Approximating Engine Tailpipe Orifice Noise Sound Quality using a Surge Tank and In-Duct Measurements 2003-01-1641

Because of the need to safely vent exhaust gases, most engine dynamometer facilities are not well suited to measuring engine exhaust orifice noise. Depending on the location of the dyno facility within the building, the exhaust system may need to be extended in order to properly vent the exhaust fumes. This additional ducting changes the acoustic modes of the exhaust system which will change the measured orifice noise. Duct additions downstream of the original orifice location also alter the termination impedance such that in-duct pressure measurements with and without the extended exhaust system can vary significantly. In order to minimize the effect of the building's exhaust system on the desired engine exhaust system measurements, the present approach terminates the engine exhaust into a large enclosed volume or surge tank before venting the gases into the building's ventilation system. The large volume of the surge tank produces acoustic reflections similar to those of an orifice open to the atmosphere. Two pressure transducers upstream of the surge tank are used to separate out the forward and backward traveling acoustic waves in the duct and approximate the particle acceleration at the exhaust orifice location using linear acoustic assumptions. The radiated orifice noise is then estimated from the particle acceleration using a simple source model. Comparisons of this approximated orifice noise with the actual external measurements reveal that this method correctly captures trends in the dominant engine orders and predicts the amplitudes of these orders within about 5 dB.