Robinson-Dadson Equal Loudness Contours; Number O F Modules - JBL 4671 Instruction Manual

The following table indicates the acoustical power
output which can be expected in two sizes of theaters.

NUMBER O F MODULES

1
2
4
8
It is essential that only the most stable amplifiers be
used for subwoofer application. The transducers can be
paralleled for 4-ohm operation.
Substantial sound pressure levels are necessary at
very low frequencies to match the 85 dB reference level
in the theater, as the equal loudness contours shown in
Figure 14B indicate. Note that with each doubling of
transducers, the maximum sound pressure level
increases 6 dB. Three dB are due to doubling the power
FIGURE 14B. ROBINSON-DADSON EQUAL LOUDNESS CONTOURS
F. Acoustical Response: The Concept of
Flat Power Response
While loudspeaker systems are fairly flat on axis,
they tend to narrow considerably in their coverage
angles at high frequencies. Their power response rolls
off naturally. If a loudspeaker maintains fairly constant
horizontal and vertical coverage angles over most of its
MAXIMUM
EFFICIENCY ELECTRICAL
POWER INPUT
300 WATTS
2 %
600 WATTS
4 %
8 % 1200 WATTS
1 6 % 2400 WATTS
MAF
CURVE
FREQUENCY IN Hz
ACOUSTIC
500-
POWER
1000 SEATS
6 WATTS 101 d B
24 WATTS 107 d B
96 WATTS 113 d B
384 WATTS 119 d B
input capability, and another three dB are due to the
increase in efficiency resulting from mutual acoustical
coupling between transducers.
Subwoofer loudspeakers should be located fairly
close together, and they should be placed at the inter-
section of a wall and floor. Careful bracing and mounting
on rubber or Neoprene pads may be necessary to mini-
mize rattling.
LOUDNESS
L E V E L (PHON)
frequency range, then flat on-axis response will also im-
ply 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.
S O U N D LEVELS
1000-
2000 SEATS
100 d B
106 d B
112 d B
118 d B
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