Siu-Kit Lau

The Acoustic Vector-Sensor's Near-Field Array-Manifold

The acoustic vector-sensor is a practical and versatile sound-measurement system, for applications in-room, open-air, or underwater. Its far-field measurement model has been introduced into signal processing over a decade ago; and many direction-finding algorithms have since been developed for acoustic vector-sensors, but only for far-field sources. Missing in the literature is a near-field measurement model for the acoustic vector-sensor. This correspondence fills this literature gap.

Beacon-Aided Adaptive Localization of Noise Sources Aboard a Pass-By Railcar Using a Trackside Microphone Array

A new adaptive “beamforming” signal-processing algorithm is developed to locate the loudest noise sources aboard a railcar that passes by a trackside immobile microphone array. This proposed microphone-array beamformer tracks the railcars spatial movement with the aid of two inaudible acoustic beacons placed aboard the railcar. The proposed scheme then localizes the noise sources with reference to the railcars coordinates. No auxiliary infrastructure (e.g., no radar or video camera) is needed besides the onboard beacons. Monte Carlo simulations and anechoic chamber experiments verify the efficacy of the proposed scheme.

Near-field/far-field array manifold of an acoustic vector-sensor near a reflecting boundary.

The acoustic vector-sensor (a.k.a. the vector hydrophone) is a practical and versatile sound-measurement device, with applications in-room, open-air, or underwater. It consists of three identical uni-axial velocity-sensors in orthogonal orientations, plus a pressure-sensor-all in spatial collocation. Its far-field array manifold [Nehorai and Paldi (1994). IEEE Trans. Signal Process. 42, 2481-2491; Hawkes and Nehorai (2000). IEEE Trans. Signal Process. 48, 2981-2993] has been introduced into the technical field of signal processing about 2 decades ago, and many direction-finding algorithms have since been developed for this acoustic vector-sensor. The above array manifold is subsequently generalized for outside the far field in Wu, Wong, and Lau [(2010). IEEE Trans. Signal Process. 58, 3946-3951], but only if no reflection-boundary is to lie near the acoustic vector-sensor. As for the near-boundary array manifold for the general case of an emitter in the geometric near field, the far field, or anywhere in between-this paper derives and presents that array manifold in terms of signal-processing mathematics. Also derived here is the corresponding Cramér-Rao bound for azimuth-elevation-distance localization of an incident emitter, with the reflected wave shown to play a critical role on account of its constructive or destructive summation with the line-of-sight wave. The implications on source localization are explored, especially with respect to measurement model mismatch in maximum-likelihood direction finding and with regard to the spatial resolution between coexisting emitters.

Beacon-aided adaptive azimuth-elevation localization of sound-sources aboard a pass-by rail-car using a track-side acoustic microphone planar array

A new adaptive ldquobeamformingrdquo signal-processing algorithm is developed to locate noise-sources aboard a pass-by rail-car, using an immobile track-side microphone-array. The microphone-arraypsilas beamforming is aided by two narrowband acoustic beacons abroad the rail-car aid to track the rail-carpsilas spatial movement and thus to locate the noise-sources with respect to the rail-car. No other auxiliary hardware (e.g., no radar nor video-camera) is needed.

An impedance-mobility model of stacked membrane-type acoustic metamaterials

Membrane-type acoustic metamaterials have been proven to exhibit high low-frequency transmission loss despite their small thickness and light weight. To date, analysis has focused primarily on experimental studies in plane-wave tubes and numerical modeling using finite element methods. These methods are inefficient when used for applications that require iterative changes to the structure of the material. In addition, high sound transmission loss with a single layer of such metamaterial only occurs in a narrow frequency range. To facilitate design and optimization of stacked membrane-type acoustic metamaterials, a computationally efficient dynamic model based on the impedance-mobility approach is proposed. Results are verified against a finite element model. Single and double layer transmission loss characteristics are compared. Wide-band high-transmission-loss acoustic metamaterials can be achieved by double layer membranes and using the proposed approach for optimization. The impedance-mobility approach...

Noise propagation through open windows of finite depth into an enclosure.

