You also have to choose a desired level at the listener location. Typical values are:
Desired SPL Level |
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Background | 85 dB |
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| 91 dB |
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Foreground | 96 dB |
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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.
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 – (Hdiff / 12) +3
To Calculate the tap setting you take [SPL at Listener] – [desired SPL] = SPLdiff
Tap clicks = round down(SPLdiff /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 SPLdiff = 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.
Hdiff = 144 – 64.4 = 79.6 in
Desired SPL = Background = 85dB
If we use the Pri665CV then the calculations for better coverage are:
