Handbook for Sound Engineers

Loudspeakers 601

for this coupling include possible increases in certain
distortion components and the requirement for a horn.
Compression drivers are not used as direct radiators.
The purpose of the phasing plug is to equalize path
lengths from the diaphragm surface to the exit. To the
extent that this is accomplished, the useful bandwidth of
the driver will be extended upward in frequency.
Fig. 17-8 is a cross-sectional view of a typical
ceramic magnet wide-range compression driver using a
dome diaphragm. The case construction is unusual and
peculiar to this design by Yamaha. The phase plug is
also a bit unusual; however, it is still of the circumferen-
tial-slit variety on the phase plug (dome) surface. The
diaphragm is aluminum and is supported by a typical
Bakelite™ or plastic support frame. The back cap has
sound-absorbing material inside to discourage inter-
fering air resonances in the cap.

Fig. 17-9 shows a 2 inch throat JBL driver, model
2440, using an alnico magnet. The phase plug is more
typical than that in Fig. 17-8, using more straight
through circumferential slits. The JBL plug is made of
cast Bakelite.

Fig. 17-10 shows another alnico driver, the 1 inch

Altec 802/808. The 802 uses an all-aluminum

diaphragm with tangential surround coupled via the

phase plug and expanding throat section to a 1 inch

diameter exit. The 808 is identical to the 802 in all

respects except for the diaphragm. From left to right in

the exploded view are the pot, the alnico magnet slug

that fits under the pole piece, in which is mounted on a

radial-slit tangerine phase plug. This unusual design is

made from glass fiber–filled plastic and is bonded to the

pole piece. Above this (to the right) is the ring that

centers the pole piece in the air gap via the top plate. It

is nonmagnetic (brass), and the holes provide a mechan-

ical load on the voice coil, which affects response and

distortion. Next are the diaphragm assembly and rear

cap.

The preceding three examples are representative of

dome diaphragm compression driver design practices,

both in concept and in practical implementation.

A large number of variations exist in the art including

a wide variety of suspension shapes and materials.

Domes are made from aluminum, titanium, and beryl-

lium. Yamaha International Corp. makes its suspension

Figure 17-7. Technics Leaf Tweeter diaphragm detail. Cour-

tesy Panasonic Industrial Corp.

Figure 17-8. Typical ceramic-magnet compression driver.

Courtesy Yamaha International Corp.

Terminals Printed voice coil

Yoke

Polyimide film

Pole piece Magnet

Rear

housing

Damping

material

Diaphragm

support

Voice coil

& form

Back plate

Ferrite

magnet

Front

plate

Front

housing

Diaphragm Connection terminal

Aluminum

spacer

Fiber

gasket

Aluminum

centering ring

Die-cast and

alloy phasing

plug & steel pole

piece

Cork gasket

Screen Gasket

Figure 17-9. JBL 2440 2 inch throat alnico compression

driver. Courtesy JBL/UREI.

Figure 17-10. Altec 802/808 alnico 1 inch throat compres-

sion driver with Tangerine radial phase plug. Courtesy Altec

Lansing Corp.