The rapid expansion of combustion products in a diesel cylinder generates high-frequency acoustic waves which cause errors in the computed heat-release signature. In this study, the effects of spline smoothing and digital filtering on the computed heat-release history were investigated. Experiments were performed on a heavy-duty single-cylinder diesel engine. In some cases the computed heat-release history was rendered unusable by the acoustic waves, which were centered at a frequency of 3.7 kHz for this engine, even after spline smoothing was applied to the pressure data. In contrast, a low-pass digital filter developed for this study was found to be very effective for removing acoustic noise from heat-release signatures without adversely affecting the computed indicated mean effective pressure.