Predicting the insertion loss of an opening backed with an enclosed space is important for building noise control. Recent research in sound transmission through apertures of finite depth in infinite rigid baffles has included the effects of propagating and evanescent modes within the aperture in order to extend models to higher frequencies. The present study extends the model to the case of the aperture backed by a cavity as opposed to sound radiating into half-space. The role of coupling between the aperture modes, radiation modes, and cavity modes in the transmission was investigated. The results were compared to those of previous models which neglected the depth of the aperture and finite element modeling using COMSOL Multiphysics. Comparisons show that the current model is effective at predicting the sound transmission loss through the aperture and the acoustic field within the cavity for an obliquely incident plane wave. By changing impedance conditions on the half-space side of the aperture and with...

Transmission characteristics of a tee-junction in a rectangular duct at higher-order modes

Numerical and experimental studies were undertaken to characterize the noise transmission and scattering properties in higher-order modes across the tee-junction of a rectangular duct used in ventilation and air-conditioning systems. To measure these properties, a formulation of a transmission matrix based on the transfer function and a two-microphone method was devised. The measurement of modal sound transmission and scattering coefficients is demonstrated for a duct element in a rectangular duct. The results of numerical simulations were verified by experiments. The results show that sound transmissions of fundamental mode and higher-order modes across the main duct are high at the eigen-frequencies of the main duct and sidebranch. Weak modal coupling of the branch-modes and the traveling wave in the main duct is observed at or very close to the eigen-frequencies of the sidebranch, which shifts excitation of the higher-order branch-modes at higher frequencies. A decrease in sound transmission and increase in sound scattering into higher-order modes occur with excitation of the axial branch-mode. Excitation of the longitudinal branch-mode due to branch-end reflection also results in lower sound transmission of higher-order modes across the junction along the main duct.

Bias and reproducibility of sound power test methods

Sound power is a useful quantity in describing the strength of an acoustic source because its value is independent of distance. However, many standardized methods exist for the measurement of sound power and comparison between methods can give rise to discrepancies. An interlaboratory study was designed according to the ISO 5725 series to quantify the bias and reproducibility of three common sound power measurement methods in the HVACR industry: free field method, diffuse field method, and sound intensity method. A loudspeaker sound source was used to generate two test signals: a broadband signal with decreasing 5 dB slope per octave band, and the same broadband signal with discrete frequency tones at 58, 120, 300, and 600 Hz. The objective of the study is to quantify repeatability, reproducibility, laboratory bias, and measurement method bias, as well as investigate the influence of tones. The design of the interlaboratory study and preliminary results will be presented. The ISO 5725 methods used to inve...

Effects of inclusion shapes within rigid porous materials on acoustic performance

The present study investigates the influence of various shapes of inclusions having same volume embedded in a porous rigid material. Previous studies showed improvement of the broadband sound absorption with particular shapes of inclusions. However, different volumes of the inclusions have been considered; therefore, the bulk densities are not the same for comparison. The present study extends the investigations of inclusions in porous materials with same volume (or bulk density) to eliminate the influence by the change of bulk density. The effects of shape will be discussed. Finite element modeling will be used for this study. Total four different shapes: circle, square, ellipse, and triangle, have been studied at various orientations. It has been found that specific configurations can be able to improve the broadband sound absorption compared with reference (no inclusion). It is being expected that a better control of sound absorption of porous materials at desired frequency range can be achieved with the results of the present study.

Cantonese speech intelligibility tests in two Hong Kong primary school classrooms

Speech transmission is a very important issue especially in early childhood education during which the children learn languages and pronunciations. There are evidences that the acoustical performance of a classroom can have significant impact on the learning progress and effectiveness of the children. The effect is not only observed in language subjects, but also in the numeracy related subjects. In this study, on-site speech intelligibility tests are conducted using newly developed phonetically balanced (in Cantonese) Chinese character lists suitable for the Year 5 Hong Kong primary school children (10–11 years old) in two classrooms. A trained speaker is recruited to speak out the characters. The classroom acoustical properties are also measured, but in the absence of the pupils because of statutory regulation. The sound of each Cantonese character in general consists of an initial consonant, a vowel, a final consonant, and a tone. A preliminary observation is that there is higher probability for the pu...