Acoustic Attenuation Due to Atmospheric Absorption, ISO 9613-1:1993

atm_absorption.py

# a function that will return the atmospheric absorption coefficient, a in dB/m - given the frequency of interest and atmospheric state
def atm_abs(frequency, air_temp_celcius, percent_relative_humidity, atm_pressure):
    import math

    T_K = air_temp_celcius + 273.15
    psat = 101.325*math.pow(10, (-6.8346*math.pow(273.16/T_K, 1.261))+4.6151) # atmospheric saturation pressure
    
    x = 1/(10*math.log(math.pow(math.exp(1),2), 10))  # 'equation shortener #1'
    h = percent_relative_humidity*(psat/atm_pressure)/100  # molar concentration of water vapor

    frO = (atm_pressure/101.325)*(24 + (4.04*math.pow(10, 4)*h*((0.02 + h)/(0.391 + h)))) # oxygen relaxation frequency
    frN = (atm_pressure/101.325)*math.pow(T_K/293.15, -0.5)*(9 + (280 *h*math.exp(-4.17*(-1*math.pow(T_K/293.15, -1/3))))) # nitrogen relaxation frequency

    z = 0.1068*math.exp(-3352/T_K)*math.pow((frN+math.pow(frequency,2))/frN, -1) # 'equation shortener #2'
    y = math.pow(T_K/293.15, -5/2)*((0.01275*math.exp(-2239.1/T_K)*math.pow((frO+math.pow(frequency,2))/frO, -1))+z) # 'equation shortener #3'


    # here's the atmospheric absorption coefficient, itself:
    a = 8.686*math.pow(frequency,2)*(1.84*math.pow(10, -11)*math.pow(atm_pressure/101.325, -1)*math.pow(T_K/293.15,0.5)+y)

    return a