Loudspeaker Enclosures 709frequencies, this shorts the speaker’s electrical input, possibly blowing up the amplifi er, or
at least blowing a fuse or shutting down the music. Under most other conditions, the ESL
appears as an almost purely capacitative load, with resistive damping across it.
24.1.1.7 The Piezo Driver
The two fundamental types of drive-unit “ motor ” looked at so far all date back (in
principle) to the early years of this century, or even to the beginnings of the modern
harnessing of electricity, 200 or more years ago.
The piezo drive-units ’ principle is the dual of the familiar household act of creating
large voltages by squeezing crystals. Although piezoelectricity precedes humankind, as
it can occur naturally, it has only been widely harnessed in the past 50 or so years, fi rst
in crystal microphones and pickups and more recently in fuel-less “ push button ” gas
fi re lighting. The dual, or reverse process, that of making a crystal vibrate by applying
electricity to it, was fi rst harnessed by Motorola, who have been producing hf drive-units
employing this principle since at least 1977. This type of drive-unit looks capacitative,
rather like an electrostatic, but has a higher DC resistance so that it can draw no long-
term power. Despite potentially useful high hf performance, since there is still a limited
range of piezo drive-units, most being fi tted to integral, out-dated horn designs, piezo
tweeters are not used much in high-performance systems, but they are occasionally used
in PA systems and may be found optimally applied in refi ned custom speaker systems.
The Motorola piezo element cannot be “ burned out ” by too much “ power ” as it presents
a high impedance. However, it is rated at about 25 V rms, and excess voltage will quickly
destroy the crystal. The crystal can even be harmed by room heating. For use with
amplifi ers having headroom above 25 V rms, operating two or three in series is suggested.
This should not degrade damping as it would with a low impedance speaker.
24.1.1.8 Inductive Coupling
Eli Boaz at Goodmans, part of the TGI Group (comprising speaker manufacturers Tannoy,
Goodmans, and Martin Audio in the United Kingdom) spearheaded the development
of two-way drive-units where the hf driver is inductively coupled ( inductive coupling
technology or ICT). It comprises a radiator with a conductive collar ( Figure 24.3 ). Placed
within the bass/midrange voice coil, it acts as a single turn transformer, picking up
magnetic fi eld most effi ciently at hf. This arrangement is limited to use at hf, but there is
no need for a crossover, and it is highly rugged—a tweeter that cannot readily burn out.