Double-leaf microperforated panel space absorbers: A revised theory and detailed analysis

A double-leaf microperforated panel space absorber (DLMPP) is composed of two microperforated panels (MPPs) placed in parallel with an air-cavity in-between, without a back wall or any backing structure. This was proposed as a space sound absorber, which can be used for a sound absorbing screen or partition. A conventional MPP absorber with a rigid back wall is effective only around its resonance frequency, which is usually at middle frequencies, and not effective at low frequencies. However, a DLMPP can be effective also at low frequencies, because an additional sound absorption is produced by its acoustic flow resistance. In the authors’ previous work, theoretical analyses on the acoustic properties of a DLMPP were carried out using a simplified electro-acoustical equivalent circuit model. However, the equivalent circuit model includes an approximation, and more sophisticated theory is required for a better prediction and detailed discussion. In this paper, a revised theory for a DLMPP is presented: A Helmholtz integral formulation is employed to obtain a rigorous solution for more precise prediction of the absorptivity of a DLMPP. The result of the present revised theory is compared with that of the equivalent circuit model, and the difference between them is discussed. A parametric survey is made through numerical examples by the present revised theory to discuss its acoustic properties.     

The acoustical properties of consolidated expanded clay granulates

The paper presents a systematic study of acoustic and non-acoustic properties of consolidated porous samples of expanded clay granulates. The effect of the particle size on the acoustic performance of consolidated expanded clays is investigated experimentally and theoretically. This work involves a comparison of the measured and predicted values of the absorption coefficient and normalised acoustic surface impedance data. It is demonstrated that the values of tortuosity and standard deviation in the pore size distribution do not depend significantly on the size of the material aggregate. An empirical expression which links the flow resistivity of the consolidated granular mix has been derived from the measured data. These results pave the way for the development of a simple practical model which will be able to link the acoustic properties of a consolidated granular mix with the characteristic particle dimension and the porosity data. These materials are structurally robust and easy to integrate in buildings and highway structures to control the levels of environmental noise and improve the acoustic quality of spaces.     

Noise generation by vacuum cleaner suction units. Part III. Contribution of structure-borne noise to total sound pressure level

Noise emitted by a vacuum cleaner suction unit consists of airborne and structure-borne noise. The airborne noise is generated mainly by the turbo blower and the structure-borne noise is generated mainly by the driving electric motor. The structure-borne noise depends on the suction unit design and on operating conditions, and is especially distinct at partial flow rates when rotating stall and surge appear. Among geometrical parameters, the stator of the blower and the electric motor, or metal shield if any, have the greatest effect on the structure-borne noise. Therefore, in this part of the paper, the effects of vibrations of the electric motor structure on the noise characteristics have been measured and analysed at the design and off-design operation. The contribution of structure-borne sound to the total sound pressure level becomes relatively less important at higher flow rates and with a vaned diffuser built-in the blower.     

Noise generation by vacuum cleaner suction units: Part II. Effect of vaned diffuser on noise characteristics

Noise emitted by a vacuum cleaner suction unit has aerodynamic, mechanical and electromagnetic noise origins. The aerodynamically generated noise within the blower mostly prevails in the total emitted noise at design and off-design operations. The vaned diffuser built into the blower has an important effect on the aerodynamically generated noise and its spectra. Therefore, in this part of the paper, the effects of the vaned diffuser on the performance and noise characteristics have been measured and analysed at the design and off-design operation. Analysis of measurement results have shown that the built-in vaned diffuser has numerous disadvantages, including: increase in the rotational noise, increase in the non-rotational turbulent noise, especially at off-design operation, increase in production costs and reduction of the flow rate. Therefore, and due to variable operating conditions of the suction units built into a vacuum cleaner, it could be omitted or replaced by a vaneless one.     

Quasi-wavelet calculations of sound scattering behind barriers

Quasi-wavelets (QWs) are a representation of turbulence consisting of self-similar, eddy-like structures with random orientations and positions in space. They are used in this paper to calculate the scattering, due to turbulent velocity fluctuations, of sound behind noise barriers as a function of the size and spatial location of the eddies. The sound scattering cross-section for QWs of an individual size class (eddy size) is derived and shown to reproduce results for the von Kármán spectrum when the scattered energies from a continuous distribution of QW sizes are combined. A Bragg resonance condition is derived for the eddy size that scatters most strongly for a given acoustic wavenumber and scattering angle. Results for scattering over barriers show that, for typical barrier conditions, most of the scattered energy originates from eddies in the size range of approximately one-half to twice the size of the eddies responsible for maximum scattering. The results also suggest that scattering over the barrier due to eddies with a line of sight to both the source and receiver is generally significant only for frequencies above several kilohertz, for sources and receivers no more than a few meters below the top of the barrier, and for very turbulent atmospheric conditions.     

