# Atmospheric Sound Absorption according to ANSI S1.26-1995

# K.-H. Frommolt 2021-12-01

# FrO - vibrational relaxation frequency for oxygen
# FrN - vibrational relaxation frequency for oxygen
# Pr  - reference atmospheric pressure = 101.325 kPa
# Tr  - reference air temperature = 293.15 K (20?C)
# pt  - pressure amplitute [Pa]
# pi  - initial pressure amplitude [Pa]
# s   - distance [m]
# a   - pure-tone sound attenuation coefficient for atmospheric absorption [dB/m]
# f   - frequency of sound [Hz]
# Ta  - (T) ambient atmospheric temperature [K]
# h   - molar concentration of water vapor [%]
# Pa  - ambient atmospheric pressure [kPa]
# RH  - relative humidity [%]
# Psat- saturation vapor pressure
# To1 - triple-point isotherm temp: 273.16 K = 273.15 + 0.01 K (0.01?C)

# Input Data
Pa<-101.325     	# actual atmospheric pressure [kPa]
Temp_C<- 10     	# actual temperature [?C]
RH=80         		# relative humidity (%)
f=5080          	# frequency of signal [Hz] for control only
Dist_Source=25.86  	# Distance between microphone and sound source animal [m]
Dist_Target=1   	# Target distance for which SPL should be corrected
Outputfile<-"C:\\Daten\\Filter_Temp7_RH80_Distance25_1000.csv"

# Constants

Pr<-101.325   # reference atmospheric pressure [kPa]
Tr<-293.15    # reference air temperature = 293.15 K (20?C)
To1<-273.16   # triple-point isotherm temp: 273.16 K = 273.15 + 0.01 K (0.01?C)
Freq<-c(0,50,63,80,100,125,160,200,250,315,400,500,630,800,1000,1250,1600,2000,2500,3150,4000,5000,6300,8000,10000,12000,24000)
Abs_Coeff<-c(0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0)


# Calculated variables

Ta<- Temp_C+273.15    # actual temperature [K]



# Calculation of h according to Improved Magnus formula according to Alduchov & Eskridge (1996)


Psat_Magnus<-6.1094*exp((17.625*Temp_C)/(Temp_C+243.04)) # [hPa]
Psat=Psat_Magnus/10
h=RH*Psat/Pa

# Calculation of relaxation frequencies FrO and FrN

P_rel<-Pa/Pr
T_rel<-Ta/Tr

FrO<-P_rel*(24+40400*h*(0.02+h)/(0.391+h))
FrN<-P_rel*T_rel^(-.5)*(9+280*h*exp(-4.170*(T_rel^(-1/3)-1)))


# calculation of a [db/m]


FrO_Term<-FrO/(FrO^2+f^2)
FrN_Term<-FrN/(FrN^2+f^2)

Part_A<-1.84*10^-11*P_rel^-1*T_rel^.5
Part_B<- 0.01275*((exp(-2239.1/Ta))*FrO_Term)
Part_C<-0.1068*((exp(-3352/Ta))*FrN_Term)

a<-8.686*f^2*(Part_A+T_rel^-2.5*(Part_B+Part_C))

idx <-(2:25)


for (i in idx) {
  FrO_Term<-FrO/(FrO^2+Freq[i]^2)
  FrN_Term<-FrN/(FrN^2+Freq[i]^2)
  Part_A<-1.84*10^-11*P_rel^-1*T_rel^.5
  Part_B<- 0.01275*((exp(-2239.1/Ta))*FrO_Term)
  Part_C<-0.1068*((exp(-3352/Ta))*FrN_Term)
  Abs_Coeff[i]<-8.686*(Freq[i])^2*(Part_A+T_rel^-2.5*(Part_B+Part_C))
}

Absorption<-cbind(Freq,Abs_Coeff)
Atten_dB<-Abs_Coeff*(Dist_Target-Dist_Source)
Ampl_dB<-Abs_Coeff*(Dist_Source-Dist_Target)
Ampl_Factor<-10^(Ampl_dB/20)
Ampl_Factor[1]<-0
Ampl_Factor[26]<-0
Ampl_Factor[27]<-0
Attenuation<-cbind(Freq,Ampl_Factor,Atten_dB)

write.csv2(Attenuation,Outputfile)