Normal Sound Incidence Research Articles

Equivalent Circuit Method Based Double Layer Micro-perforated Panel (MPP) Design to Widen the Sound Absorption Bandwidth

Acoustical solutions that occupy minimum space and simultaneously allow for flexibility in interior spaces are not feasible by using thick absorbers like conventional porous and fibrous materials. Micro-perforated panels are an excellent alternative to those classical materials due to their environmental friendliness, design flexibility on various materials, and easy installation. However, they come with the handicap of having a very narrow absorption frequency bandwidth. This study explores two inhomogeneous microperforated layers arranged in a cascade with different back cavities in order to widen the sound absorption bandwidth. This serial-parallel architected MPP system is investigated by mathematical models using Maa's model for a single MPP and the Equivalent Circuit Method (ECM). The numerical results were validated through experiments for normal sound incidence in the impedance tube using the transfer function method. Increasing the back cavity length of the MPP having a higher perforation ratio and larger holes results in a higher absorption in the high-frequency range. In contrast, combinations of larger cavity lengths and smaller perforation ratios are more effective in the low-frequency spectrum. By tuning the design parameters of those structures, it is possible to achieve wideband absorption by fibreless materials for room acoustic applications.

Angle-Dependent Absorption of Sound on Porous Materials

Sound-absorbing materials are usually measured in a reverberation chamber (diffuse field condition) or in an impedance tube (normal sound incidence). In this paper, we show how angle-dependent absorption coefficients could be measured in a factory-type setting. The results confirm that the materials have different attenuation behavior to sound waves coming from different directions. Furthermore, the results are in good agreement with sound absorption coefficients measured for comparison in a reverberation room and in an impedance tube. In addition, we introduce a biofiber-based material that has similar sound absorption characteristics to glass-wool. The angle-dependent absorption coefficients are important information in material development and in room acoustics modeling.

Enhancement in fire retardant Properties of Coconut Coir/Polyurethane acoustic composites

ABSTRACT This study aims to enhance the flammability properties of coconut coir and polyurethane resin sound-absorbing material. Sound absorption properties of the material for normal sound incidence were measured using impedance tube setup up based on ISO 10534-2 and ASTM E1050-98 standards whereas fire-retardant tests include limiting oxygen (ASTM D2863 standard) and flame extinguish tests. Results illustrate that coconut coir fiber with polyurethane resin has greater sound absorption characteristics; moreover, it has poor fire-retardant properties. Effect of different synthetic and natural fire retardants like diammonium phosphate (DAP), urea, and aloe vera were investigated. Fire-retardant tests show that material treated with DAP and aloe vera shows excellent self-extinguish properties. Morphological study of the acoustic material has been performed by using scanning electron microscope. Also, acoustic tests show that the effect of fire retardants on sound absorption characteristic of the material was not too deteriorating.

Optimization of micro-perforated sound absorber using Particle Swarm Optimization (PSO)

This study investigates the acoustic properties of micro-perforated panel (MPP) as an alternative to passive noise control. In order to obtain high sound absorption over broad frequency band, the analysis of sound absorption performance of MPP sound absorber should be conducted based on 3 major design parameters, such as perforation diameter, d, air cavity depth, D, and distance between perforations, b, of MPP. Next, the optimization of sound absorption coefficient for MPP sound absorber need to be done by using the Particle Swarm Optimization (PSO) algorithm and the optimum parameters of a single layer MPP sound absorber should be obtained for achieving higher sound absorption which the sound absorption coefficient, α + 0.5 with a wider frequency band. The PSO is used in order to optimize the panel construction to provide maximum sound absorption coefficient in a determined wide band frequency range. Experiments are carried out at normal sound incidence and plane waves. A good agreement is achieved between the theory and the experiments.

Analytical coupled modeling of a magneto-based acoustic metamaterial harvester

Membrane-type acoustic metamaterials (MAMs) have demonstrated unusual capacity in controlling low-frequency sound transmission, reflection, and absorption. In this paper, an analytical vibro-acoustic-electromagnetic coupling model is developed to study MAM harvester sound absorption, energy conversion, and energy harvesting behavior under a normal sound incidence. The MAM harvester is composed of a prestressed membrane with an attached rigid mass, a magnet coil, and a permanent magnet coin. To accurately capture finite-dimension rigid mass effects on the membrane deformation under the variable magnet force, a theoretical model based on the deviating acoustic surface Green’s function approach is developed by considering the acoustic near field and distributed effective shear force along the interfacial boundary between the mass and the membrane. The accuracy and capability of the theoretical model is verified through comparison with the finite element method. In particular, sound absorption, acoustic-electric energy conversion, and harvesting coefficient are quantitatively investigated by varying the weight and size of the attached mass, prestress and thickness of the membrane. It is found that the highest achievable conversion and harvesting coefficients can reach up to 48%, and 36%, respectively. The developed model can serve as an efficient tool for designing MAM harvesters.