A sound quality-based investigation of the HVAC system noise of an automobile model

Using methods and techniques of sound quality engineering, the noise of the heating, ventilation and air-conditioning system (HVAC) of an automobile model was studied. Such noise has a great influence on vehicle acoustical comfort and on overall quality perception of a vehicle. The study was divided into two steps. The first step aimed to identify the most significant attributes that contribute to the perception of similarity or dissimilarity of this kind of noise, using the paired comparison technique and correlation of the results with psychoacoustic models. Loudness, spectral composition and tonality, represented by the psychoacoustic models of loudness, sharpness, tone-to-noise ratio and prominence were found to be the most important dimensions for the perception of similarity and dissimilarity of HVAC-noise.In the second step of the study a model to predict subjective response to HVAC sounds using the semantic differential technique was developed. In particular the perception of annoyance was studied and it is shown that the annoyance caused by the HVAC noise can be satisfactorily described by Zwicker’s stationary loudness model, provided that the HVAC noises do not present tonal components. The loudness model also predicts scores on a quiet/loud scale. Both results confirm the power of the loudness dimension and its model introduced by Zwicker for the overall quality of stationary broadband sounds without slow fluctuations or tonal components. From the annoyance model developed in this study a maximum acceptable loudness level for HVAC-systems can be determined.     

A simple empirical model of polyester fibre materials for acoustical applications

A new empirical model has been developed by the authors to predict the flow resistivity, acoustic impedance and sound absorption coefficient of polyester fibre materials. The parameters of the model have been adjusted to best fit the values of airflow resistivity and sound absorption coefficient measured over a set of 38 samples. Calculated results are compared with normal incidence measurements carried out using two different techniques: the transfer-function method in an impedance tube (ISO 10534-2) and the free-field impulse response method (ISO 13472-1). Measurements performed on polyester fibre materials with different density and thickness values, and diameter ranging from 18 to 48 μm, are in good agreement with the predictions of the new model. It is concluded that the new model can predict the basic acoustic properties of common polyester fibre materials with any practical combination of thickness and density².     

复杂封闭空间结构声辐射的有源消声机制

采用两种控制目标函数,通过比较控制前后两个弹性板及空腔内声压模态坐标幅值与相角的变化,分析了复杂封闭空间结构声辐射的有源消声机制.结果表明,在一般情况下,空腔内噪声的降低是由两个弹性板的模态重组来实现;当扰动频率为顶板的一个固有频率时,有源消声则由顶板模态幅值抑制和底板模态重组的共同作用来完成.     

Directing ultrasound at the cemento-enamel junction (CEJ) of human teeth: I. Asymmetry of ultrasonic path lengths

The diagnosis of degenerative changes in human teeth is of general interest because early detections can avoid greater health problems and further weakening effects. Since the wear of teeth determines their stability and lifetime in relation to the physiological load, an ultrasonic survey of any dimensional changes of the enamel layer and especially of the dentin wall thickness may be very helpful. However, an ultrasonographic diagnosis requires first to determine the anisotropic human tooth properties at clinically relevant locations and to simulate wave propagation phenomena in inhomogeneous tooth models with proper dimensions. The first article of a series that provides modular data of mineralized tissues in human teeth at the cemento-enamel junction (CEJ) deals with an ultrasonic method for measuring the asymmetry of dimensional characteristics of extracted human teeth and their ultrasonic path lengths (UPL). Heavily attenuating tooth halves were investigated with respect to the symmetry of normal and inclined oppositely directed radial ultrasonic paths. The measured UPLs ranged from 1.2 mm to 4.4 mm. The relative difference in inclined UPLs between the left and the right tooth halves reaches almost 30%. This reveals a large asymmetry. The mean difference of angles that represent fastest path lengths was 2.2+/-8.1 degrees, which indicates large asymmetry and anisotropy. Several aspects, which are required for a proper integration of asymmetric data into models designed for medical element engineering and simulation (MEES), are discussed.     

