710 Chapter 24
Also, without having the capacitative loading region of a conventional tweeter (and the
load dip of any passive crossover), an ICT driver’s load impedance at hf is benign.
24.1.2 Loudspeaker Sensitivity versus Effi ciency
Loudspeaker drive-units have to be “ packaged ” to be usable in the real world. Together,
enclosures and drive-units defi ne the effi ciency of the resultant loudspeaker. Effi ciency
(or its derivative, sensitivity) then decides the scale of amplifi er power needed. With
different high-performance loudspeaker types, effi ciency varies over an unusually wide
range of at least a hundredfold, from about 20% down to 0.2%.
E f fi ciency 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 effi ciency that makers use to specify “ how much SPL for a given excitation. ” In part,
sensitivity is universally specifi ed because it’s easier to measure. It is given as an SPL
with a given input (nearly always 1 W) at a given distance at close range (1 m normally).
So the spec is the one that reads: “ Sensitivity 96 dB @ lw @ 1 m. ”
Figure 24.3 : The exploded hf dome above this Tannoy drive-unit has no ohmic connections
and cannot be burnt out. It employs inductive coupling technology, the fi rst completely new
type of drive-unit to enter mass production for many years. Each new drive-unit type has its
loading peculiarities, which add a new layer of variables to the considerations of amplifi er
users and designers alike.
(Courtesy of Tannoy Ltd.)