Sound field of a baffled sound source covered by an anisotropic rigid-porous material

A theoretical approach for the sound field of a piston sound source covered by a finite thickness layer of anisotropic rigid-porous material is presented. The formulation is an extension of the method worked out by Amedin et al. [Sound field of a baffled piston source covered by a porous medium layer. J Acoust Soc Am 1995;98(3):1757]. First, in the present study the sound field of a point source is described by cylindrical waves. Then, with the proper boundary conditions, the sound pressure radiated from a piston source covered by a layer of anisotropic porous material can be calculated. The effects of frequency and bulk density of material on the sound propagation in an anisotropic porous material are studied. Finally, the effect of anisotropy is discussed.     

Vortex sound under the influence of a piecewise porous material on an infinite rigid plane

The vortex dynamics and the sound generation by an inviscid vortex in the presence of a finite length porous material on an otherwise rigid plane are studied numerically in the present study in an attempt to understand the sound generation near the surface of a wall lining in a lined duct. The combined effects of the effective fluid density and flow resistance inside the porous material, and the length and thickness of the porous material on the sound generation process are examined in detail. Results obtained demonstrate the sound pressure is longitudinal dipole and show how seriously the above-mentioned parameters are affecting the vortex sound pressure under the influence of the porous material.     

Vibration damping material as a means to reduce steel noise barrier cost

A preliminary analysis has been made of the use of vibration damping material as a means to reduce the cost of steel noise barriers in primarily highway applications. For cost-effective barriers, the sound transmission through, as well as over, a noise barrier must be considered. The through-barrier sound transmission characteristics of sample panels from a Toronto noise barrier were measured with and without damping material. It was found that a given through-barrier sound transmission performance could be achieved at less cost with the damping material than without it. Further study is recommended.     

高分子颗粒孔隙结构材料的吸声特性研究

在噪声控制与治理工程中，许多场合了消除由物体表面产生的反射声波，往往需要在物体表面敷设具有吸声性能的材料或结构，以吸收入射声波的声能，本文对一种新型的以高分子材料为基底的颗粒型微孔结构进行了吸声特性研究，首先对颗粒微孔结构的吸声机理进行分析，认为由于受材料自身化学性质与结构特征所决定，这种颗粒微孔结构除了具一般多孔吸声材料的空气粘滞阻力作用吸声外，还存在颗粒材料的内部的弹性弛豫效应吸声，因而吸怕系     

Sound absorption characteristics of a double-leaf structure with an MPP and a permeable membrane

As for the sound absorbing system using an MPP (microperforated panel), a double-leaf MPP sound absorber has been studied so far. However, this structure uses two MPPs, which are still expensive, and is disadvantageous when its cost is concerned. Therefore, it is considered that it can be advantageous if one of the leaves can be replaced with a less expensive material keeping high sound absorption performance. In this study, the possibility of producing a useful sound absorbing structure with an MPP and a permeable membrane as an alternative less expensive material is examined. The acoustic properties of this MPP and permeable membrane combination absorber are analysed theoretically with a Helmholtz integral formulation. The absorption performance and mechanism are discussed through the numerical examples. Also, the effect of a honeycomb in the air cavity, which is to be used for reinforcing the structure, is also discussed through a theoretical analysis.     

Evaluation of the acoustic and non-acoustic properties of sound absorbing materials using a three-microphone impedance tube

This paper presents a straightforward application of an indirect method based on a three-microphone impedance tube setup to determine the non-acoustic properties of a sound absorbing porous material. First, a three-microphone impedance tube technique is used to measure some acoustic properties of the material (i.e., sound absorption coefficient, sound transmission loss, effective density and effective bulk modulus) regarded here as an equivalent fluid. Second, an indirect characterization allows one to extract its non-acoustic properties (i.e., static airflow resistivity, tortuosity, viscous and thermal characteristic lengths) from the measured effective properties and the material open porosity. The procedure is applied to four different sound absorbing materials and results of the characterization are compared with existing direct and inverse methods. Predictions of the acoustic behavior using an equivalent fluid model and the found non-acoustic properties are in good agreement with impedance tube measurements.     

