Intermittent combustion engine exhaust noise

A general aviation intermittent combustion engine noise test facility, is used to investigate the tonal characteristics of engine exhaust noise using a Lycoming 0-320 D1D engine with a standard Cessna 172, exhaust system. Propeller noise is eliminated because this facility incorporates a hydraulic dynamometer to absorb the power normally delivered to a propeller. Engine tests are performed at different delivered powers, shaft rotational speeds (rpm), and the far-field radiated noise measured as well as recorded. As expected, the magnitude and frequency of the primary engine firing frequency (EFF) tone and multiples scale with engine shaft rotational speed. It is determined that the EFF tone scales as the 4th power, 2EFF as the 12th power, 3EFF as the 16th power, 4EFF as the 20th power, and 5EFF as the 24th power of RPM. In an attempt to collapse the tonal data to a single curve, a dimensionless acoustic power and frequency, in terms of a Strouhal number, is successfully developed.