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Efficiency

Efficiency is classically defined as energy out / energy in, and in the case of loudspeakers it is defined as the sound power output divided by the electrical power input. Loudspeakers are actually very inefficient transducers. Only about 1% of the electrical energy put into a typical home loudspeaker is converted to the acoustic energy we know as sound - the remainder being converted to heat. The main reason for this low efficiency is the difficulty of achieving proper impedance 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 today's hi-fi speaker standards.

Measuring the sound power output is not easy, but the sound pressure is relatively simple to measure. The sound pressure level (SPL) that a loudspeaker produces is measured in decibels (dBSPL). Ratings based on the sound pressure level are known as sensitivity ratings. The sensitivity is often defined as dB/(Wm)decibels output for an input of one nominal watt measured at one metre from the loudspeaker and on-axis or directly in front of it, given that the loudspeaker is radiating into an infinitely large space and mounted on an infinite baffle. Sensitivity then does not correlate precisely with efficiency as it also depends on the directivity of the source and the acoustic environment in front of the speaker. As an example, a simple cheerleader's horn makes more sound output in the direction it is pointed than the cheerleader could by herself, but the horn did not improve or increase the cheerleader's total sound power output much, it mainly focused it over a smaller area.

  • Normal loudspeakers have a sensitivity of 85 to 95 dB/(Wm) - an efficiency of about 0.5-4%.

  • Nightclub speakers have a sensitivity of 95 to 102 dB/(Wm) - an efficiency of about 4-10%.

  • Rock concert, stadium speakers have a sensitivity of 103 to 110 dB/(Wm) - an efficiency of about 10-20%.

Current state-of-the-art loudspeakers can approach apparent efficiencies of 70% or higher under very special circumstances. This is partly due to a very high magnetic field and partly to a high amplitude displacement (speaker cone movement 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) (a nominal efficiency of 0.1%). However at full power in an enclosed automobile may achieve 160+ decibels at 20% to 40% apparent efficiency. In general a low frequency speaker designed for high SPL's will have a larger and or heavier magnet, and a higher Xmax.

It should be noted that a higher power driver will not necessarily be louder than a lower power one. In the examples which follow, it is implicit that the drivers being compared have the same impedance. For the first example, a speaker that is 3 dB more sensitive than another will produce double the sound pressure (or play 3 dB louder) for the same power as the other. Thus a 100W speaker (call it A) rated at 92 dB/(W.m) sensitivity will be twice as loud as a 200W speaker (call it B) rated at 89 dB/(W.m) when both are driven with 100W of input power. For this particular example, when driven at 100W, speaker A will produce the same SPL or loudness that speaker B would produce with 200W input. Thus a 3 dB increase in sensitivity of the speaker means that it will need half the power to achieve a given SPL, and this translates into a smaller power amplifier and some cost savings.

 
 
 

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