User Guide

Sound System Design Reference Manual
The three HF sections in each main array
should be powered by one section of an MPX600
amplifier (200 watts into 16 ohms).
Each one of the four main arrays should also
have an SP128S subwoofer module. These would
be powered the same way as the LF sections of the
SP128S units. Level calculations are given:
Level Power Distance
102 dB 1 W 1 meter
133 dB 1200 W 1 m
113 dB 1200 W 10 m
The electrical diagram for the main array is
shown in Figure 7-8.
3.2 Delay Rings:
The first delay ring of loudspeakers should be
composed of fairly robust loudspeakers, such as the
8340A. Each one of these eight loudspeakers has a
far throw range of about 6 meters and can
accommodate 150 watts input. We can calculate the
level output of each one as follows:
Level Power Distance
96 dB 1 W 1 meter
118 dB 150 W 1 m
102 dB 150 W 6 m
As an ensemble, these eight loudspeakers will
provide added high frequency coverage of the
balcony seats, ensuring good articulation.
The under balcony soffit system consists of 16
transducers located 2 meters above the seated
listener’s ear height. The model 2142 has a
sensitivity of 97 dB and a power rating of 90 watts.
A seated listener directly under one of these
transducers, if it is powered by 1 watt, will hear a
level of 91 dB. A listener mid-way between a pair of
them will hear a level of about 90 dB. The 90-watt
per transducer rating means that peak levels of
about 110 dB can be developed under the balcony.
Case Study B: A Distributed Speech
Reinforcement System for a Large
Liturgical Church
1. General Information and Basic Performance
specifications:
The system to be studied here is typical of what
may be found in just about every large religious
edifice in Europe and in many large cities in the U. S.
The plan and front section views are shown in Figure
7-10. The building under consideration here has an
internal volume of 12,000 cubic meters and surface
area of 4000 square meters. A mid-band empty
house reverberation time of 2.5 seconds indicates a
value of 800 square meters of absorption units (Sα),
as extrapolated in Figure 5-10.
Our major concerns with a system in such a
space as this are the net speech direct-to-
reverberant ratio and the reverberation time itself.
Each loudspeaker will contribute to the overall
reverberant level behind the amplified speech, and
our first step is to determine the number of
loudspeakers that will be required to cover the entire
seating area. Studying the plan view of the building,
we can see that 8 loudspeakers will cover the
transept seating, while 10 systems will cover the
nave seating.
2. Analysis:
The longest “throw” that will be required of any
single loudspeaker is to cover a listener seated at
the center aisle, a distance of about 7 meters. Let
us now specify a JBL Control 28 and power it to
produce a level of 85 dB at a distance on-axis of 7
meters.
We can do this directly by setting up the familiar
level/power/distance chart as follows:
Level Power Distance
92 dB 1 W 1 meter
75 dB 1 W 7 m
85 dB 10 W 7 m
We now want to make an estimate of the
reverberant level that will exist in the room when the
direct sound from a single loudspeaker at the listener
is 85 dB. To do this, we must determine the efficiency
of the loudspeaker. Taking data from the Control 28
specification sheet, and averaging the DI over the
200 to 2000 Hz range, we now use the following
equation:
Sensitivity (1 W @ 1 m) = 109 + DI + 10 log Efficiency.
7-12