Yat-Sze Choy

Hybrid noise control in a duct using a light micro-perforated plate

A plate silencer consists of an expansion chamber with two side-branch cavities covered by light but extremely stiff plates. It works effectively with a wide stopband from low-to-medium frequencies only if the plate is extremely stiff, to ensure a strong reflection of acoustic wave to the upstream in the duct. However, a plate with a slightly weak bending stiffness will result in non-uniform transmission loss (TL) spectra with narrowed stopband. In this study, a hybrid silencer is proposed by introducing micro-perforations into the plate to elicit the sound absorption in order to compensate for the deficiency in the passband caused by the insufficient sound reflection in a certain frequency range due to weaker plate stiffness. A theoretical model, capable of dealing with the strong coupling between the vibrating micro-perforated plate and sound fields inside the cavity and the duct, is developed. Through proper balancing between the sound absorption and reflection, the proposed hybrid silencer provides a more flattened and uniform TL and a widened stopband by more than 20% while relaxing the harsh requirement on the bending stiffness of the plate. Theoretical predictions are validated by experimental data, with phenomenon explained through numerical analyses.

Noise suppression of a dipole source by tensioned membrane with side-branch cavities

Reducing the ducted-fan noise at the low frequency range remains a big technical challenge. This study presents a passive approach to directly suppress the dipole sound radiation from an axial-flow fan housed by a tensioned membrane with cavity backing. The method aims at achieving control of low frequency noise with an appreciable bandwidth. The use of the membrane not only eliminates the aerodynamic loss of flow, but also provides flexibility in controlling the range of the stopband with high insertion loss by varying its tension and mass. A three-dimensional model is presented which allows the performance of the proposed device to be explored analytically. With the proper design, this device can achieve a noise reduction of 5 dB higher than the empty expansion cavity recently proposed by Huang et al. [J. Acoust. Soc. Am. 128, 152-163 (2010)]. Through the detailed modal analysis, even in vacuo modes of the membrane vibration are found to play an important role in the suppression of sound radiation from the dipole source. Experimental validation is conducted with a loudspeaker as the dipole source and good agreement between the predicted and measured insertion loss is achieved.

Sound propagation in and low frequency noise absorption by helium-filled porous material

Low-frequency noise is difficult to deal with by traditional porous material due to its inherent high acoustic impedance. This study seeks to extend the effective range of sound absorption to lower frequencies by filling a low density gas, such as helium, in the porous material. Compared with conventional air-filled absorption material, the helium-filled porous material has a much reduced characteristic impedance; hence, a good impedance matching with pure air becomes more feasible at low frequencies. The acoustic properties of a series of helium-filled porous materials are investigated with a specially designed test rig. The characteristic of the sound propagation in a helium-filled porous material is established and validated experimentally. Based on the measured acoustic properties, the sound absorption performance of a helium-filled absorber (HA) of finite thickness is studied numerically as well as experimentally. For a random incidence field, the HA is found to perform much better than the air-filled absorber at low frequencies. The main advantage of HA lies in the middle range of oblique incidence angles where wave refraction in the absorber enhances sound absorption. The advantage of HA as duct lining is demonstrated both numerically and experimentally.

Reactive control of subsonic axial fan noise in a duct.

Suppressing the ducted fan noise at low frequencies without varying the flow capacity is still a technical challenge. This study examines a conceived device consisting of two tensioned membranes backed with cavities housing the axial fan for suppression of the sound radiation from the axial fan directly. The noise suppression is achieved by destructive interference between the sound fields from the axial fan of a dipole nature and sound radiation from the membrane via vibroacoustics coupling. A two-dimensional model with the flow effect is presented which allows the performance of the device to be explored analytically. The air flow influences the symmetrical behavior and excites the odd in vacuo mode response of the membrane due to kinematic coupling. Such an asymmetrical effect can be compromised with off-center alignment of the axial fan. Tension plays an important role to sustain the performance to revoke the deformation of the membrane during the axial fan operation. With the design of four appropriately tensioned membranes covered by a cylindrical cavity, the first and second blade passage frequencies of the axial fan can be reduced by at least 20 dB. The satisfactory agreement between experiment and theory demonstrates that its feasibility is practical.

Multi-feature entropy distance approach with vibration and acoustic emission signals for process feature recognition of rolling element bearing faults:

To accurately reveal rolling bearing operating status, multi-feature entropy distance method was proposed for the process character analysis and diagnosis of rolling bearing faults by the integration of four information entropies in time domain, frequency domain and time–frequency domain and two kinds of signals including vibration signals and acoustic emission signals. The multi-feature entropy distance method was investigated and the basic thought of rolling bearing fault diagnosis with multi-feature entropy distance method was given. Through rotor simulation test rig, the vibration and acoustic emission signals of six rolling bearing faults (ball fault, inner race fault, outer race fault, inner ball faults, inner–outer faults and normal) are gained under different rotational speeds. In the view of the multi-feature entropy distance method, the process diagnosis of rolling bearing faults was implemented. The analytical results show that multi-feature entropy distance fully reflects the process feature of rolling bearing faults with the change of rotating speed; the multi-feature entropy distance with vibration and acoustic emission signals better reports signal features than single type of signal (vibration or acoustic emission signal) in rolling bearing fault diagnosis; the proposed multi-feature entropy distance method holds high diagnostic precision and strong robustness (anti-noise capacity). This study provides a novel and useful methodology for the process feature extraction and fault diagnosis of rolling element bearings and other rotating machinery.