Reverberation time measurement by the product of two room impulse responses

The decay function for the evaluation of the reverberation time is often obtained by the method of the backward integration of a squared room impulse response as suggested by M.R. Schroeder more than four decades ago. Since then much work has been published about its implementation. However, soon after the initial exploitation of the method, it was realized that the effects of the background noise contaminating the room impulse response required a careful consideration for accomplishing better results.This paper describes an alternative method dealing with the problem of the backward integration of noisy room impulse responses. This method is based on the processing of two impulse responses sequentially recorded for a fixed source and receiver arrangement in a room. Statistical criteria are proposed to identify how the effect of the noise corrupts the level decay curve using a noise-free synthesized room impulse response as well as measurements performed in a real room.     

Numerical and experimental studies on the effects of stage risers

The acoustic effects of stage risers, especially on the sound of lower string instruments, are numerically and experimentally analyzed. To discuss the effects of the vibration of riser’s boards due to the mechanical force from an instrument, a structural-acoustical coupling approach is applied based on the mode expansion and the boundary element technique. Measurement results of the mechanical force from real instruments are used in the numerical study. The vibration of the top board of a riser affects the sound field only around the natural frequencies of the board and the cavity of the riser. In contrast, the acoustic diffraction due to the riser affects the sound field in a wide frequency range. The riser’s sideboard that faces to receiving points increases the sound pressure levels because it reflects waves diffracted at the riser’s edge to the front. To verify the numerical results, the effects of acoustic diffraction due to risers are especially investigated in detail through a scale model experiment.     

Measurement of noise associated with model transformer cores

The performance of a transformer core may be considered in terms of power loss and by the noise generated by the core, both of which should be minimised. This paper discusses the setting up of a suitable system for evaluation of noise in a large model transformer core (500 kV A) and issues associated with noise measurement. The equivalent continuous sound pressure level (LAeq) was used as a measure of the A-weighted sound level and measurements were made in the range 16 Hz–25 kHz for various step lap core configurations. The selection of optimum sound insulation materials between core and ground support and for enclosing the transformer is essential for minimisation of background noise. Core clamping pressure must be optimised in order to minimise noise. The use of two laminations per layer instead of one leads to an increase in noise arising from the core. Provided care is taken in building the core, good reproducibility of results can be obtained for analysis.     

Measurement and modeling of diffraction from an edge of a thin panel

The diffraction of sound from an edge of a thin chipboard panel was measured in an anechoic chamber, and compared to simulations based on the diffraction formulation developed by Svensson et al. [Svensson UP, Fred RI, Vanderkooy J. An analytic secondary source model of edge diffraction impulse responses. J Acoust Soc Am 1999;106(5):2331-44]. The measurements and simulations were performed for a line of receiver positions below the panel to include cases for which the direct sound had an unobstructed propagation path to the receivers, as well as cases for which the direct sound was occluded by the panel. Comparison of the measured and simulated responses is provided in both the time and frequency-domains, and shows that the differences between them are small over the entire audible frequency range. This case study verifies that the applied diffraction-modeling method gives accurate results, and that the assumptions of ideal source and wedge characteristics inherent in the method do not preclude its use in simulations of realistic scenarios.     

A new method to determine the frequency response of enclosures using masked tones

Existing methods for equalization of enclosures are traditionally carried out in an empty room due to the annoying effect of the signals employed (pink noise, 1/3 octave tones, etc.) over the audience. Under these conditions, the frequency response and the subsequent equalization are determined from results that do not take into account the presence of people nor any other change that may occur in the room while the musical event takes place. In addition, this kind of equalization cannot be objectively readjusted during the event. In this contribution, a method to determine the frequency response of enclosures in an imperceptible way for the audience is presented. It involves including a test signal within a musical track that, thanks to the behaviour of human hearing, can be masked and successfully recovered using reference microphones. Hence, it is possible to obtain the room transfer function in the presence of public and perform the equalization adjustments while the music track is being played. The results agree completely with traditional methods, the great advantage being the possibility of performing the equalization at any moment during the musical event and in a transparent fashion with respect to the audience.     

