Where \ omega is the angular frequency, " P " the pressure, \ Delta x the wave propagation distance, \ alpha ( \ omega ) the attenuation coefficient, \ alpha _ 0 and frequency dependent exponent \ eta are real non-negative material parameters obtained by fitting experimental data and the value of \ eta ranges from 0 to 2.
42.
Q in cm & minus; 1 ) is variously called barns or units of 10 & minus; 24 cm 2 ), density mean free path ( e . g . ? in grams / cm 2 ), and its reciprocal the mass attenuation coefficient ( e . g . in cm 2 / gram ) or " area per nucleon " are all popular, while in electron microscopy the inelastic mean free path ( e . g . ? in nanometers ) is often discussed instead.
