Handbook for Sound Engineers

Loudspeakers 611

case with coated synthetic textiles, or hard, in the case

of metals or composite materials (e.g., carbon

fiber/epoxy). Dome radiators are most popular for high-

frequency elements, although a number of dome-shaped

midrange elements are also available. As with the cone

radiator, the convex shape of the typical dome radiator

has acoustic motivations. Since the excitation is at the

edge, the dome’s mechanical motion will propagate

inward. As a result, if the shape were flat, radiation

from the inner portion would arrive at an on-axis obser-

vation point later than radiation from the edge. The

convex shape helps deliver a more coherent wavefront

to an on-axis listening position. It is common practice to

suspend a small round cover just in front of the center of

the dome.

17.7.4 Ring Radiators

Yet another form of direct radiator is the ring radiator. In

a typical ring radiator, a flexible ring-shaped diaphragm

is rigidly captured along its inner and outer circumfer-

ences and driven along a concentric circular line

between those two circles. There is, then, no distinction

between the diaphragm and the suspension, as a single

part fills both functions. A dome tweeter with a cover

over the center of the dome functions as a ring radiator.

Ring radiators can also be used to drive short horns.

The JBL 075 Bullet is an example of a ring radiator.

Intended for use above 7 kHz, the diaphragm is a

V-shaped ring of aluminum attached to a voice coil and

former.

Fig. 17-27 shows a ceramic version of the ring,

which is made by Yamaha. The phase plug is a simple

slit ending in a large enough mouth to project the

desired low end of the driver. The suspension is the

diaphragm itself, and it is quite stiff. Ring radiators are

typically operated in or above the principal resonance

frequency of the diaphragm assembly.

17.7.5 Panel Radiators

Both electrostatic and planar electrodynamic speakers
fall into this category. As with the ring radiator, there is
a mixing of functionality between the diaphragm and
the suspension. The acoustic advantage, at least in prin-
ciple, of a panel radiator, is that the driving force is
applied uniformly over a large portion of the diaphragm.
For this reason, diaphragm rigidity is not an essential
design element, as is the case with cone radiators. An
interesting characteristic of a large panel radiator is that
it will essentially project a shadow of its shape as a
listening pattern; this shadow of the speaker’s radiation

pattern will take up a large part of a typical listening

area, particularly at close listening distances. This is

claimed to produce a wider “sweet spot” compared to

conventional cone systems.

17.7.6 Horns

Horns are used to increase the efficiency of a transducer

and to control the directivity of the sound that is radi-

ated. Horns are characterized by a number of parameters.

The earliest approach to a predictive model, and the one

still employed in acoustics texts, is characterization by

the rate of increase of cross-sectional area with longitu-

dinal position in the horn. Other means of characteriza-

tion are related to the shapes formed by the horn walls.

Of all possible expansion (or flare) rates, a relative

few have found use in horn design and analysis. Those

most commonly encountered are exponential, hyper-

bolic, conic, and catenary. In general, the change of

cross-sectional area with position in a horn can be

expressed as

(17-4)

where,

A(x) is the cross-sectional area at a point x along the

axis of the horn,

F(x) is some function of x.

For example, in an exponential horn,

(17-5)

where

A 0 is the area of the horn at its throat or entry,

m is a constant called the flare rate.

Figure 17-27. Yamaha ceramic-magnet ring radiator cross

section. Courtesy Yamaha International Corp.

Aluminum Diffraction plug

ring radiator.

Paper voice

coil form and

edgewound

coil

Front housing

horn

Aluminum

shorting ring

Front plate

Ferrite magnet

Backplate/

polepiece

Rear housing

Connection terminal

Ax =Fx

Ax =A 0 emx