Distance for a Target SPL
The inverse of the spl-distance tile: the distance at which the sound pressure level falls to a target, d2 = d1 x 10^((L1 + mode_factor + 10 log10(N) - L2) / 20). A 110 dB source at 1 ft drops to 84 dB at about 20 ft in free field (every doubling of distance loses 6 dB). Answers 'how far to the hearing-safe or spill line' instead of the level at one spot. Includes the mode adjustment and N incoherent sources; a target above the source level is flagged. A planning estimate; the room and the measurement govern.
Formula and source
d2 = d1 x 10^((L1 + mode_factor + 10 log10(N) - L2) / 20); the inverse of L2 = L1 - 20 log10(d2/d1).
Classical acoustics (inverse-square law); ISO 9613-2 (Acoustics - Attenuation of sound during propagation outdoors) by name.
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