Vortex sound radiation in a flow duct with a dipole source and a flexible wall of finite length

The noise attenuation of fan-ducted noise at low blade-passage frequency remains a challenge. The present study investigates the noise reduction mechanism of a tensioned membrane housing device that directly controls the sound radiation from the doublet which is enclosed in an infinitely long duct with a point vortex. The time dependent sound radiation mechanism and the vibro-acoustics coupling mechanism of the systems are studied by adopting the potential theory and matched asymptotic expansion technique. The silencing performance of such a passive approach depends on the amplitude and phase of the sound field created by the doublet and the acoustic pressure induced by the membrane oscillation in order to achieve sound cancellation. Results show that the response of membrane vibration is strongly associated with the flow field induced by the grazing uniform flow and also the fluid loading generated by the inviscid vortex. The geometrical property of the cavity and the mechanical properties of the flexible membranes play important roles in controlling the performance of the proposed device.

Reliability-Based Design Optimization Method of Turbine Disk with Transformed Deterministic Constraints

AbstractTo improve the computational efficiency of the reliability-based design optimization (RBDO) of a complex structure with nonlinear and implicit limit-state function, the single-loop-single-vector (SLSV)-limit-state factor (LSF) (SLSV-LSF) method was developed by fully considering the advantages of the SLSV approach and the LSF method to transform uncertain constraints into deterministic constraints. The mathematical models of SLSV and LSF were established and the basic RBDO process of the SLSV-LSF method is presented. The shape optimization of an aeroengine turbine disk was completed based on the proposed method. From the reliability sensitivity analysis of the turbine disk, it is revealed that an uncertain constraint of average circumferential stress can be transformed into a deterministic constraint and material density can be regarded as a deterministic variable. Through the min-mass shape design of the turbine disk based on different approaches, it is demonstrated that the developed method main...

Acoustic behaviors of the microperforated panel absorber array in nonlinear regime under moderate acoustic pressure excitation

The acoustic performance of a microperforated panel (MPP) absorber array in the nonlinear regime is investigated both numerically and experimentally. The MPP absorber array is constructed by three parallel-arranged MPP absorbers with different cavity depths. A finite element model is used to simulate the acoustic response of the MPP absorber array by adopting the nonlinear impedance model. The results show that the absorption of the MPP absorber array is affected by the incident sound pressure when it is beyond around 100 dB. With appropriate structural and perforation property of MPP, the MPP absorber array in non-linear regime outperforms that in linear regime due to the improvement of equivalent acoustic impedance matching with ambient air over wide frequency range. However, when the sound pressure excitation is too high, the local resonance effect of the resonating component MPP absorber is diminished and the sound absorption is decreased. With the carefully chosen properties of MPP, the performance degradation induced by panel vibration can be avoided. An optimal set of MPP properties to avoid the performance degradation induced by panel vibration is determined. The measured normal absorption coefficients of a prototype MPP absorber array compare well with the numerical prediction in both linear and nonlinear regimes.

Sound quality control of axial fan noise by using microperforated panel housing with a hollow tube

This study presents a novel passive noise control approach to directly suppress sound radiation from an axial-flow fan, which involves micro-perforated panels (MPP) backed by cavities and a hollow tube. Apart from the sound suppression performance in terms of insertion loss, sound quality of axial fan with a dipole nature is also investigated which serves as a significant supplementary index for assessing the noise control device. The noise suppression is achieved by the sound cancelation between sound fields from the fan of dipole nature and sound radiation from a vibrating panel via vibro-acoustic coupling and interference from the hollow tube boundaries, as well as by sound absorption in micro-perforations. A two-dimensional theoretical model, capable of dealing with strong coupling among the vibrating micro-perforated panel, sound radiation from the dipole source, and sound fields inside the cavity and the duct is developed. The theoretical results are validated by both finite element simulation and e...

Normal incidence sound absorption of parallel absorber at high sound pressure

A finite element model is developed to simulate the acoustic response of the parallel absorber array at high sound intensity. The device consists of an MPP and a rectangular backing cavity which is divided into two sub-cavities with different cavity depths. At high acoustic excitation, a nonlinear impedance model instead of the traditional linear model is adopted such that the effects of jets and vortex rings formed at the exit of orifices on the acoustic properties are taken into account. The normal incident pressure is considered as a main variable of the nonlinear acoustic impedance model. The performance of the MPP absorber array with different designed geometric parameters are studied. Based on the parallel absorption mechanism, the preliminary results show that the MPP absorber array provides good absorption performance with a wider frequency range by comparing with the single MPP absorber. Also, compared with the results obtained in linear regime, a better absorption performance is achieved by the ...