Investigations on Whistle Noise in Automotive Exhaust System Mufflers

Automotive exhaust mufflers are not only attenuating noise, but can, in fact, along with other exhaust components, generate noise within the exhaust system. This type of noise is commonly called “flow generated noise” to distinguish it from the “pulsation noise” excited by the combustion engine. Flow generated noise is normally very broad band but it can also have tonal regions. This tonal flow noise can sound like a whistle. In this contribution an experimental study of the generation and prevention of such whistle tones will be presented. The study was focused on perforated pipes in mufflers with different perforation patterns, hole sizes and hole shapes. The noise spectra have been measured on a flow bench at room temperature as well as under real (hot) conditions on a vehicle. The results will be correlated in terms of frequency and Strouhal number. Concepts to avoid or suppress these tones will also be discussed.