Specifications

11
EXAMPLE
A loudspeaker (sensitivity: 94 dB @ 1W, 1 meter) in a busy office covers an area with an ambient noise
level of 67 dBA, measured at a seated person’s ear position at the desks. The client wants superb
intelligibility, so your goal is to provide an SPL of 92 dB (25 dB above ambient SPL) to the intended
listeners, the office workers. The ceiling-mounted speaker is about 6 feet, or 1.8 meters, from the
workers’ ears. The attenuation for D of 1.8 meters is 5.2 dB; this means that the loudspeaker would
have to produce 97.2 dB (92 plus 5.2) at a distance of 1 meter to achieve 92 dB at a distance of 1.8
meters.
The loudspeaker will require a power level of 3.2 dBW (97.2 minus 94); this works out to 2.1 watts.
The speaker transformer taps are 0.5, 1, 2, 4, and 8 watts; the closest choice would be the 2-watt tap,
an almost exact match.
Calculating total power requirements
After you’ve determined the power taps for each loudspeaker transformer on the distributed line, add them up.
The sum will be a start toward calculating the amplifier power requirements. If you have 16 loudspeakers tapped
at 2 watts, seven at 1 watt, and eight at 10 watts, the total audio power the loudspeakers want is 119 watts.
Thanks to a transformer phenomenon called insertion loss, though, your amplifier actually needs to provide more
power than the loudspeakers will get. High-quality speaker transformers typically have an insertion loss of about
1 dB or less, meaning that it takes as much as 1.25 watt going into the transformer to put 1 watt into the
loudspeaker. A lower-quality transformer may have a loss of 2 dB, which requires approximately 1.6 watts for
every watt that the loudspeaker receives. Poor-quality transformers may have even higher losses, but they will
probably degrade the system’s audio performance severely even if you allow extra amplifier power to overcome
the losses.
EXIT
97.2 dB
1
.
8
m
e
t
e
r
s
92 dB
Ambient noise
= 67 dBA
2.1 watts (3.2 dBW) in
(sensitivity = 94 dB @ 1W, 1 m)