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You also have to choose a desired level at the listener location. Typical values are:
Desired SPL Level
Background 85 dB
Mid-ground 91 dB
Foreground 96 dB
Note: Background is usually in quiet rooms that people do not want to shout to be heard or that the music is
to provide ambience and is not a focal point. Mid-ground is when you have a lot of ambient noise and want
the music to be heard over this noise. Foreground type applications are usually where the music or voice being
amplified are a focal point and either must be heard to entertain, inform or enlighten.
A general rule of thumb is that you lose about 1dB per foot. The Max SPL is at the highest Tap setting allowed.
Therefore, to determine the tap setting you use Hdiff to subtract the SPL difference from 3 feet to the listener ear
location. Once you have this you have the Max SPL at the listener location. Now you lower the tap until you get
the nearest value above the desired SPL.
SPL at listener = Speaker Max SPL – (H
diff
/ 12) +3
To Calculate the tap setting you take [SPL at Listener] – [desired SPL] = SPL
diff
Tap clicks = round down(SPL
diff
/3)
So starting with the speakers max tap setting (64 for all speakers) and going down tap clicks gives you the tap
setting for that speaker.
Example: If SPL
diff
= 10 then tap clicks = 3 and the tap setting is 64 => 32 => 16 => 8 is the correct setting of the
transformer.
The last step in the system design is to calculate the amplifier power needed. This calculation must take into ac
-
count a 20% loss of power to the speakers due to transformer losses.
Amp Power = [total number of speakers] x [tap setting] x 1.2
As an example:
Assume the room is 60 ft x 40 ft x 12 ft high. Listeners are usually standing.
W = 40 x 12 = 480 in.
L = 60 x 12 = 720 in.
Ceiling H = 12 x 12 = 144 in.
H
diff
= 144 – 64.4 = 79.6 in
Desired SPL = Background = 85dB
If we use the Pri665CV then the calculations for better coverage are:
Spacing max = 79.6 x 2.606 = 207.4 in
Columns = Roundup( 720 / 207.4) = Roundup(3.47) = 4
Rows = Roundup( 480 / 207.4) = Roundup(2.31) = 3
Spacing L = 720 / 4 = 180 in = 15 ft
Spacing W = 480 / 3 = 160 in = 13 ft 4 in
142-002698-D OWNERS MANUAL PRICV6 6 10/4/07 3:24:07 PM