where ω {\displaystyle \omega } is the angular frequency, P the pressure, Δ x {\displaystyle \Delta x} the wave propagation distance, α ( ω ) {\displaystyle \alpha (\omega )} the attenuation coefficient, α 0 {\displaystyle \alpha _{0}} and frequency dependent exponent η {\displaystyle \eta } are real non-negative material parameters obtained by fitting experimental data and the value of η {\displaystyle \eta } ranges from 0 to 2. Acoustic attenuation in water, many metals and crystalline materials are frequency-squared dependent, namely η = 2 {\displaystyle \eta =2} .