Speech therapy aiming at improving voice quality and speech intelligibility is often hampered by the lack of knowledge of the underlying deficits. One way to help speech therapists treating patients would be to supply synthetic bench-marks for pathological speech. In a listening experiment testing perceived intelligibility, three types of manipulations of tracheoesophageal speech were evaluated by experienced speech therapists. It was found that modeling the intensity contour of the voice source signal improved speech quality over plain analysis-synthesis. Replacing the voicing source with fully synthetic source periods decreased the perceived intelligibility markedly. Making the source fully periodic with a regular pitch had no effect on perceived intelligibility. Low quality speech benefited more from manipulations, or deteriorated less, than high quality speech.