Lixi Huang

Biomechanics of snoring.

A large proportion of the population either snores or suffers the snoring of others. Recent advances with the use of fibre-optic endoscopes have enabled surgeons to observe the inside of the pharynx while a patient is asleep and snoring. In this article we look at the underlying structure of the upper airway and explain, with the use of simple mechanical models, the aerodynamic events occurring inside the upper airway during snoring.

On the acoustic properties of parallel arrangement of multiple micro-perforated panel absorbers with different cavity depths.

The acoustic properties of a compound micro-perforated panel (MPP) absorber array are investigated. The absorber array consists of three parallel-arranged MPP absorbers with different cavity depths. A finite element procedure is used to simulate its acoustic behaviors under normal incidence. Experimental studies are carried out to verify the numerical simulations. Due to different reactance matching conditions in the absorber array, strong local resonance occurs and the corresponding local resonance absorption dominates. Compared with single MPP absorber, the absorber array requires lower acoustic resistance for good absorption performance, and the resonance frequencies shift due to inter-resonator interactions. The different acoustic resistance requirement is explained by considering the reduced effective perforation rate of the MPP in the absorber array. The performance of the absorber array varies with the sizes and spatial arrangement of the component absorbers. When the distance between component absorbers is larger than a quarter-wavelength, the above-mentioned parallel absorption mechanism diminishes. In the experimental study, the normal incidence absorption coefficients of a prototype MPP absorber array are tested. The measured results compare well with the numerical predictions. The experimental study also shows that although other absorption mechanisms may exist, dissipation by the MPP is dominant in the MPP absorber array.

Characterizing computer cooling fan noise

Computer cooling fan noise is studied theoretically, focusing on the radiation from the interaction between rotor blades and motor struts. The source is decomposed into axial thrust, circumferential drag, and radial force. There is no sound-power coupling among the three components. The index of spatial spinning pressure mode plays the key role in noise radiation. The leading modes are the zeroth, or coincident, mode for thrust and the first mode for the drag and radial force. The effect of source noncompactness is quantified and found to be substantial only for higher-order radiation modes. The sound powers of the leading modes follow a sixth-power law, while the next high-order modes follow an eighth-power law. Quantitative analysis shows that the drag force can be equally noisy as the coincident thrust force. Based on an empirical aerodynamic model of rotor-strut interaction, it is found that the total sound power is more sensitive to the number of struts than rotor blades. Numerical examples are given to demonstrate how the struts can be optimized for typical cooling fan conditions.

Broadband sound reflection by plates covering side-branch cavities in a duct

When a segment of a rigid duct is replaced by a plate backed by a hard-walled cavity, grazing incident sound waves induce plate bending, hence sound reflection. The mechanism is similar to the drumlike silencer with tensioned membranes [L. Huang, J. Acoust. Soc. Am. 112, 2014–2025 (2002)]. However, the logarithmic bandwidth over which the reflection occurs is much wider than that of a drumlike silencer of the same cavity geometry, the typical difference being nearly one octave band. The difference in the silencing performance is explained in terms of the intermodal acoustic interference between the odd and even in vacuo vibration modes. For a given cavity volume, the widest stopband for noise in air is obtained by using long plates with two free lateral edges parallel with the duct axis. The optimal material should be stiff and light, and the critical parameter is found to be the ratio of the Young’s modulus over the cube of density. Typically, this ratio is 250 times higher than those of common metallic ...

Parametric study of a drum-like silencer

An optimization study is carried out for a silencer consisting of two side-branch, rectangular cavities covered by membranes highly stretched in the direction of the duct axis. Stopband is defined as the range of frequency where the transmission loss is everywhere higher than the peak value of that in an expansion chamber which occupies three times as much cavity volume as does the present silencer. The logarithmic bandwidth is optimized with respect to the length-to-depth ratio of the cavity, the mass and the tension of the membrane. For two cavities each with a dimensionless volume of 5 (the duct height being the length scale), the optimal cavity aspect ratio is 6.6, and the lower stopband frequency is 0.09 times the first cut-on frequency of the rigid duct. This is compared favourably with the traditional duct lining modelled as an equivalent fluid. As the membrane mass increases, the stopband shifts to lower frequencies but it also narrows. The widest stopband is around 1.6 octaves for a massless membrane. The membrane tension plays a delicate role of setting the intervals between adjacent spectral peaks.

