Diffusers as used in gas transport systems have an optimal pressure recovery but are unstable due to marginal flow separation. Coupling of diffuser flow oscillation with acoustic modes in a pipe has been demonstrated in a recent work by Kwong and Dowling [J. Fluids Eng. 116, 842 (1994)] to drive flow unsteadiness. Considered here in addition to the diffuser at a pipe termination is the aeroacoustic response of a diffuser in a long pipe. In both cases reflection coefficient measurements show that at moderate and low amplitudes of the acoustical particle velocity compared to the main flow velocity, diffusers are aeroacoustic sources similar to the whistler nozzle and the horn. This confirms the observations of Kwong and Dowling. At higher acoustical velocity amplitudes diffusers become strong absorbers, which can be explained in terms of a quasistationary flow model. Finally, an indication is provided for possible remedial measures when a stable flow is needed.