handling, and the remaining three come from
mutual coupling, the tendency of two loudspeakers
to behave as a single, more efficient loudspeaker.
F. Acoustical Response:
1. Concept of flat power response:
to ship the components complete so that the
systems can be readily assembled. It is a wise
dealer, however, who will keep a good stock of
replacement components and mounting hardware.
The systems do not travel well in their assem-
bled form, so they should always be assembled at
their final site. All transducers should be checked
prior to being sent out, however.
While most loudspeaker systems are fairly
flat on axis, they tend to narrow considerably in
their coverage angles at high frequencies. Their
power response is said to roll off. If a loud-
speaker maintains fairly constant horizontal and
vertical coverage angles over most of its fre-
quency range, then flat on-axis response will also
imply flat power response. Since about half of the
sound heard in the theater has been reflected at
least once, it is important that loudspeakers
exhibit fairly flat power response if the reproduced
sound is to be natural.
Another consequence of using loudspeakers
with flat power response is that little system equal-
ization is required. Figure 7 illustrates this. At
A, we see the boundary absorption in a typical
well-designed theater. Note that it is quite flat
over the frequency range. High-frequency losses
due to air absorption are shown at B, and typical
high-frequency screen losses are shown at C.
When we add up all of these, we get the response
shown at D, and this is exactly the acoustical
response we would observe in the room if we
were using a loudspeaker that had flat power
response.
Figure 8 shows the standard "house curve"
to which most theater systems are equalized.
Note that it closely resembles the summed
curve shown in Figure 7,D, indicating that flat
power response systems will require little, if any
added equalization in most houses.
JBL's theater systems using Bi-Radial horns
exhibit essentially flat power response as well
as flat on-axis response. The smaller systems
exhibit flat on-axis response, but their power
response rolls off slightly at high frequencies.
JBL's passive dividing networks provide for a
high-frequency boost which effectively makes any
of the systems power-flat at high frequencies.
In the smaller systems, this may be accompanied
by a slight on-axis rise at high frequencies.
Nevertheless, we recommend that the high-
frequency boost be used in all theater applica-
tions.
H. System Assembly:
2. Details of assembly:
We present in Figure 9 detailed wiring dia-
grams of each of the theater systems, along with
a complete listing of components in each system.
Figure 10 shows details of mounting LF trans-
ducers in the various 4500-series LF enclosures.
The models 4507, 4508 and 4518 all require front-
mounting of transducers. There are T-nuts
mounted on the back side of the baffle, and the
four mounting screws for each transducer engage
these as shown at A.
Both the 4550BKA and 4560BKA have
demountable back panels. These are removed and
the LF transducers mounted in place with screws
engaging T-nuts, as shown at B.
High-frequency horns are mounted as shown
in Figure 11. The 2370 horn-2425 HF driver com-
bination is placed directly on top of the 4507 LF
enclosure, and its mouth is parallel with the front
of the LF enclosure. It should be mounted flush
with the front of the LF enclosure. Two angle irons
may be used to secure the front of the horn to the
enclosure, as shown at
A.
An alternate way to
mount the smaller Bi-Radial horns (2370 and 2380)
is to screw the bottom front flange into the upper
front edge of the LF enclosure, as shown in Figure
11C. The user should note in general that Bi-Radial
horns, because of their extended vertical coverage
at high frequencies, do not normally have to be
tilted with respect to the LF enclosure in order to
provide proper coverage of the audience area.
When the 4560BKA enclosure is used, the
2370 horn is mounted in a space provided at the
top of the LF enclosure. The upper panel is
removed, and the horn is screwed into place, as
shown at B.
The 2380 horn is mounted on top of the LF
enclosure in the 4670A, 4673, and 4674A systems.
With the 2445 HF driver in place, the horn-driver
combination sits on top of the enclosure with its
mouth parallel to the front of the enclosure. As
in the case shown at A, the front of the horn
should be secured to the enclosure top with angle
irons, or by screwing into the front edge of the LF
enclosure.
1. General comments:
JBL's theater systems are shipped to the
dealer in component form. Every attempt is made
