Handbook for Sound Engineers

Loudspeakers 605

reached only in theory. In reality, under continuous high
drive levels, mechanical stress on the surface of the
ceramic wafers generates cracks in the microstructure
that eventually penetrate the entire wafer. This is espe-
cially severe around the area where solder connections
are made to the wafers, since the soldering operation
tends to prestress the material at this point. The net
result is that the 35 V maximum drive level is an inter-
mittent specification, with the continuous drive level
recommended at a 15 V maximum up to 20 kHz. For
use above that frequency, it has been recommended that
the level be reduced further through the addition of a
series attenuation resistor so as to safeguard the ceramic
element from absorbing excessive high-frequency
power. Here again, when using larger, thinner ceramic
wafers, these problems are further aggravated. Using
this ceramic is an area for future development.
Fig. 17-17 shows the Motorola KSN 1001A.
Although Motorola manufactures a wide variety of
other piezo-driven loudspeakers, the one illustrated here
is the most widely used.

One near-optimal application of piezoelectric drive
is underwater use. This is due to the excellent imped-

ance match of the piezoelectric material to water via a

waterproof barrier. Lubell Labs manufactures the under-

water loudspeaker shown in Fig. 17-18. Although

swimming pool loudspeakers using standard electro-

magnetic drivers are also available, the piezoelectric

configuration is more efficient due to its mechanical

impedance match to water. The loudspeaker is fixed to

the side of the pool and driven like a conventional loud-

speaker. Lubell Labs makes high-power arrays of these

devices and a portable swim coach system with a noise-

canceling microphone for underwater communications

in various pool athletic events.

17.6 Motor Design Considerations

The most common means of coupling amplifier output

to the diaphragm in an electrodynamic transducer is via

a cylindrical voice coil. This configuration is used on all

magnetic cone loudspeakers and compression drivers.

This is commonly known as a linear motor. The coil,

made of round or rectangular wire (edgewound), is

wound around a hollow cylinder called a former.

Formers may be made of paper, plastic (e.g., Kapton

polymer, Mylar™), or aluminum. The voice coil

assembly is bonded to the diaphragm. Fig. 17-19 shows

the construction of a typical cone loudspeaker.

One novel approach to motor design involves

printing or etching a conductor onto a thin sheet of

Mylar™ (0.0005 inch) then folding it to produce and

if

Figure 17-16. Typical coupling circuit and high-voltage
power supply for electrostatic loudspeakers.

Figure 17-17. Motorola KSN 1001A piezoelectric ultra-
high-frequency driver/horn. Courtesy Motorola, Inc.

40 MF

(nonpolarized) 17

Stepup

4:1

Electrostatic

loudspeakers

8/16 (^7) 1 k 7
1.5 mH
7 7
117 Vac
0.0047MF 22 M7 22 M^7
1100 V
22 M 7
Four 0.218" (5.5 mm) diameter holes equally
spaced on a 3.94" (100.1 mm) diameter B.C.
Weight: 75 grams
3.34
84.8
3.34
84.8
0.070
1.8
0.5
12.7
2.69
73.7
Figure 17-18. Lubell Labs underwater piezoelectric loud-
speaker. Courtesy Lubell Laboratories, Inc.