Handbook for Sound Engineers

610 Chapter 17

17.7.1 Direct Radiators

The simplest form of radiator is the direct radiator, in

which the diaphragm is directly coupled to the air. Most

hi-fi loudspeakers consist of combinations of different

sizes of direct radiators. Various forms of direct radia-

tors were described in the previous section. In this

section, we will outline their acoustic attributes.

17.7.2 Cone Radiators

In a cone radiator, the diaphragm is in the shape of a

truncated cone. The concave surface of the cone is

usually, but not always, the one which radiates sound.

The cone shape is partially dictated by expediency: it

allows the magnet structure to reside at the rear of the

transducer assembly, while at the same time allowing

for the use of a spider and a surround to suspend the

diaphragm. This dual-element suspension provides

positive centering of the voice coil in the magnet gap,

and it helps constrain the motion of the cone to the

desired linear path.

The above notwithstanding, there are also acoustic

motivations tend to favor the cone shape. At first

glance, one might expect a flat piston to offer superior

on-axis response and directivity to a cone. A cone shape

is generally preferable, however, when the excitation

will be applied near the center of the diaphragm. Due to

the fact that sound propagates at a finite velocity in a

solid, the motion of the outer portion of the diaphragm

will follow the initial excitation by some amount of

time. If the diaphragm were flat, radiation from the

outer portions of its surface would arrive at an on-axis

observation point at later times than radiation from the

center. The cone shape reduces the distance must be

traveled by sound radiated from the outer portions to

on-axis listening positions. Since the velocity of sound

in the cone material is typically greater than the velocity

of sound in air, a cone shape having the optimum

included angle will tend to synchronize on-axis radia-

tion from the outer portions of the diaphragm with that

from near the center. Given a judicious choice of angle,

the useful range of response of a cone transducer can be

extended to a significantly higher frequency than would

otherwise have been the case.

17.7.3 Dome Radiators

Another variant on the direct radiator theme is the dome

radiator. Most often, this radiator takes the form of a

convex dome driven and suspended at its periphery. The

material used to form the dome may be soft, as is the

Figure 17-26. Various coil/gap geometries showing the
evolution of heat transfer designs in modern woofers.

Heat escapes at top

and bottom of coil

Thermo-inductive ring

(EV’s “TIR”) aluminum

pole extension

Heat escapes here

from top of coil

Aluminum

Aluminum pole

extension

C. Improved full-coil air conduction

in EV “EVX” woofers.

A. Straight pole heat transfer.

B. Aluminum pole extension on EV “DL” woofers.

D. JBL “vented gap” natural forced-convection

heat transfer.