The pressure time history in the combustion chamber of an IC-engine is seen as a sound phenomenon which - attenuated by the engine structure - is radiated as combustion noise. A distinction has been made between the direct and indirect combustion noise. The subject “transmission rate/direct combustion noise” has been investigated by the example of an air-cooled single cylinder diesel engine. To this end short-range noise intensity measurements as well as cylinder pressure measurements at several points in the combustion chamber have been carried out simultaneously. The evaluation method chosen considers the considerable cyclic fluctuations from working cycle to working cycle of the cylinder pressure level to check the applicability of a transmission rate. Within a frequency range of 0.5 to 9.5 kHz the correlation coefficients of sound intensities and cylinder pressure excitation have been determined in 500 Hz band-width for 50 successive working cycles. The results of the investigation can be used for evaluation of the acoustic properties of the combustion process.