PreSonus StudioLive 328AI - Manual (Page 26)

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4 System Configuration
4.2 Delay Systems
StudioLive ™ AI-Series Loudspeaker
Owner’s Manual
4.1.3 Maintain a High Direct-to-Reverberant Ratio
Another way a room interacts with sound waves is through reflectivity. Like
most room anomalies, reflections can be good and bad. Consider the effect of
a cathedral’s reflections on a choir or a piano. This type of reverberation (reverb)
is quite desirable. But not all reverb is good reverb. Reflections can also cause
comb filtering. For example, if a speaker is placed near a reflective surface (such
as a concrete wall), the direct sound coming from the speaker and the reflected
sound coming from the wall can arrive at the listener’s ears out of phase with
each other, causing cancellation and reinforcement. If they’re 180 degrees
out of phase with respect to each other, they will cancel each other out.
If you are using your loudspeakers in a reverberant environment, position them
so that as much sound as possible is focused on the audience area and steered
away from reflective surfaces. StudioLive AI-series full-range loudspeakers have
a nominal 90-degree horizontal and 60-degree vertical coverage pattern. When
you do your placement and positioning, it’s a good idea take some time to do
a “walk around” of your loudspeakers, playing either pink noise or program
material, so you get a feel for how the sounds are translating into the room.
4.2 Delay Systems
In most situations, a PA system relies on two main speaker systems
positioned at the front of the room to reproduce audio for the
entire performance space. As a result, the level of the system is
considerably louder at the front line then it is at mix position.
With a point source, horn-loaded loudspeaker (such as a single,
powered loudspeaker), sound intensity is lost at a rate of -6 dB per
doubling of distance. This is true regardless of tuning, amplification,
power rating, or any other speaker specifications. So if your signal
level is 106 dB SPL at 1 foot, at 8 feet away it’s down by 18 dB!
Here’s a simple chart that illustrates the math:
DISTANCE dB SPL DISTANCE dB SPL
1 ft. 106 dB 8 ft. 88 dB
2 ft. 100 dB 16 ft. 82 dB
4 ft. 94 dB 32 ft. 76 dB
In situations where sound must be reproduced outside of the main system’s
optimum range, well-placed delay systems offer support by extending the
intelligible range of the PA. Rather than relying on a pair of front-of-house
speakers to fill the entire room, you can create listening zones throughout the
room so that your front-of-house system only needs to be loud enough to
cover the front of the room. This allows you to lower the level, give the front-
row listeners’ ears a break, and get better fidelity from your speakers.
However, it’s not as easy as just bringing an extra pair of speakers. Since
electricity travels much faster than sound, listeners in the rear of the room
are likely to hear the sound coming from the nearest set of speakers
before they hear the sound from stage, which can dampen the attack and
intelligibility of the sound and create an unpleasant phasing effect.
To compensate, you need to delay the signal going to the additional speakers.
For example, it takes about 55 ms for sound to travel 50 feet. So if you put
your speakers 50 feet back, you need to delay the signal by that much.
Fortunately, with StudioLive AI-series loudspeakers and SL Room Control, you
can delay each speaker by up to 300 ms. All you need is a reasonably accurate
measurement of the distance between your main speakers and delays.