Modelling of a city canyon problem in a turbulent atmosphere using an equivalent sources approach

The sound propagation into a courtyard shielded from direct exposure is predicted using an equivalent sources approach. The problem is simplified into that of a two-dimensional city canyon. A set of equivalent sources are used to couple the free half-space above the canyon to the cavity inside the canyon. Atmospheric turbulence causes an increase in the expected value of the sound pressure level compared to a homogeneous case. The level increase is estimated using a von Kármán turbulence model and the mutual coherences of all equivalent sources' contributions. For low frequencies the increase is negligible, but at 1.6 kHz it reaches 2-5 dB for the geometries and turbulence parameters used here. A comparison with a ray-based model shows reasonably good agreement.     

Infrared diode laser absorption features of N2O and CO2 in a laval nozzle

Design and operation of a pulsed Laval nozzle and the characterization of molecular flow through such a nozzle using IR tunable diode laser (TDL) is the central theme of this work. The results here deal with He diluted N₂O and CO₂ gaseous systems. Boltzmann type plots of the spectral intensity data of both N₂O and CO₂ show non-linear behaviour. We have attempted to understand this non-linear behaviour of Boltzmann plots in terms of (1) instability in the jet and (2) a two-temperature model for the flowing gas, a cold central core and a hot boundary layer close to the nozzle walls. The model based on jet instability represents the data somewhat poorer than the two-temperature model. The parameters derived from fitting our experimental data to the former model could be used to calculate the thermodynamic parameters only through further approximations. Measured absorption line profile of the P(15) line of the v ₂ band of N₂O as a function of axial distance from the nozzle exit gradually shifts from a Lorentzian to a Gaussian type. Velocity distribution of N₂O molecules in a Laval nozzle is determined by differentiating the absorption line profile of the P(15) line (v ₀=576.235 cm^–1) of the v ₂ band of N₂O. Translational temperature of N₂O molecules is determined from the observed spectral profiles.     

外力源和外声源共同作用下复杂封闭空间的有源消声

对于具有两个弹性板的复杂封闭空间,建立了在外声源和外力源以及控制电压共同作用下有源消声系统建模的新方法。长方体封闭空间耦合系数数值计算的结果,验证了这一方法的正确性。采用全局控制和局部控制方案,进行了计算机仿真,取得了良好的消声效果。该方法拓宽了利用结构声辐射进行有源消声的范围,可适用于任意形状的封闭空间。     

Numerical modelling of the radiation efficiency of asymmetrical structures

The radiation efficiency of a structural element is required by some models in order to predict its sound insulation. A common assumption is that the radiation on both sides of the element is the same. This is not true for asymmetrical structural elements like lightweight floors consisting of a beam-supported flat board. The radiation efficiency is larger on the beam side, because the beams act as exciters and increase the pressure level in the room. These different radiation efficiencies are calculated here for a two-dimensional cross-section by using finite elements and boundary elements. The obtained preliminary results illustrate that considering a single radiation efficiency can be a source of errors and that further investigation is required in order to improve predictions.     

Periodic shunted arrays for the control of noise radiation in an enclosure

This work presents numerical and experimental investigations of the application of a periodic array of resistive-inductive (RL) shunted piezoelectric patches for the attenuation of broadband noise radiated by a flexible plate in an enclosed cavity. A 4×4 lay-out of piezoelectric patches is bonded to the surface of a rectangular plate fully clamped to the top face of a rectangular cavity. Each piezo-patch is shunted through a single RL circuit, and all shunting circuits are tuned at the same frequency. The response of the resulting periodic structure is characterized by frequency bandgaps where vibrations and associated noise are strongly attenuated. The location and extent of induced bandgaps are predicted by the application of Bloch theorem on a unit cell of the periodic assembly, and they are controlled by proper selection of the shunting circuit impedance. A coupled piezo-structural-acoustic finite element model is developed to evaluate the noise reduction performance. Strong attenuation of multiple panel-controlled modes is observed over broad frequency bands. The proposed concept is tested on an aluminum plate mounted in a wooden box and driven by a shaker. Experimental results are presented in terms of pressure responses recorded using a grid of microphones placed inside the acoustic box.     

Effect of changing data size on eigenvalues in the Korean and Japanese stock markets

In this study, we attempted to determine how eigenvalues change, according to random matrix theory (RMT), in stock market data as the number of stocks comprising the correlation matrix changes. Specifically, we tested for changes in the eigenvalue properties as a function of the number and type of stocks in the correlation matrix. We determined that the value of the eigenvalue increases in proportion with the number of stocks. Furthermore, we noted that the largest eigenvalue maintains its identical properties, regardless of the number and type, whereas other eigenvalues evidence different features.