JBL 4671, 4673, 4672A, 4674A, 4670B Instruction Manual

Instruction Manual

Motion Picture Loudspeaker Systems:

A Guide To Proper Selection And installation

A. Introduction:

B. Systems for Smaller Houses:

In recent years, JBL has emerged as the predomi-
nant manufacturer of loudspeaker systems for the mo-
tion picture theater. Our products are to be found in the
most prestigious screening rooms and dubbing theaters
in Hollywood, including Warner Hollywood Studios, Glen
Glenn Sound, The Burbank Studios, Universal Studios,
and Todd-AO. The recent installation of a JBL system in

the Goldwyn Theater of the Academy of Motion Picture
Arts and Sciences represents a benchmark for our com-
pany. Our components have also been selected by
Lucasfilm, Ltd., for their THX theater system.

There are several reasons for JBL's success in mo-
tion picture sound. Foremost here is our concept of flat
power response, which results in an order of improve-
ment in sound coverage in the theater. Second, our
components are of proven reliability and have extended,
inherently smooth response. In addition, we manufac-

ture components for all aspects of theater sound. JBL's
systems are very cost effective when compared to the
vented horn/multicell designs of earlier years.

This manual is intended for both theater dealers and

prospective purchasers of theater systems. It will be use-
ful as a guide to the selection and installation of systems,
and it will provide considerable theoretical background
for the specifying engineer.

1.

A Summary of House Types:

By far, most of the theaters constructed today are
relatively small rectangular rooms. They are usually built
in groups known as multiplex theaters for economic rea-

sons.

Most of these theaters are monophonic, with a

sin-
gle loudspeaker behind the screen, and seating capacity
ranges from 200 up to 500.

2.

General Acoustical Characteristics:

Motion picture theaters are generally acoustically

"dead"

rooms, with reverberation time, even in the

larg-
est houses, not exceeding 1.25 seconds. In most multi-
plex theaters, reverberation times are on the order of 0.5
seconds. The short reverberation time ensures clear dia-
log throughout the house.

The most common interior treatment of theaters is
double-fold velour, which may be placed on the side and
rear walls. This will provide freedom from unwanted re-
flections, but it does nothing to isolate one multiplex
space from the adjacent one. Here, the architect must
rely on solid block walls for good isolation.

Any unusual acoustical problems should be referred

to a qualified acoustical consultant.

3. Sound Level Requirements:

Normal peak levels of 85 dB SPL are common in the
theater. A minimum headroom level above this figure
would be 10 dB; that is, the system should be capable of
handling peaks up to 95 dB-SPL Wherever possible,
JBL recommends that a headroom figure of 15 dB be
designed into the system.

The following factors have been taken into account
in determining system and amplifier choice for various
house sizes:

a. Room volume
b. Room boundary
c. Reverberation time
d.

System sensitivity and power rating

e. System directivity index

4.

Recommendations for Various Size Theaters:

A. For theaters seating up to 200 patrons; volume
approximately 1700 cubic meters (60,000 cubic feet):

*Preferred system: Model 4673 (direct radiator LF)

Power needed for 100 dB: 46 watts

Recommended power amplifier: JBL 6230 in
bridged mode (300 watts into 8 ohms)

Characteristics: Broad-band sensitivity 97 dB, one
watt at one meter; 500-Hz crossover into Bi-Radial horn;
integral power response correction; smooth response
down to 45 Hz; ideal quality for both dialog and music;
shallow profile.

* Alternate system: Model 4671 (direct radiator LF)

B. For theaters seating up to 350 patrons: volume

approximately 2800 cubic meters (100,000 cubic feet):

*Preferred system: Model 4670B

Power needed for 100 dB: 30 watts

Recommended power amplifier: Single section of
JBL 6230 (75 watts into 4 ohms, the impedance of the
low-frequency section of the 4670B system.)

Characteristics: Sensitivity, 100 dB, one watt at one
meter; 500-Hz crossover for most natural dialog and
music quality; integral power response correction; LF
response smooth down to 45 Hz; shallow profile

'Alternate vented horn systems: Models 4672A and
4674A

Power needed for 100 dB: 30 watts

Characteristics: Mid-band sensitivity, 103 dB, one

watt at one meter; LF response limited due to horn

load-

ing;

to be specified where available amplifier power is

limited (typically, 50 watts or less)

C. For theaters seating up to 500 patrons: volume

approximately 4500 cubic meters (160,000 cubic feet):

'Preferred system: Model 4670B

Power needed for 100 dB: 50 watts

Recommended power amplifier: single section of

JBL 6260 (300 watts can be delivered into 4 ohms, the
impedance of the low-frequency section of the 4670B
system)

Characteristics: Sensitivity, 100 dB, one watt at one
meter; 500-Hz crossover for most natural dialog and
music quality; integral power response correction;
smooth response down to 45 Hz; shallow profile

Power needed for 100 dB: 46 watts
Recommended power amplifier: JBL 6230 in

bridged mode (300 watts into 8 ohms)

Characteristics: Broad-band sensitivity, 97 dB, one
watt at one meter; 800-Hz crossover with integral power
response correction; smooth response down to 45 Hz;
slightly less realistic on dialog than recommended sys-
tem;

shallow profile

'Alternate vented horn system: Model 4674A

Power needed for 100 dB: 50 watts
Characteristics: Mid-band sensitivity, 103 dB, one

watt at one meter; LF response limited due to horn

load-

ing;

should be specified only where available amplifier

power is limited (100 watts or less)

* Alternate vented horn systems:

Models 4672A and 4674A

Power needed for 100 dB: 23 watts

Characteristics: Mid-band sensitivity, 103 dB, one
watt at one meter; LF response limited due to horn

load-

ing;

to be specified where available amplifier power is

limited (typically, 50 watts or less)

The systems discussed above are shown in Figure

1

A, and typical free-field response curves are shown at

B and C.

