Handbook for Sound Engineers

Loudspeakers 607

stationary one or when a suspension element is made
unacceptably nonlinear (either temporarily or perma-
nently) by deformation beyond its design range. Elec-
trical displacement limiting occurs when the motor is
operated outside its range of linear travel. This is a func-
tion of the length of the windings on the voice coil and
the thickness of the plates that form the magnet gap.
Fig. 17-24 shows three typical voice coil configura-
tions: equal length, overhung, and underhung coils.

When any of these coils reaches a displacement that

causes a reduction in the current sensitivity of the

motor, higher distortion will result.

It has been empirically determined that, due to a

magnetic fringe or leakage field at the pole tips, an

excursion of 15% farther than the gap length results in a

reasonable distortion level (approximately 3% harmonic

distortion at low frequencies). The equal length voice

coil, Fig. 17-24C, has the greatest potential for

motor-generated distortion. However, it also yields the

highest motor strength (the greatest total conductive

mass in the highest density magnetic field). The equal

length voice coil is a common configuration for

compression drivers, where maximum excursion is

intrinsically low. The underhung coil, Fig.17-24B,

allows greater excursion but requires a larger magnet

due to the longer gap. For moderate flux density levels

(10,000 to 15,000 G), this design, as compared to the

equal length design, requires approximately twice the

magnet weight (twice the area and the same length) for

a doubling of the gap length. This approximately

doubles the excursion capacity, giving four times the

acoustic power output capability (6 dB) for a doubling

of magnetic weight (3 dB). The overhung coil, Fig.

17-24A, is capable of the greatest motor linearity, all

else being equal. It is commonly seen on woofers used

as direct radiators, where higher excursion is required.

The major disadvantage here is that the coil that is not

in the gap does not participate in transduction. The extra

coil length does add both mass and dc resistance,

however, reducing motor efficiency. In spite of this,

there are numerous examples of successful commercial

woofers using overhung coils. The transducer designer

must take into account the often conflicting demands of

high-efficiency, high-output, and low-frequency exten-

sion to arrive at an optimum design for a given range of

applications.

The thermal limit of a magnetic loudspeaker motor is

a function of the temperature limits of the materials

used and heat transfer from the coil assembly to the

outside world. Most adhesives used in the loudspeaker

industry have an upper limit between 120°C and 177°C

(250°F and 350°F). Some epoxy adhesives will tolerate

higher temperatures, but they can require special curing

processes and are therefore potentially more difficult to

use. Wiring insulation may tolerate temperatures as high

as 218°C (425°F). Anodized aluminum wire has the

melting point of aluminum as a limit. Voice coils oper-

ated at high temperatures have higher resistance. A 1°C

rise produces approximately a 0.4% rise in dc resistance

in both copper and aluminum. Therefore, operating a

voice coil 100°C above ambient (127°C or 261°F) will

Figure 17-23. Position of the SDL diaphragms on a folded
horn.

Figure 17-24. Three basic voice coil/magnetic gap
configurations.

C. Equal length coil and gap.

A. Overhung coil.

B. Underhung coil.