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Speakers used for emergency voice/alarm communication system are wired as “Constant Voltage”
systems, where the maximum power output of the amplifier is obtained at a certain speaker
voltage, such as 25 V or 70.7 V. The power output of a speaker, and thus the resulting SPL is
controlled by wattage taps on the speakers themselves. The minimum wattage tap for a
UL Standard 1480 listed speaker is 1/4 W. For example, the Simplex 4902 Series speakers have
wattage taps in 3 dB increments: 1/4 W, 1/2 W, 1 W and 2 W. Each wattage tap doubles the
power delivered by the speaker, and so increases the SPL output by 3 dB for each increasing tap.
An increase of 3 dB is considered a just noticeable increase in SPL, changing the wattage tap from
1/4 W to 2 W increases the perceived loudness by slightly less than double.
Many rooms require more than one speaker, the question then becomes, “how many speakers do I
need, and how far apart should they be placed?” The answer invariably is “that depends.” There are
many layout patterns to choose from, each suitable for different room geometries, background noise
and reverberation times. See Table 2-2 “Layout Pattern Selection Guide” for some recommendations.
The most common layout pattern is the rectangular pattern shown in Figure 2-7 below.
Table 2-2. Layout Pattern Selection Guide
Pattern Description
2x Edge-to-
Edge
Not recommended except for
tone only signaling or small rooms with
low noise and low reverberation.
1.4x Edge-to-
Edge
Uses fewer speakers than Edge-to-
Edge pattern. Only appropriate for
rooms with low noise and low
reverberation.
Edge-to-Edge
Preferred layout pattern for most
areas.
Minimum
Overlap
Use with areas of high reverberation
and/or high ceilings.
Full Overlap
For the worst areas, generally provides
excellent intelligibility for even difficult
areas. Use with caution, this type of
pattern can result in lower than
expected intelligibility due to multiple
speaker interaction. Modeling is
recommended for areas that would
need this layout pattern.
Speaker Basics, Continued
Power Rating
Speaker Layouts
Speaker Layout
Pattern
Increase in SPL
Over Single
Speaker
Variation in
Coverage
2x Edge-to-Edge 0.2 dB –10.4 dB
1.4x Edge-to-
Edge
0.4 dB –6.8 dB
Edge-to-Edge 0.7 dB –4.4 dB
Minimum Overlap 2.0 dB –2.0 dB
Full Overlap 5.2 dB –1.4 dB
Table 2-3. SPL Variation by Layout Pattern
Figure 2-7. Speaker Layout Patterns
Edge to Edge
(Standard layout )
Minimum Overlap
Full Overlap
1.4x Edge-to-Edge
2x Edge-to-Edge
Speaker layout patterns
(rectangular placement)