2

FIGURE 1. SYSTEMS FOR SMALLER HOUSES
FIGURE 1A. LINE DRAWINGS

4671

PMAX =

150 W

SENSITIVITY:

97 dB,
1 W, 1 M

4673

PMAX =

150 W

SENSITIVITY:

97 dB,

1 W, 1 M

4670B

PMAX =

300 W

SENSITIVITY:

100 dB,
1 W, 1 M,

4672A

PMAX =

150 W

SENSITIVITY:

103 dB,
1 W, 1 M

4674A

PlVlAX =

150 W

SENSITIVITY:

103 dB
1 W, 1 M

FIGURE 1B. ON-AXIS RESPONSE OF 4670B

(2TT

LOADING);
HIGH-FREQUENCY POWER RESPONSE
CORRECTED.

3

dB

Relative

Response

Frequency in Hz

FIGURE 1C. ON-AXIS RESPONSE OF 4674A

(2TT

LOADING);
HIGH-FREQUENCY POWER RESPONSE
CORRECTED.

5. Installation

of

the systems:

FIGURE 2. MOUNTING THE SYSTEM

A. General Comments:

A. Position Behind The Screen

JBL's theater systems

are

shipped

to the

dealer

in

component form. Every attempt

is

made

to

ship

the

components complete so that the systems can

be

readily

assembled.

All

components should

be

inspected prior

to

being sent

out to

the job site. Wiring instructions

for all

theater systems

are

included

in the

dividing network

package.

Many dealers will want

to

take advantage

of

the fact

that certain JBL theater low-frequency systems

are

available loaded from

the

factory. These

are:

4647 (consisting

of

4507 loaded with 2225H)

4648 (consisting

of

4508 loaded with

2-

2225H)

Further,

the

4671

OK

is a

factory preassembled ver-

sion

of

the utility system with

an

oak vinyl enclosure

and

brown grille cloth.

V3

•SCREEN

B. Physical Considerations:

, ABSORPTIVE
' MATERIAL

STAND

IF

NECESSARY

MOUNTING
OF SCREEN
LOUDSPEAKER

Mounting

the

system behind

the

screen

is

shown

in

Figure 2A.

The

preferred location

is

two-thirds the dis-

tance

up the

screen, and this

may

require a platform.

Details

of

this

are

shown

at B.

4

LF

Horn Loading

LF

Reflex Loading

dB,

Relative Response

Frequency in

Hz

EL Platform Details

Frame dimensions
slightly larger than
bottom of enclosure.

Top covered
V2"

thick plywood.

Frame and legs, 2x4
lumber
Braces 1 x 4 lumber.

Bracing required on
four sides.

All JBL theater systems must be oriented as shown

in Figure 1 A. In particular, the models 4670B and 4675A
should NOT be assembled with the low-frequency enclo-
sure lying on its side.

FIGURE 3- INSTALLING TRANSDUCERS
AND COMPONENTS IN SYSTEMS

The loudspeaker should be placed on a dense foam
rubber pad or section of carpet so that it cannot possibly
rattle when driven at high levels. When the loudspeaker
is placed on an elevated support, it is advisable to
anchor the loudspeaker with four or more angle irons so
that it cannot possibly shift position.

The back wall behind the screen should be covered

with a sound absorptive material.

1.

High-frequency Horn Aiming:

Figure 3 shows details of mounting the components
in the various systems. Note that the high-frequency
horns are aimed straight out into the theater. JBL has
found that this mounting will be best for most installa-
tions for the reasons shown in Figure 4A. Since the off-
axis response of our new horns is quite uniform out to 12
kHz, we can trade off the inverse square losses in the
room with the off-axis horn losses to provide more even

front-to-back coverage in the seating area.

ANGLE IRON

5

A.

FIGURE 3, Cont'd

6

HORN/DRIVER

SCREWED IN PLACE

PANEL REMOVED

B.

C. 4 WOOD SCREWS*

INTO FRONT EDGES

OF LF ENCLOSURE

"Use #10x2 flat-head Philips wood screw; pre-drill holes with Va" bit.

D. MOUNTING LF TRANSDUCERS IN 4507, 4508,

AND 4518

E. MOUNTING LF TRANSDUCERS IN 4550BKA

AND 4560BKA

FIGURE 3, Cont'd

H. (See note in text concerning polarity)

INCLUDED

WITH

NETWORK

INSERT
T-NUTS

#6
SCREWS

TERMINAL BOARD

PLATE REMOVED
FROM INSIDE

F.

G.

For 4671,4672A, 4673, 4674A:

For4670B:

3115A
311 OA

J.

7

FIGURE 4. PROPER AIMING OF HIGH-FREQUENCY HORNS
A. INVERSE SQUARE, OFF-AXIS TRADE-OFF

0° (Maximum output)

FIGURE 4B. CADP DISPLAY; HF HORN AIMED STRAIGHT AHEAD

Screen!

HF Horn
10'

high

12'

2kHz

8