Comment 5 for bug 1373404

"At 80 dB(A) exposure time shall be limited to 40 hours/week, whereas at 89 dB(A) exposure time shall be limited to 5 hours/week. For other exposure levels a linear intra- and extrapolation applies."

I'll put this here for others to check while I do the rest of the design:

maximum time per week = a function of volume
40 h = f(80 dB(A))
5 h = f(89 dB(A))
linear intra- and extrapolation ∴ maximum time per week = m × volume + c
m = (40h – 5h) ÷ (80 dB(A) – 89 dB(A)) = 35 h ÷ –9 dB(A) = –3.889 h/dB(A)

40h = –3.889 h/dB(A) × 80 dB(A) + c
c = 40h – (–3.889 h/dB(A) × 80 dB(A)) = 351.111 h

∴ maximum time per week = –3.889 h/dB(A) × volume + 351.111 h.

A quick graph of this formula looks right, at least.