Conventional silencers have extensively been used to attenuate airborne pressure pulsations in the breathing system of internal combustion engines, typically at low frequencies as dictated by the crankshaft speed. With the introduction of turbocharger compressors, however, particularly those with the ported shroud recirculating casing treatment, high-frequency tones on the order of 10 kHz have become a significant contributor to noise in the induction system. The elevated frequencies promote multi-dimensional wave propagation, rendering traditional silencing design methods invalid, as well as the standard techniques to assess silencer performance. The present study features a novel high-frequency silencer designed to target blade-pass frequency (BPF) noise at the inlet of turbocharger compressors. The concept uses an acoustic straightener to promote planar wave propagation across arrays of quarter-wave resonators, achieving a broadband attenuation. The effectiveness of the silencer is evaluated on a turbocharger gas stand where the compressor is the noise source. The experiment utilizes a unique rotating inlet duct upstream of the silencer to perform a modal decomposition of the acoustic field in order to compute sound power levels across the operating flow range at various compressor rotational speeds. The results are then compared to those from an earlier experiment with a straight duct installed at the compressor inlet to determine silencer insertion loss, defined here as the difference between upstream (with respect to flow direction) sound power levels without and with the silencer. The study also addresses the resulting compromise in compression system performance and noise generated due to flow-acoustic coupling within the silencer. Hence, the current effort demonstrates the effectiveness of a novel silencer in terms of insertion loss derived from a modal decomposition of the multi-dimensional acoustic field at the compressor inlet.
