In this model, 3 interfaces were considered; thus, 3 sets of Equations 10 and 11 were used to define the observed pressure as follows: i) determine the incident pressure at interface with Equation (P 1,i = P 0 exp (-α 1 x 1 ) at the first interface), ii) apply Equation (P 1,t = (1 + r 1 )P 1,i ), iii) substitute P 1,i in the expression for P 1,t (P 1,t = (1 + r 1 ) exp (-α 1 x 1 )P 0 ) and iv) calculate the incident pressure at the next interface by substituting P 1,t in Equation for j = 2 and repeat for every interface until the transmission of the signal into the transducer (j = 3 in this case). Assuming the transmitted wave at the air-receiver interface, P 3,t , is the observed pressure designated as P obs , the equation for this quantity following substitutions using the transmitted pressure and pressure attenuation is given in Equation .