User Guide

Sound System Design Reference Manual
1. That when a single loudspeaker produces a
level of 85 dB SPL at the farthest listener, the
resulting reverberant level is 96 dB SPL.
2. That the mid-band reverberation time in the
room is 2.5 seconds.
As our final step in the analysis, we can check
the probable system speech intelligibility
performance, according to Peutz’ Articulation loss of
consonants (Al
cons
) by means of the chart shown in
Figure 6-13.
Locating a direct-to-reverberant ratio of -11 dB
along the bottom axis, and then moving up to the
horizontal line corresponding to a reverberation time
of 2.5 seconds, we see that the system’s
performance is slightly above the borderline of 15%
Al
cons
. This indicates that we have barely succeeded
in our design goal of acceptable performance. In
other words, our analysis has shown that we have
marginal acceptability in terms of speech intelligibility.
In reality, there are three factors that will work in
our favor:
1. The Peutz criteria are based on acoustical
relationships at 2 kHz. With mid-band (500 Hz)
reverberation at 2.5 seconds we can reasonably
assume that reverberation time at 2 kHz will be about
2.2 seconds due to excess air attenuation, as can be
read from Figure 5-13.
2. We have not yet considered that the surface
area (as opposed to the average absorption
coefficient) on which most of the initial acoustical
power is aimed is more absorptive than the average
absorption coefficient would indicate. At 2 kHz, the
audience area will have an absorption coefficient
conservatively estimated at about 0.5, and this
indicates that the actual reverberant level generated
by the reinforcement system could be a good 3 dB
less than our calculations indicate. This would easily
move our data point in the graph of Figure 6-13 very
close to the “GOOD” zone. Of course we must take
into account the actual number of persons present in
the audience area before this assumption can be
made. On any given Sunday, if all the attendees at a
service can be coaxed into the front pews, the
loudspeakers behind them are unnecessary and can
be turned off, further reducing the level of the
reverberant field.
3. The final factor working in our favor would be
the reduction of reverberation time in the space due
to the presence of the congregation. Remember that
the reverberation time of 2.5 seconds is for the
empty room.
3. Delay Zoning:
Suggested delay settings are:
Zone 1. Loudspeakers 2, 3, 6, and 7 10 msec
Zone 2. Loudspeakers 1, 4, 5, and 8 22 msec
Zone 3. Loudspeakers 9 and 10 40 msec
Zone 4. Loudspeakers 11 and 12 55 msec
Zone 5. Loudspeakers 13 and 14 70 msec
Zone 6. Loudspeakers 15 and 16 85 msec
Zone 7. Loudspeakers 17 and 18 100 msec
4. General comments:
The system described in this section
emphasizes the complex inter-relations between
acoustics and electroacoustics that are inherent in
basic sound reinforcement design in large, live
spaces. We strongly urge that all of the basic
relationships presented here be carefully studied and
understood. The fundamental principles we would
like to stress are:
1. Whenever possible, use distributed
loudspeakers that cover the intended seating area,
but that have rapid cutoff beyond their nominal
coverage angles; in other words, keep the on-axis DI
as high as possible consistent with required
coverage.
2. Try to minimize the longest throw distance
within a given loudspeaker zone. Loudspeakers have
been placed in overhead chandeliers in the attempt
to do this. Pewback systems take this approach to
the limit.
3. Seat the congregation toward the front of the
room and turn off unnecessary loudspeakers.
4. Many large spaces were designed during a
time when few people cared about speech
intelligibility, and many liturgical spaces are simply
too live for modern requirements. A careful
assessment should be made here, and no live
liturgical space should be altered acoustically without
the advice and counsel of an experienced acoustical
consultant.
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