Milani, E., Paze, C., Ambrosino, M., and Pagliano, P., "Reduction of Exhaust Noise by Means of Thermal Acoustics," SAE Int. J. Passeng. Cars - Mech. Syst. 5(2):956-961, 2012, doi:10.4271/2012-01-0804.
It is well known that mufflers attenuate the engine noise essentially through dissipative and reflective effects. There is however another alternative technique for noise attenuation that has not been deeply explored, i.e. thermal acoustics. In fact the temperature of the gas influences the acoustic behaviour of the exhaust system; reducing the exhaust gas temperature, the sound pressure of the acoustic waves is reduced. This phenomenum could be used to improve the sound attenuation.We propose an experimental study of this phenomenum and of how it could be used to reduce the exhaust noise. We measured that, using in underfloor position passive heat exchangers like corrugated pipes, the exhaust gas quickly exchanges heat with the external environment and arrives to the rear muffler significantly colder. We observe about 2 dB decrease of the OA dB value when the gas temperature decreases of about 100°C. With colder exhaust gas the noise can be heavily reduced; in particular the flow noise improves the sound attenuation at high engine speed.