Public Address - Audio Principles 4
Loss and Gain Calculators
Formula for calculating loss over a given distance is:

20Log x where x is the distance from the loudspeaker or source of sound. The loss chart below can be used instead of a calculator. A nomograph provides a quick method of calculating loss in dB over a given distance.
20 Log table
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Example: A loudspeaker has a sensitivity of 99dB at 1m with 1 watt of input power. What will the level be at 10m?
1. Relative dB at 10m is 20
2. Relative dB at 1m is 0
3. Deduct 20 from the starting level of 99dB (99-20 = 79dB)
Formula for calculating gain from a loudspeaker with a given power input is:

SPL = (10*(log LSMax input power)) + LS 1W/1m rating
or SPL = (10*(log B)) + A

10 Log table
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Example: If you look at the specification for most loudspeakers they have both a 1W/1M SPL figure and a maximum input power figure quoted.  These are the only things you need to calculate the maximum output SPL.

If  A= The loudspeaker 1W/1M rating (for this example we'll say 90dB SPL) and  B= The loudspeaker maximum input power (for this example we'll say 32 Watts) If we substitute our values above, the calculation becomes: (10*(log 32))+90 which calculates up to 105dB SPL