcursion maximum) as opposed to high (dB/(Wm)) rating. A higher Xmax indicates that the driver can move a larger volume of air as power increases. A few top of the line woofers have a very low "sensitivity" rating i.e. 80 to 86 dB/(Wm) (sensitivity efficiency of 0.01%). However at full power may achieve 160+ decibels at 20% to 40% "true" efficiency.

In general a higher quality speaker will have a higher sensitivity rating, larger and or heavier magnet, and a higher Xmax.

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Electrical characteristics

A dynamic loudspeaker presents a complex load to the amplifier as opposed to a pure resistance. It is a combination of resistive, capacitive, inductive as well as mechanical effects. A typical amplifier is most usually quoted for a given power into a resistive load. However a loudspeaker of say, a rated impedance of 8O/100W can easily overload an amp designed purely with a resistive load of 8O/100W as a target.