Sound absorption of porous materials – Accuracy of prediction methods

The normal incidence sound absorption coefficient of porous materials can be predicted when the specific acoustic impedance of the material is known. Impedance prediction methods have been published by Delany and Bazley, Qunli, Miki, Mechel, Mechel and Vér, Komatsu, and Allard and Champoux. The independent variables of these methods are specific flow resistivity and material thickness. However, the existing literature fails to provide proper knowledge regarding the prediction accuracy of these methods. The aim of this study was to determine the prediction accuracy of the sound absorption coefficient of these seven published impedance prediction methods. The study focused on normal sound incidence. Accuracy was determined by comparing the predicted and the measured absorption coefficients of 82 mineral wool configurations. The transfer matrix method was used to calculate the absorption coefficient within 100 and 3150Hz. Allard and Champoux’s method reached the best prediction accuracy. A simplification of this method was developed to enable fast implementation. The results are very useful for researchers and developers who employ impedance prediction methods.

Sound absorption modeling of thin woven fabrics backed by an air cavity

A theoretical model for the oblique incidence sound absorption coefficient of thin woven fabrics backed by an air cavity is presented where the fabric is acoustically described by its specific airflow resistance and its surface mass density. The theoretical model is illustrated by an equivalent electrical circuit and validated in the case of normal sound incidence by experimental results obtained from impedance tube measurements on three fabric types. The influence of the surface mass density on the absorption coefficient is discussed and recommendations for practical applications are derived. Further, a simple formula to predict the specific airflow resistance of woven fabrics based on geometrical parameters is deduced. The normal incidence absorption coefficient and geometrical parameters of a set of 24 fabrics with a large range of interyarn porosities and specific airflow resistances were measured and used to validate the proposed geometry-based model to predict the absorption coefficient. Measured and estimated absorption coefficients show excellent agreement, with mean value and standard deviation of the differences of 0.03 ± 0.10. The model is therefore suitable for the design of new fabrics with an intended absorption coefficient.

2D Phononic Crystal Sensor with Normal Incidence of Sound

This contribution shows the sensor application of a resonance-induced extraordinary transmission through a regular phononic crystal consisting of a metal plate with a periodic arrangement of holes in a square lattice at normal incidence of sound. The characteristic transmission peak has been found to strongly depend on liquid sound velocity. The respective peak maximum frequency can therefore serve as measure for liquid analysis. Experimental verification has been performed with mixtures of water and propanol as model system. Numerical calculations based on COMSOL and EFIT reveal more insides to the wave propagation characteristics. Experimental investigations with Schlieren method and laser interferometry support the theoretical findings.

Multi-frequency characterization of the speed of sound and attenuation coefficient for longitudinal transmission of freshly excised human skulls

For medical applications of ultrasound inside the brain, it is necessary to understand the relationship between the apparent density of skull bone and its corresponding speed of sound and attenuation coefficient. Although there have been previous studies exploring this phenomenon, there is still a need to extend the measurements to cover more of the clinically relevant frequency range. The results of measurements of the longitudinal speed of sound and attenuation coefficient are presented for specimens of human calvaria. The study was performed for the frequencies of 0.27, 0.836, 1.402, 1.965 and 2.525 MHz. Specimens were obtained from fresh cadavers through a protocol with the Division of Anatomy of the University of Toronto. The protocol was approved by the Research Ethics Board of Sunnybrook Health Sciences Centre. The specimens were mounted in polycarbonate supports that were marked for stereoscopic positioning. Computer tomography (CT) scans of the skulls mounted on their supports were performed, and a three-dimensional skull surface was reconstructed. This surface was used to guide a positioning system to ensure the normal sound incidence of an acoustic signal. This signal was produced by a focused device with a diameter of 5 cm and a focal length of 10 cm. Measurements of delay in time of flight were carried out using a needle hydrophone. Measurements of effective transmitted energy were carried out using a radiation force method with a 10 µg resolution scale. Preliminary functions of speed of sound and attenuation coefficient, both of which are related to apparent density, were established using a multi-layer propagation model that takes into account speed of sound, density and thickness of the layer. An optimization process was executed from a large set of random functions and the best functions were chosen for those ones that closest reproduced the experimental observations. The final functions were obtained after a second pass of the optimization process was executed, but this time using a finite-difference time-difference solution of the Westervelt equation, which is more precise than the multi-layer model but much more time consuming for computation. For six of seven specimens, measurements were carried out on five locations on the calvaria, and for the other specimen three measurements were made. In total, measurements were carried out on 33 locations. Results indicated the presence of dispersion effects and that these effects are different according to the type of bone in the skull (cortical and trabecular). Additionally, both the speed of sound and attenuation showed dependence on the skull density that varied with the frequency. Using the optimal functions and the information of density from the CT scans, the average values (±s.d.) of the speed of sound for cortical bone were estimated to be 2384(± 130), 2471(± 90), 2504(± 120), 2327(± 90) and 2053(± 40) m s−1 for the frequencies of 270, 836, 1402, 1965 and 2526 kHz, respectively. For trabecular bone, and in the same order of frequency values, the speeds of sound were 2140(± 130), 2300(± 100), 2219(± 200), 2133(± 130) and 1937(± 40) m s−1, respectively. The average values of the attenuation coefficient for cortical bone were 33(± 9), 240(± 9) and 307(± 30) Np m−1 for the frequencies of 270, 836, and 1402, respectively. For trabecular bone, and in the same order of frequency values, the average values of the attenuation coefficient were 34(± 13), 216(± 16) and 375(± 30) Np m−1, respectively. For frequencies of 1.965 and 2.525 MHz, no measurable radiation force was detected with the setup used.

