7
SPL of 70V/100V vs. Low Impedance Speakers – 70V/100V speakers produce lower maximum output
due to a number of factors. First, they are tapped at lower power levels than the low-impedance speaker’s
capabilities. Second, there is always some “insertion loss” through the speaker transformer. In addition,
speakers operated from a 70V or 100V line will produce substantially lower overall SPL capabilities if the
driving amplifier clips short-term peaks immediately at 70V or 100V and if it doesn’t have much dynamic
(short-term) headroom. Amplifiers with higher clipping limits and greater dynamic headroom can result in
substantially higher output from 70V/100V loudspeakers.
Note: All of the computations in Table 6 assume JBL MPC amplifiers which exhibit high clipping limits
AND good dynamic headroom.
Power Compression -- One must also take into consideration the fact that speakers compress the sound at
high levels (as their voicecoil temperatures rise, causing the impedance to go up, resulting in less draw of
audio power from the same voltage drive signal). This is called “power compression”. We have left
power compression out of the formulas because speakers differ in their power compression, and because
the degree of compression is highly dependent on operational factors such as the peak-to-average ratio of
the signal source. A general guideline is to assume around 2 or 3 dB of compression for good quality
speakers (such as those with large diameter voicecoils) with typical music or speech and to assume as much
as 5 or 6 dB of compression with inexpensive speakers (especially those with small diameter voicecoils
and/or low power ratings) operating with compressed music sources.
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There are a number of different patterns that can be used for laying out ceiling speakers, and each pattern
can be implemented with various density factors. The pattern and density selected for an installation affect
the:
•
Evenness of coverage
•
Sound level capability
•
Intelligibility
•
Cost of the system
•
Power amplification requirements
Layout Patterns
– Although there may be many ways to lay out the speakers in a distributed system,
two basic patterns that have gained wide acceptance are SQUARE and HEXAGONAL. The choice of a
square or hexagonal layout is usually a function of the room dimensions and shape.
Square Layout Pattern
Hexagonal Layout Pattern
