A new measurement technique for duct characterization

Devices such as silencers or filters are commonly used by the automotive industry to reduce the propagation of noise in ducts. The acoustical characterization of these devices is usually performed on transfer matrix measurement rigs equipped with microphone doublets. One drawback of this widespread technique is the difficulty of fitting of the test specimen to the test rig duct. It is required that the shape and cross-section of the test rig duct be as alike as possible as that of the test specimen. Such requirement complicates the testing and can become the source of uncertainty should the fitting be inadequate. When the qualification procedure does not require an air flow to blow through the test specimen, and alternate measurement technique allows avoiding the issue of the mechanical fitting. Said technique uses an acoustic impedance sensor developed by LAUM and CTTM. First the principles of this alternate technique are reminded, then some experimental results are discussed in order to highlight the pros and cons of both methods.