Handbook for Sound Engineers

606 Chapter 17

pleated diaphragm that is forced in the magnetic field.
In another implementation, continuous lengths of wire
are bonded to a large panel of Mylar™, which is oper-
ated over a field of bar magnets. The leaf tweeter is
similar, etching a conductor field on Mylar™. They are
identical in principle to Fig. 17-19 and are discussed
more thoroughly elsewhere in this text. The ribbon
loudspeaker, Fig. 17-20, is a special case in which the
voice coil serves as both conductor and diaphragm.

One notable departure from conventional linear
motor design is the Servo-Drive loudspeaker. This
patented drive system uses a rotary servomotor that
drives a woofer cone and suspension assemblies via a
pulley-belt mechanism, alternately pushing and pulling
the diaphragms in response to the input signal. Two
opposing diaphragms are driven in a push-pull arrange-
ment so as to yield a balanced axial force on the drive
mechanism. The motor is configured so that it presents
a typical impedance load to an amplifier. SDL (for
servo-drive loudspeakers) speakers come in a variety of
sizes and power capacities, but typically they are in the
form of low-frequency horns. Fig. 17-21 shows the

mechanism employed to translate rotational motion of

the servomotor to the linear motion needed to drive the

opposing diaphragm assemblies. The opposing rein-

forced blastomeric belt mechanisms are used in the rota-

tion-to-linear conversion, and the result is noiseless and

free of slip. The opposing diaphragms drive the throats

of conventional wood-fabricated folded bass horns. The

positions of the diaphragms are shown in Fig. 17-22.

Fig. 17-23 shows the positioning of the servo-driven

diaphragms in a typical folded bass horn.

17.6.1 Output Limitations

The maximum usable output of an electromagnetic

loudspeaker is a function of a number of parameters,

including diaphragm displacement, heat transfer, sound

quality (maximum acceptable nonlinearity), and/or wear

life due to fatigue of moving parts.

There are two fundamental limitations on a magnetic

driver, a displacement limit and a thermal limit.

Displacement limits may be caused by either mechan-

ical or electrical factors. Mechanical displacement

limiting occurs when a moving part contacts a

Figure 17-19. Cross section of a typical cone loudspeaker
showing construction (alnico magnet at center under pole
piece). Courtesy JBL.

Figure 17-20. Ribbon loudspeaker.

Input

current

Ribbon in air gap (may be corrugated or taut)

Force and ribbon

diaphram motion

Magnet

poles Figure 17-21. Belt drive system of the SDL loudspeaker.

Figure 17-22. Position of SDL belt drive and opposing

diaphragms.

Linear drive to

diaphragm #1

Rotational input drive

from servo motor

Linear drive to

diaphragm #2

Drive belts

Servomotor

Belt drive

mechanism

Diaphragms