Acoustic waves propagating within a duct containing a subsonic mean flow are of interest in many practical applications such as aeroengines or gas turbines. In these systems, sudden cross section area increases where flow separation occurs are widespread and the acoustic energy balance is known to be affected for such conditions. On the other hand, sudden cross section area decreases are usually assumed to be isentropic and the acoustic energy balance is unaffected. The objective of this work is to determine the acoustic absorption coefficient for various models of sudden cross section area increase and decrease commonly implemented in low order network tools used for thermoacoustic stability prediction. Analytical expressions in the low Mach number limit are also provided and compared with numerical predictions. It is shown that for a sudden cross section area increase with flow separation, the acoustic absorption coefficient depends on the upstream Mach number, cross section area ratio and boundary acoustic reflection coefficient only. For certain values of these parameters, all of the acoustic energy is damped across the area change. On the other hand, the acoustic energy is amplified across the area change for other values of these parameters.