An analytical solution for the Wilson point in homogeneously nucleating flows

The calculation of conditions at the Wilson point is the key to both theoretical and numerical studies of the condensation of pure vapours by homogeneous nucleation. Nucleation and droplet growth occur in a very short period of time, during which the changes of many vapour properties due to the normal thermofluid dynamic processes are negligible compared with the change of the heat release rate. This feature is exploited in an analysis leading to an approximate solution for the maximum subcooling and other properties at the Wilson point. The analysis is general but attention is focused on the main application of interest, which is the condensation of steam in high-speed flows by homogeneous nucleation. Crucial approximations are justified over a wide range of steam pressures and the analytical results reveal the dependency of steam properties at the Wilson point on controlling parameters such as the rate of pressure decrease. A direct link is established between the steam properties at the saturation point and those at the Wilson point, which, when used in multidimensional condensation flow calculations, should remove the need for very fine meshes and excessive computing resources which are otherwise required.

Point vortex model for prediction of sound generated by a wing with flap interacting with a passing vortex.

Acoustic signature of a rigid wing, equipped with a movable downstream flap and interacting with a line vortex, is studied in a two-dimensional low-Mach number flow. The flap is attached to the airfoil via a torsion spring, and the coupled fluid-structure interaction problem is analyzed using thin-airfoil methodology and application of the emended Brown and Michael equation. It is found that incident vortex passage above the airfoil excites flap motion at the system natural frequency, amplified above all other frequencies contained in the forcing vortex. Far-field radiation is analyzed using Powell-Howe analogy, yielding the leading order dipole-type signature of the system. It is shown that direct flap motion has a negligible effect on total sound radiation. The characteristic acoustic signature of the system is dominated by vortex sound, consisting of relatively strong leading and trailing edge interactions of the airfoil with the incident vortex, together with late-time wake sound resulting from induced flap motion. In comparison with the counterpart rigid (non-flapped) configuration, it is found that the flap may act as sound amplifier or absorber, depending on the value of flap-fluid natural frequency. The study complements existing analyses examining sound radiation in static- and detached-flap configurations.

Thin broadband noise absorption through acoustic reactance control by electro-mechanical coupling without sensor

Broadband noise with profound low-frequency profile is prevalent and difficult to be controlled mechanically. This study demonstrates effective broadband sound absorption by reducing the mechanical reactance of a loudspeaker using a shunt circuit through electro-mechanical coupling, which induces reactance with different signs from that of loudspeaker. An RLC shunt circuit is connected to the moving coil to provide an electrically induced mechanical impedance which counters the cavity stiffness at low frequencies and reduces the system inertia above the resonance frequency. A sound absorption coefficient well above 0.5 is demonstrated across frequencies between 150 and 1200 Hz. The performance of the proposed device is superior to existing passive absorbers of the same depth (60 mm), which has lower frequency limits of around 300 Hz. A passive noise absorber is further proposed by paralleling a micro-perforated panel with shunted loudspeaker which shows potentials in absorbing band-limit impulse noise.

Membrane covered duct lining for high-frequency noise attenuation: Prediction using a Chebyshev collocation method

A spectral method of Chebyshev collocation with domain decomposition is introduced for linear interaction between sound and structure in a duct lined with flexible walls backed by cavities with or without a porous material. The spectral convergence is validated by a one-dimensional problem with a closed-form analytical solution, and is then extended to the two-dimensional configuration and compared favorably against a previous method based on the Fourier-Galerkin procedure and a finite element modeling. The nonlocal, exact Dirichlet-to-Neumann boundary condition is embedded in the domain decomposition scheme without imposing extra computational burden. The scheme is applied to the problem of high-frequency sound absorption by duct lining, which is normally ineffective when the wavelength is comparable with or shorter than the duct height. When a tensioned membrane covers the lining, however, it scatters the incident plane wave into higher-order modes, which then penetrate the duct lining more easily and get dissipated. For the frequency range of f=0.3-3 studied here, f=0.5 being the first cut-on frequency of the central duct, the membrane cover is found to offer an additional 0.9 dB attenuation per unit axial distance equal to half of the duct height.

Active control of drag noise from a small axial flow fan

Noise sources in an axial flow fan can be divided into fluctuating axial thrust forces and circumferential drag forces. For the popular design of a seven-blade rotor driven by a motor supported by four struts, drag noise dominates. This study aims to suppress the drag noise globally by active control schemes. Drag noise features a rotating dipole and it has to be cancelled by a secondary source of the same nature. This is achieved experimentally by a pair of loudspeakers positioned at right angles to each other on the fan rotational plane. An adaptive LMS feedforward scheme is used to produce the control signal for one loudspeaker and the time derivative of this signal is used to drive the other loudspeaker. The antisounds radiated by the two loudspeakers have a fixed phase relation of 90° forming a rotating dipole. An open-loop control scheme is also implemented for the purpose of comparison and easier implementation in real-life applications. The results show that the globally integrated sound power is ...