Public Address Information - 6
100V Line Public Address

Effects of using 100v, 70v or 50v tapps on amplifiers with 100v line speaker circuits.
Amplifier Tapping
Speaker output
100v
100% Full Power
70v
50% Half Power (-3dB)
50v
25% Quarter power (-6dB)
   
 
Loss & Distance Effects
1m
2m
4m
SPL (dB) @ 1w
97
91
85
SPL (dB) @ 2w
100
94
88
SPL (dB) @ 4w
103
97
91
SPL (dB) @ 8w
106
100
94
SPL (dB) @ 16w
109
103
97
       
       
Add 3dB when power is doubled
Deduct 6dB when distance is doubled
 
SPL Levels & Ambient Noise
Ideal PA levels for a listener should be a minimum of 6dB, up to an ideal of 10dB higher than ambient sound levels in the location to be covered. Distance then has to be factored into the calculation to compensate for loss.

Impedance readings and wattages when using an Impedance meter

Calculation for 100v circuit loading
   
100 (volt)/ohms x 100 = wattage
eg 100/200 x 100 = 50 watts
eg 100/100 x 100 = 100 watts
eg 100/1000 x 100 = 10 watts
   
or simplified as 10,000/ohms = wattage
or 10,000/watts = impedance (ohms)
   
Reading
Loading
   
10000 ohms
1 watt
4000 ohms
2.5 watts
2000 ohms
5 watts
1333 ohms
7.5 watts
660 ohms
15 watts
330 ohms
30 watts
165 ohms
60 watts
83 ohms
120 watts
42 ohms
240 watts
28 ohms
360 watts
21 ohms
480 watts
 
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