Engineering Calculators

Fire Alarm Audibility Calculator

Public mode requires the greater of average ambient +15 dB or 60-second max ambient +5 dB. Private mode uses narrower +10 dB / +5 dB offsets (e.g. staff-alert-only zones). Sleeping areas require a 75 dBA absolute floor at the pillow, overridden upward if the relative rule would demand more.

Optional — leave blank if unmeasured. 60-second maximum sustained ambient level.

Formula Reference

Public Mode Threshold (NFPA 72 §18.4.4)

T=max⁡(Lavg+15,  Lmax,60s+5)T = \max\left(L_{avg} + 15,\ \ L_{max,60s} + 5\right)

T = required threshold (dB), L_avg = measured average ambient level, L_max,60s = measured 60-second max sustained ambient level (candidate skipped if not measured). Sleeping areas require a 75 dBA absolute floor at the pillow, overridden upward if the public-mode relative rule would demand more.