User Guide

Sound System Design Reference Manual
single loudspeaker will, at a distance of 7 meters,
produce a level of 105 dB. The added contribution of
the eight neighboring loudspeakers will increase this
by 3 dB, making a maximum level capability of 108
dB. Level variations will be 1.4 dB.
Because of the wide-band capability of the
loudspeakers and relatively high power required, a
low impedance distribution system should be used.
Each 8-ohm loudspeaker should be driven from a
section of a JBL MPX 300 amplifier, making a total of
6 amplifiers. This will provide 200 watt capability into
each loudspeaker, which will more than exceed the
specification.
JBL Professional provides a program for
determining layout density for distributed ceiling
loudspeakers. It is called Distributed System Design,
version 1.1, and runs on Windows 95 and is
available from JBL Professional.
3. Zoning requirements: Measure the average
distance from center stage to a center listening
position directly under each zone. Subtract from that
the value of 7 meters. For each meter difference,
calculate 3 milliseconds of delay:
Zone Difference Delay
1 negligible 0 msec
2 12 meters 36 msec
3 20 meters 60 msec
4 26 meters 78 msec
In normal cases, the calculated delay values
will have to be adjusted slightly on-site for best
overall sound quality.
3.2 Meeting Room System:
1. Use the hexagonal array, with center-to
center overlap, for best coverage. Twelve
loudspeakers will be required, and the JBL Control
26C/CT will be specified because of its nominal
coverage angle of 110°. With a sensitivity of 89 dB,
one watt input will produce a level of 83 dB at a
distance of 2 meters (ear height). With one watt fed
to the six nearest neighboring loudspeakers, the
level will increase to 87 dB, which is 2 dB higher than
the design requirement.
It is obvious that normal usage will require only
about one watt per loudspeaker. However, we should
provide the system with a nominal 10 dB headroom
factor for undetermined applications. The total power
in this case would be 120 watts, and a single section
of a JBL MPX300 amplifier, operating in series-
parallel, would more than meet this requirement. The
resulting load impedance of 12 ohms could easily
accept 120 watts from one section of the MPX300
amplifier.
3.3 Social Area System:
1. The foreground stereo system is shown as
alternating L and R loudspeakers around the main
bar area. The JBL Control 5 would be a good
candidate for this application. In order to see if 12 of
the units can meet the specification, we will pick a
point midway in the room (marked X) and sum the
individual levels of the loudspeakers at that point.
Taking the 1-watt, 1 meter sensitivity as a reference
point, we can set up a table as follows for summing
the individual contributions:
7-19
Spkr Distances S - 20 log Dist = Net level
11089-20=69
>73
2 8 89 - 18 = 71
>77.2
3 7 89 - 17 = 72
>74.5
4 8 89 - 18 = 71
>81
5 8 89 - 18 = 71
>75.7
6 6 89 - 15 = 74
>76.3
7 5 89 - 14 = 72
>77.5 Total: 82 dB
8 6 89 - 15 = 74
9 8 89 - 18 = 71
>74.5
10 7 89 - 17 = 72
>77.5 >77.5
11 8 89 - 18 = 71
>74.5
12 7 89 - 17 = 72