Effective wide-band, low-frequency multiple resonator sound absorber

Resonator-type sound absorbers are incorporated to provide relatively large sound attenuation in narrow bands. They are applied in several environments and have great practical value for the attenuation of low frequencies. A detailed experimental study leads to the construction of a multiple resonator sound absorber. It is made of metal sheet and has a total volume of 0.0016 m3. The acoustic performance of the designed device, namely, the sound absorption coefficient and the normalized input acoustic impedance, were measured in 20-Hz steps at normal sound incidence in room temperature and under heating conditions up at 4000<th>°C. The designed device provides good sound-absorption-frequency characteristics not only in the high values of the sound-absorption coefficient, but also in covering a wide frequency band (60 to 1000 Hz) without attenuation breaks. Also, its acoustic characteristics are changed very little under heating conditions. Applying earlier results obtained by Lord Rayleigh and simplified by Crandall for the propagation of a sound wave in a tube, and recently applied by Maa to characterize microperforated sound absorbers, the acoustic performance of the designed multiple resonator was predicted. The measured and predicted results are in good agreement.

Measurements of the spherical reflection coefficient at grazing sound incidence above sediment

Liquid undissolved fluid pollutants covering the sea bottom, in general, form a thin layer of a few cm, which cannot be detected acoustically by normal sound incidence due to the small difference of impedance compared to water. Such layers, however, can strongly affect the spherical reflection coefficient at grazing sound incidence. Using broadband chirp signals, the interference pattern at the receiving hydrophone is found to provide information on the physical properties of the sediment surface and its coating. Laboratory measurements in a small water tank with flat and rippled sand sediments coated by various different chemical substances revealed a systematic dependence of the measured excess attenuation function (EAF) on the acoustic impedance of the chemical, the layer thickness and the grazing angle of the incident sound. Results of numerical modeling performed with a fast field program (fflags) show good agreement with the measurement results. In order to prove the applicability of the new detection method, measurements in shallow water will be presented. [Work supported by BMBF.]

Determination of acoustic absorber parameters in impedance tubes

A computer-aided test method was developed to measure the acoustic parameters of absorber specimens in impedance tubes. A specially written Windows program, implementing the single microphone fast Fourier transform method, enables quick and precise measurement of the impedance and absorption coefficient of samples at normal sound incidence. By means of specific modifications a high precision is achieved even under critical test situations. This is demonstrated in different test cases. The measured parameter of newly designed resonance absorbers (one reactive and one active) underline the improvements made. Additional care was taken to keep the system simple, robust and easy-to-use.

Sound absorbing double curtains from local textile materials

A new sample made of local textile material (100% cotton) (Misr Spinning and Weaving Company, Mehalla El Kubra) is produced from a specially woven structure which possesses high porosity for use as a sound absorbing curtain. A double-layered curtain was designed for high efficiency, not only in sound absorption, but also light insulation. One layer was made of 100% cotton, while the other was made of cotton/polyester 35:65. The two layers are separated by an air space, which is varied to obtain a construction of optimum effectiveness. Using the standing wave apparatus (Bruel and Kjaer type 4002), the sound absorption coefficient α of such samples was measured at normal sound incidence. The effect of air space, behind and/or between the layers of curtain, on the absorption coefficient has been studied in order to achieve the best construction for a specific application.