Loudspeaker drive units have to be ‘packaged’ to be usable in the real world. Together, enclosures and drive-units define the efficiency of the resultant loudspeaker.

Efficiency (or its derivative, sensitivity) then decides the scale of amplifier power needed. With different high performance loudspeaker types, efficiency varies over an unusually wide range of at least a hundredfold, from about 20% down to 0.2%.

Efficiency is not often cited, but can be inferred from the vertical and horizontal polar radiation patterns, the impedance plot, and the sensitivity. Sensitivity is the derivative of efficiency that makers use to specify ‘how much SPL for a given excitation’.

In part sensitivity is universally specified because it’s easier to measure. It is given as an SPL with a given input (nearly always 1 watt) at a given distance at close range (1 metre normally). So the spec is the one that reads:
‘Sensitivity 96dB @ 1w @ 1m’.

96dB is a high sensitivity for most domestic speakers but low for professional types. The sensitivity is but a broad measure of efficiency differences, since two factors are missing.

One is how the sound energy is spread in space. If it is all focused forwards, sensitivity (dB SPL @ 1w @ 1m) is raised as the sound ‘density’ at the measuring position increases. At low frequencies, rated sensitivity commonly falls as the sound radiation becomes more nearly spherical, while efficiency is unaffected.

Factor two is the impedance. Where mainly resistive, efficiency is about the norm, as computed by integrating the SPL over all the solid angles. But around the (or a) resonant frequency where the impedance changes rapidly from capacitative to inductive, efficiency is high, as little energy is dissipated.

With these 4 dimensions of variables (3D space + 1D impedance), converting efficiency into sensitivity figures and vice-versa is not straightforward. But as a rough idea, an 86dB@1w@1m rated domestic speaker is about 0.5% efficient.

While with a two sided (planar) speaker, the efficiency might be the same 0.5% but sensitivity would ideally halve towards 80dB.

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