Mathematically trivial control of sound using a parametric beam focusing source

By exploiting a case regarded as trivial, this paper presents global active noise control using a parametric beam focusing source (PBFS). As with a dipole model, one is used for a primary sound source and the other for a control sound source, the control effect for minimizing a total acoustic power depends on the distance between the two. When the distance becomes zero, the total acoustic power becomes null, hence nothing less than a trivial case. Because of the constraints in practice, there exist difficulties in placing a control source close enough to a primary source. However, by projecting a sound beam of a parametric array loudspeaker onto the target sound source (primary source), a virtual sound source may be created on the target sound source, thereby enabling the collocation of the sources. In order to further ensure feasibility of the trivial case, a PBFS is then introduced in an effort to meet the size of the two sources. Reflected sound wave of the PBFS, which is tantamount to the virtual sound source output, aims to suppress the primary sound. Finally, a numerical analysis as well as an experiment is conducted, verifying the validity of the proposed methodology.     

单层传声器阵列重建相干声场的近场声全息方法研究

针对传声器阵列两侧存在相干声源的非自由声场重建问题,提出基于球面谐波函数扩展近场声全息理论的相干声场重建方法。该方法在已知测量面两侧声源几何位置时,使用单层传声器阵列获取测量面处的声压分布,通过最小二乘法获得与目标声源和干扰噪声源响应对应的最优球波函数扩展项数和最优系数向量,结合测点位置的空间坐标进行声波分解,并分别重建出各声源在测量面上的声压分布。为了验证方法的有效性,分别给出了相干噪声源为球形声源和非球形声源的仿真验证,并在全消声室内对双扬声器产生的相干声场的重建进行了实验验证。结果表明:该方法对球形声源和非球形声源干扰下的声场重建都具有较好的效果,球形声源干扰下的重建精度更高。      

Definition of a labelling code for the noise emitted by machines

It has been increasingly realised that traditional parameters, such as A-weighted sound levels, sound power and noise spectra in octaves or third octaves are not enough to describe a sound and that machine labels should show more subjective information about the emitted noise. A new labelling system based on empirical data is proposed. The methodology of the study used as the basis for the proposal is explained, as well as the objective and subjective characteristics to be included in the new label. The uses and applications of the label are discussed briefly.     

Acoustic evolution of ancient Greek and Roman theatres

In this investigation the acoustic properties of ancient performance spaces for Greek and Roman drama were studied, from the viewpoint of material and design evolution. The paper first reviews the theatre evolution in antiquity. Following a systematic examination of computer simulation methodology for ancient Greek and Roman theatres, it then presents the results of a series of acoustic simulations in six typical theatre forms, using a beam-tracing program. Particular attention is paid to the scattering and diffraction from seat risers, which are important in outdoors theatres. Acoustic indices considered include sound pressure level, reverberation time and speech transmission index. The results show that in the evolution of ancient Greek and Roman theatres, changes in materials and theatre design generally resulted in acoustic improvements – the reverberation time had been increased and the sound level had been enhanced. Since the exact material characteristics are unknown, possible variations in acoustic performances are discussed based on simulations using a range of boundary absorption and scattering coefficients.     

Sound absorption characteristics of a high-temperature sintering porous ceramic material

This article is dedicated to sound absorption properties of porous zeolite with macropores, a ceramic material fabricated by high-temperature sintering. Acoustical properties of this ceramic material are studied by two analytical models, Delany–Bazley model and Johnson–Allard model, where the latter one shows a better fit to the experimental results. Moreover increasing the thickness of samples would improve the sound absorption in the low frequency ranges. Raising the porosity could increase the highest sound absorption coefficient. The resonance frequencies of the materials with 3–5 mm particles are more obvious. Comparing with glass wool, porous zeolite has a better sound absorption.     

