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I don't know if this is what you are looking for, but....

... there is an equation that goes something like this:

Where factor definitions are:

x = sensitivity rating of speakers in dB
y = amplifier power in dBW (not simple Watts)
z = distance from speaker in metres

The dB peak listening level = (x+y)-5(z-1)

(Remember that the ‘5’ comes from the 5dB loss for every metre moved away from the speaker and the ‘-1’ allows for the fact that the speaker’s dB sensitivity was measured at a distance of 1m).

Then there is a table that helps to convert Watts (RMS values) to dBW:

10 = 10
20 = 13
50 = 17
63 = 18
79 = 19
100 = 20
126 = 21
156 = 22
200 = 23
251 = 24
316 = 25
400 = 26

Now, by way of example, here is the calculation I used for my set-up:

a) Speakers : Tannoy D700

- Nominal Impedance : 6 ohms
- Quoted Sensitivity: 93dB at 1m
- Listening Distance: ~3 metres

b) Amplifier: Jeff Rowland Model 201 monoblocks (x2)

- Output into 8ohms : 250 watts per channel
- Output into 4ohms : 500 watts per channel
- Output into 6ohms : 375 watts per channel (interpolated)

c) Calculation as per the formula:

- From Amp Power Table: 25dB
- Speaker Sensitivity : 93dB
- Maximum SPL (1metre): 118dB (ie: 25 + 18)
- Distance Attenuation: ((3-1) x -5) = (2 x -5) = -10 dB
- Peak Listening Level: 108dB (ie: 118 - 10)

Where "Peak Listening Level" is at 3 metres from the plane of the speakers.

I hope that helps....
DevillEars

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