602 Chapter 17
out of stamped beryllium-copper cantilever fingers, and
JBL and Ramsa stiffen their suspensions with rhombic
and diamond patterns, respectively; this is actually a
redistribution of diaphragm breakup resonances.
Midrange compression drivers are useful where there
is a requirement to supply very high levels of acoustic
power with low distortion. Community Professional
Loudspeakers, M4 midrange driver is shown in Figs.
17-11 and 17-12. It is intended for use from 200 Hz to
2000 Hz. The diaphragm is approximately 7 inches in
diameter. Originally it was fabricated from specially
formed aluminum skins and a light, stiff foam core,
about 0.090 inch thick. More recent versions had
diaphragms made of a carbon fiber composite.
Another compression driver configuration is the
screw-on driver. The University 7110XC (explosion
proof) is shown in Fig. 17-13. This type of unit is often
used on a reentrant horn in public address systems.
Throat diameter is usually ¾ inch. Diaphragms are most
often made of phenolic resin-impregnated domes with
integral convoluted suspensions. Voice coils are usually
round copper wire.17.4 Electrostatic TransducersElectrostatic transducers make use of the fact that two
static electrical charges placed at a distance from each
other will experience a force directed along a line
between them. The force is attractive if the charges have
the opposite sign (positive and negative) and repulsive
if the charges have the same sign (positive and positive
or negative and negative). In practical loudspeaker
designs, the forces are attractive, due to the complemen-
tary nature of the charge transfer from the amplifier
output to the speaker plates. The magnitude of the force
is inversely proportional to the distance between the
charges and directly proportional to the magnitude of
the charges.
A typical electrostatic loudspeaker consists of a
diaphragm made of two pieces of metallic foil separated
by a sheet of dielectric, or nonconductive, material. By
itself, the application of a pure ac signal (i.e., one with
no dc component) to an electrostatic loudspeaker would
cause attractive forces for both positive- and nega-
tive-going signal excursions, since the induced charges
are opposite in both cases. This would create a
frequency-doubled signal containing extremely high
levels of harmonic distortion.
For this reason, a dc polarizing voltage is applied to
the foil diaphragms, maintaining a steady attraction
between them. The audio (ac) signal is superimposed on
this dc offset, modulating the attractive force. InFigure 17-11. Community M4 4 inch throat midrange
driver. Courtesy Community Professional Loudspeakers.
Figure 17-12. Community M4 cross-section view. Courtesy
Community Professional Loudspeakers.
Figure 17-13. University 7110XC ¾ inch throat public
address driver.