Objective assessment of the sound paths through earmuff components

This paper proposes an objective measurement methodology to assess the sound transmission paths through various earmuff components (cup, cushion, back-plate, foam insert, silicone flesh) and estimate the vibroacoustic couplings between them. The measurement methodology is applied to two different commercial earmuffs. For each Hearing Protection Device (HPD), the different sound paths are assessed, by analyzing separately the vibroacoustic behavior of each HPD component. In particular, the Insertion Loss (IL) of the lateral walls of the cushion is assessed, with and without pumping motion. The effect of the coupling between the different parts is also investigated from the comparison of the separate components with the complete earmuff. As shown in past studies, results confirm that the acoustical behavior of an earmuff is governed by the pumping motion and air leaks at low frequency and by the sound path through the cup and the cavity resonances at high frequency. However, at mid frequency, new counter-intuitive results are revealed. It is found that earmuffs do not behave as their weakest element, but rather as the one which has the highest attenuation. It is also observed that the foam insert can also decrease the IL. Complementary tests using other absorbing materials show that this effect could mainly be attributed to the absorption behaviors of each material at medium frequencies.     

Use of a hybrid adaptive finite element/modal approach to assess the sound absorption of porous materials with meso-heterogeneities

The paper discusses the sound absorptive performance of a porous material with meso-perforations inserted in a rectangular waveguide using a numerical hybrid adaptive finite element-modal method. Two specific applications are investigated: (i) the improvement of porous materials noise reduction coefficient using meso-perforations (ii) the effects of lateral air gaps on the normal incidence sound absorption of mono-layer and two-layer porous materials. For the first application, a numerical design of experiments is used to optimize the sound performance of a porous material with meso-perforations with a reduced number of numerical simulation. An example in which the optimization process is carried out on the thickness and size of the perforation is presented to illustrate the relevance of the approach. For the second application, a set of twenty fibrous materials spanning a large flow resistivity range is used. Practical charts are proposed to evaluate the influence of air gaps on the average sound absorption performance of porous materials. This is helpful to both the experimenter regarding characterization of porous material based on Standing Wave Tube measurements and for the engineer to quantifying the impact of air gaps and for designing efficient absorbers.     

Noise reduction on a Rhine-cruise ship due to damping material

In order to attenuate the structure-borne sound generated by the resiliently mounted auxiliary diesel engines and propagated via the decks to the passenger cabins of a Rhine-cruise ship, two decks and the hull between them were treated with 400 m² elasto-dissipative material. Measurements of the sound pressure levels in a cabin, situated between these decks, and vibration measurements, carried out both before and after the application of the damping material, indicated only small level reductions. The reduction caused by completion of the fitting out of the ship, however, appreciably exceeded the influence of the damping material.     

An experimental procedure to obtain sound power level of tyre/road noise under Coast-By conditions

The rolling noise from tyre–pavement interaction represents the greatest sound contribution from a vehicle when cruising at a high speed. To evaluate the sound levels from this source, existing standardized methods that establish different measurement procedures in both the immediate tyre surroundings, for example the Close-Proximity method, as well as at greater distances, as the Coast-By method. A fundamental parameter that can quantify the sound generation of a source is its sound power level. The standardized methods establish procedures to measure the sound pressure level but not the power level of a tyre as a noise source. For this reason, this paper presents a novel methodology based on sound pressure measurements to obtain the sound power level that a vehicle emits in Coast-By conditions, where noise is generated at tyre/road interaction. The paper describes the testing procedure used to obtain the sound power level, and it is accompanied by a mathematical simulation that studies the feasibility of the proposal. Finally, the proposed methodology is further validated through a field study.     

Experimental estimation of the transmission loss contributions of a sound package placed in a double wall structure

The objective of this paper is to propose a practical impedance tube method to optimize the sound transmission loss of double wall structure by concentrating on the sound package placed inside the structure. In a previous work, the authors derived an expression that breakdown the transmission loss of a double wall structure containing a sound absorbing blanket separated from the panels by air layers in terms of three main contributions; (i) sound transmission loss of the panels, (ii) sound transmission loss of the blanket and (iii) sound absorption due to multiple reflections inside the cavity. The sound transmission loss contributions of the blanket can thus be estimated from three acoustic measurements using impedance tube techniques: two reflection coefficients at the front face and the rear face of the blanket placed in specific positions characteristic of its position inside the double wall structure and its sound transmission coefficient. The method is first validated in the case of a double wall structure filled with a 2 in. foam material. Next, it is applied to investigate (i) the effect of frame compression of a 2 in. fibre glass in an aeronautic-type double wall structure and (ii) the effect of double porosity with or without porous inclusions in a building-type double wall structure.     

