SPL with Atmospheric Absorption (ANSI S1.26)
Far-field SPL with per-octave-band atmospheric absorption (alpha dB/m) per ANSI S1.26-2014 (R2019).
Example
You enter
- Source SPL at reference distance (dB) 95
- D ref m 1
- D far m 30
- Temperature (°C) 20
- Relative humidity (percent) 50
- Pressure kpa 101.325
You get
- Inverse-square attenuation (dB) 29.54
- Far-field SPL at 1 kHz (dB) 65.3
Details, formula, and sources
Octave breakdown 125 Hz to 8 kHz; companion to the inverse-square-only distance loss.
SPL_far = SPL_ref - 20*log10(d_far/d_ref) - alpha(f, T, RH, P)*d_far. Per-octave alpha (dB/m) from the ANSI S1.26 relaxation-frequency formula: alpha = 8.686 * f^2 * { 1.84e-11 * (p_r/p_a) * sqrt(T/T_0) + (T/T_0)^-2.5 * [ 0.01275 * exp(-2239.1/T) / (frO + f^2/frO) + 0.1068 * exp(-3352/T) / (frN + f^2/frN) ] }.
ANSI S1.26-2014 (R2019) Method for Calculation of the Absorption of Sound by the Atmosphere. Inverse-square law from classical acoustics.
ansi.org for TOC; full standard is licensed.
Estimate. Head rigger and manufacturer working-load-limit charts govern. Inspect every piece of hardware before the show.
Field names used by the API: source_SPL_dB, d_ref_m, d_far_m, temperature_C, RH_percent, pressure_kPa, inverse_square_dB, SPL_far_1kHz_dB
- Octave bands 125 / 250 / 500 / 1000 / 2000 / 4000 / 8000 Hzspec-v9 §H.2
- Saturation vapor pressure ANSI S1.26 IAPWS-style approximation: log10(p_sat/p_r) = -6.8346 * (273.16/T)^1.261 + 4.6151ANSI S1.26-2014
- Relaxation frequencies frO and frN per ANSI S1.26 closed-form expressions in molar concentration of water vaporANSI S1.26-2014
- Summary band 1 kHz reported as the operator-grade voice-band referencespec-v9 §H.2 (operator-grade summary)
- Companion tile v1 spl-distance is inverse-square only; this tile adds atmospheric absorptionspec-v9 §H.2 discipline