A time-domain approach for acoustic analysis of perforated tube silencers

A time domain computational approach is applied to investigate the acoustic characteristics of automotive-type perforated tube silencers. The investigation includes single- and multiple-pass silencer geometries for both linear and nonlinear perforate behavior. Silencers are modeled in an impedance tube configuration with single-frequency excitation and zero mean flow. The model employs a nonlinear finite-difference method that includes flows through perforated interfaces based on a coupled one-dimensional approach. With suitable lumped parameter perforate models for the operating conditions of interest, the approach is capable of modeling silencers within an integrated intake, engine, and exhaust system, thereby avoiding many of the restrictions in typical frequency domain analyses.; Oscillating flow through circular orifices is investigated experimentally to address the effects of nonlinear perforate behavior and develop perforate submodels for the time domain approach. Experiments are performed with single-frequency excitation and zero mean flow for a frequency range of 100--800 Hz. An approximate method is developed to investigate the instantaneous pressure/flow relationship in addition to the more traditional impedance measurements. The large changes in perforate behavior due to nonlinearity are demonstrated in terms of both perforate impedance and temporal variation in the pressure/flow relationship. Empirical expressions for coefficients in the orifice model equation are formulated for linear, nonlinear time-invariant, and quasi-steady behaviors.; Computational predictions of transmission loss are compared to measurements for single- and multiple-pass silencers in an anechoically-terminated impedance tube. Within the limit of one-dimensional disturbances, agreement between predictions and experiments is good. The measurements show that multidimensional effects may become significant before the cut-on frequency of the first higher duct mode. The model predictions demonstrate that nonlinear perforate behavior can be expected to noticeably affect silencer attenuation: Resonances are changed dramatically, with broadband behavior changing more moderately. The approximate quasi-steady model is applied to address the effects of time-varying coefficients in the orifice model equation. Time variation in the pressure/flow relationship for perforations is shown to be of secondary importance, but not necessarily negligible.