Multi-domain active sound control and noise shielding

This paper describes an active sound control methodology based on difference potentials. The main feature of this methodology is its ability to automatically preserve "wanted" sound within a domain while cancelling "unwanted" noise from outside the domain. This method of preservation of the wanted sounds by active shielding control is demonstrated with various broadband and realistic sound sources such as human voice and music in multiple domains in a one-dimensional enclosure. Unlike many other conventional active control methods, the proposed approach does not require the explicit characterization of the wanted sound to be preserved. The controls are designed based on the measurements of the total field on the boundaries of the shielded domain only, which is allowed to be multiply connected. The method is tested in a variety of experimental cases. The typical attenuation of the unwanted noise is found to be about 20 dB over a large area of the shielded domain and the original wanted sound field is preserved with errors of around 1 dB and below through a broad frequency range up to 1 kHz.     

Sound propagation above a porous road surface with extended reaction by boundary element method

Acoustic impedance of an absorbing interface is easily introduced in boundary element codes provided that a local reaction is assumed. But this assumption is not valid in the case of porous road surface. A two-domain approach was developed for the prediction of sound propagation above a porous layer that takes into account the sound propagation inside the porous material. The porous material is modeled by a homogeneous dissipative fluid medium. An alternative to this time consuming two-domain approach is proposed by using the grazing incidence approximate impedance in the traditional single-domain boundary element method (BEM). It can be checked that this value is numerically consistent with the surface impedance calculated at the interface from the pressure and surface velocity solutions of the two-domain approach. The single-domain BEM introducing this grazing incidence impedance is compared in terms of sound attenuation with analytical solutions and two-domain BEM. The comparison is also performed with the single-domain BEM using the normal incidence impedance, and reveals a much better accuracy for the prediction of sound propagation above a porous interface.     

Schlieren photography to study sound interaction with highly absorbing materials

Strong absorption of sound is often caused by the conversion of sound energy into heat. When this happens, it is not possible to study the interaction of sound with the absorbing material by means of reflected sound characteristics, because there is no reflected sound. Detecting for example the distance that sound travels in a strongly absorbing material, can be done by heat detection systems. However, the presence of temperature detectors in such materials interferes with the sound field and is therefore not really suitable. Infrared measurements are a possible option. Another option is the use of Schlieren photography for simultaneous visualization of sound and heat. This technique is briefly outlined with a 3 MHz sound beam incident on a highly absorbing sponge.     

A scale value for the balance inside a historical opera house

In the framework of opera house acoustics, the term "balance" refers to the acoustical competition between the singer on the stage and the orchestra in the pit. The mechanism allowing the operatic singers to be heard over the orchestra has to do with their skill in enhancing the vocal emission by a peculiar use of the formant frequencies. This vital factor is sensed by the listeners and, apart from the obvious sound power ratio of the stage and the pit sources, is the main cue that helps to formulate a subjective impression of the balance. To achieve its objective qualification, two calibrated sound sources can be placed on the stage and in the pit, respectively, and their sound level difference is measured at the listeners' seats. The scope of this work is to investigate the relationship between the subjective impression and the objective indicator of the balance and to develop a scale value for the parameter in the case of a historical opera house. For this scope a set of acoustical data from the Teatro Comunale in Ferrara will be used to create synthetic sound fields with controlled conditions of the balance between the stage and the pit. This methodology employs an anechoic piece for soprano (with piano accompaniment) and is implemented in a dead room equipped with an acoustical rendering system. The sound fields are used to investigate the appropriate balance values by means of listening tests. The results of the scaling exercise show that a suitable range of values can be extracted and that the sound from the stage and the pit is perceived as balanced when the loudness difference between the two is comprised within -2.0 dBA and +2.3 dBA.