Behrooz Yousefzadeh

Energy- and wave-based beam-tracing prediction of room-acoustical parameters using different boundary conditions.

A beam-tracing model was used to study the acoustical responses of three empty, rectangular rooms with different boundary conditions. The model is wave-based (accounting for sound phase) and can be applied to rooms with extended-reaction surfaces that are made of multiple layers of solid, fluid, or poroelastic materials-the acoustical properties of these surfaces are calculated using Biot theory. Three room-acoustical parameters were studied in various room configurations: sound strength, reverberation time, and RApid Speech Transmission Index. The main objective was to investigate the effects of modeling surfaces as either local or extended reaction on predicted values of these three parameters. Moreover, the significance of modeling interference effects was investigated, including the study of sound phase-change on surface reflection. Modeling surfaces as of local or extended reaction was found to be significant for surfaces consisting of multiple layers, specifically when one of the layers is air. For multilayers of solid materials with an air-cavity, this was most significant around their mass-air-mass resonance frequencies. Accounting for interference effects made significant changes in the predicted values of all parameters. Modeling phase change on reflection, on the other hand, was found to be relatively much less significant.

An experimental study of Sound Transmission Loss (STL) measurement techniques using an impedance tube

A comparison between the two sound transmission loss (STL) measurement techniques using an impedance tube (i.e. two‐load method and anechoic termination method) is presented. A modified B&K type 4206 impedance tube has been designed and built. STL tests have been carried out for three homogeneous and isotropic materials with disk‐type test samples of identical diameters and different thicknesses. In addition, the results have been compared with those of the classical and more reliable method of two‐room. For both methods, the effect of downstream (tube termination) boundary conditions have been completely studied. The two‐load method yields results which matches with two‐room measurements, especially when the two boundary conditions are considerably different. The anechoic termination method, on the other hand, is significantly dependant on the termination boundary conditions.

Supratransmission in a disordered nonlinear periodic structure

Abstract We study the interaction among dispersion, nonlinearity, and disorder effects in the context of wave transmission through a discrete periodic structure, subjected to continuous harmonic excitation in its stop band. We consider a damped nonlinear periodic structure of finite length with disorder. Disorder is introduced throughout the structure by small changes in the stiffness parameters drawn from a uniform statistical distribution. Dispersion effects forbid wave transmission within the stop band of the linear periodic structure. However, nonlinearity leads to supratransmission phenomenon, by which enhanced wave transmission occurs within the stop band of the periodic structure when forced at an amplitude exceeding a certain threshold. The frequency components of the transmitted waves lie within the pass band of the linear structure, where disorder is known to cause Anderson localization. There is therefore a competition between dispersion, nonlinearity, and disorder in the context of supratransmission. We show that supratransmission persists in the presence of disorder. The influence of disorder decreases in general as the forcing frequency moves away from the pass band edge, reminiscent of dispersion effects subsuming disorder effects in linear periodic structures. We compute the dependence of the supratransmission force threshold on nonlinearity and strength of coupling between units. We observe that nonlinear forces are confined to the driven unit for weakly coupled systems. This observation, together with the truncation of higher-order nonlinear terms, permits us to develop closed-form expressions for the supratransmission force threshold. In sum, in the frequency range studied here, disorder does not influence the supratransmission force threshold in the ensemble-average sense, but it does reduce the average transmitted wave energy.

Complete delocalization in a defective periodic structure

We report on the existence of stable, completely delocalized response regimes in a nonlinear defective periodic structure. In this state of complete delocalization, despite the presence of the defect, the system exhibits in-phase oscillation of all units with the same amplitude. This elimination of defect-borne localization may occur in both the free and forced responses of the system. In the absence of external driving, the localized defect mode becomes completely delocalized at a certain energy level. In the case of a damped-driven system, complete delocalization may be realized if the driving amplitude is beyond a certain threshold. We demonstrate this phenomenon numerically in a linear periodic structure with one and two defective units possessing a nonlinear restoring force. We derive closed-form analytical expressions for the onset of complete delocalization, and we discuss the necessary conditions for its occurrence.

Nonlinear Energy Transmission in a Finite Dissipative Periodic Structure

We study nonlinear transmission of energy in a finite dissipative periodic structure, which is externally driven at one end with a harmonic force. We show that when the driving frequency is within the stop band of the periodic structure, there is a threshold for the driving amplitude, above which there is a sudden increase in the energy transmitted through the finite structure. This is a generic phenomenon for discrete nonlinear periodic systems. The onset of transmission is due to loss of stability of the harmonic solutions localized at the driven end of the structure. The transmission threshold is predicted analytically based on the underlying bifurcation. The influence of both hardening and softening types of nonlinearity on the transmission phenomenon is discussed.

Comparison of room‐acoustical parameters predicted using different surface‐reaction models.

This paper presents the development of a beam‐tracing model for calculating the transient responses of rooms. The model is wave‐based (i.e., includes phase changes due to distance traveled and wall reflections), and can be applied to rooms with extended‐reaction surfaces. Room surfaces can be modeled as multiple layers of solid, fluid, and poroelastic materials; their acoustical properties are calculated using a transfer‐matrix approach. The beam‐tracing model calculates the complex transfer function of a room. Pressure impulse responses are then computed via Fourier transformation, and the room‐acoustical parameters derived. Since pressure impulse responses are calculated, the model can also be used for auralization. The model has been applied to different room configurations in order to study the effects of different surface‐reaction models on the predicted steady‐state characteristics and temporal variations of sound‐pressure fields in various room configurations. In particular, the audibility of using...

Wave transmission in finite dissipative nonlinear periodic structures

Spatially periodic structures exhibit intriguing dynamic characteristics, contributing to their growing applications as phononic crystals, acoustic metamaterials and lightweight lattice materials. A striking feature, employed in many engineering applications, is their filtering effect, whereby waves can propagate only in specific frequency intervals known as pass bands. Other frequency components (stop bands) are spatially attenuated as they propagate through the structure. This thesis studies nonlinear wave transmission in periodic structures of finite extent in the presence of dissipative forces and externally induced nonlinear forces. Perfectly periodic structures with identical units are considered, as well as nearly periodic structures with small deviations from periodicity extended throughout the structure. At high amplitudes of motion, nonlinear forces gain significance, generating qualitatively new dynamic phenomena such as supratransmission. Supratransmission is an instability-driven transmission mechanism that occurs when a periodic structure is driven harmonically at one end with a frequency within its stop band. The ensuing enhanced transmission contrasts the vibration isolation characteristic of the same structure operating in the linear regime. In the context of engineering applications, three factors play a significant role: dissipative forces, symmetry-breaking imperfections induced by manufacturing constraints (disorder) and the finite size of the structure. This thesis systematically investigates the influence of these parameters on supratransmission in a onedimensional periodic structure, studying the competition between the effects of dispersion, dissipation, nonlinearity and disorder-borne wave localization (Anderson localization).

Acoustical modeling of the transient response of rooms using a beam-tracing model

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Prediction of sound propagation in rectangular ducts by a ray‐tracing model with phase.

A ray‐tracing model which includes phase changes due to distance traveled and surface reflection has previously been developed for predicting sound field in rectangular rooms. Including phase has removed the limitation associated with energy‐based models, that room dimensions must be larger than the sound wavelength. The model has been applied to a rectangular room with dimensions not large compared to the wavelength. The room dimensions are that of a real‐world ventilation duct, with the boundary conditions idealized as a point source at one end and anechoic termination at the other end. Predictions have been made of the pressure field inside the duct and the results have been compared with analytical solutions. The model is found to be capable of predicting the modal characteristics of the duct.