Loudspeaker Enclosures 711For most domestic speakers, 96 dB is a high sensitivity but low for professional types. The
sensitivity is but a broad measure of effi ciency differences, since two factors are missing.
One is how the sound energy is spread in space. If it is all focused forward, sensitivity
(dB SPL @ 1 W @ 1 m) 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 effi ciency is unaffected.
Factor two is the impedance. Where mainly resistive, effi ciency is about the norm, as
computed by integrating the SPL over all the solid angles. But around the resonant
frequency where the impedance changes rapidly from capacitative to inductive, effi ciency
is high, as little energy is dissipated.
With these four dimensions of variables (3D space ID impedance), converting
effi ciency into sensitivity fi gures and vice versa is not straightforward. However, as a
rough idea, an 86-dB @ l-W@ l-m rated domestic speaker is about 0.5% effi cient. With
a two-sided (planar) speaker, the effi ciency might be the same 0.5% but sensitivity would
ideally halve toward 80 dB.
24.1.3 Loudspeaker Enclosure Types and Effi ciencies
Horn loading is by far the most effi cient technique. It is between 10 and over 100 times
more effi cient than any others. “ Effi ciency ” means it gives the most acoustic intensity
for a given power input, from the amplifi er. Only when a horn (or “ fl are ” ) is coupled to
a transducer with a low output (e.g., a ribbon driver) is the overall effi ciency not “ streets
ahead ” of all the other driver-t-enclosure combinations.
The most effi cient drivers are the familiar electrodynamically driven cone, dome, and
compression types, particularly those with an optimum balance between the strength of
magnetic and electric coupling, the levity of the moving parts, and the compliance of the
suspension. In the midrange, some ESLs can be as effi cient as the cone driver, both in the
context of a refi ned domestic speaker.
The least effi cient enclosures are:
None (this holds true at low frequencies only),
The sealed box (SB) or “ infi nite baffl e ” (IB), and
The transmission line (TL)—used to extend bass response.