Diesel powered vehicles, in compliance with the more strict exhaust emission standards such as Euro V, is likely to require a diesel particulate filter （DPF）. A DPF used on a vehicle will affect the acoustic emissio...Diesel powered vehicles, in compliance with the more strict exhaust emission standards such as Euro V, is likely to require a diesel particulate filter （DPF）. A DPF used on a vehicle will affect the acoustic emission of the diesel engine, so it is important to investigate the sound propagation rule in DPF and further to propose the optimum DPF design. However, due to the geometrical complexity of the DPF, the traditional analysis method, such as analytical method, can not assess the acoustic performance of DPF accurately in medium and high frequency band. In this paper, a combined approach of finite element analysis and viscosity correction is proposed to predict acoustic performance of DPF. A simplified model of the full DPF is established and is used to analyze the sound propagation characteristic of the DPF. The distribution of the sound pressure and velocity, the transmission matrix of the DPF are obtained using the finite element method. In addition, the method of the viscosity correction is used in the transmission matrix of the DPF to evaluate the acoustic performance of DPF. Based on the FEM computation and the viscosity correction, the transmission losses under the rated load and idle condition of a diesel engine are calculated. The calculation results show that DPF can effectively attenuate exhaust noise, and sound attenuation increase with the rise of the frequency. Sound attenuation is better under rated condition than idle condition of diesel engine, particularly in frequency above 1 000 Hz.展开更多
This paper reports results of the authors’ studies on the virtual design method used in the development of low noise intake system of I.C. engine. The resulting high pass-by noise at level above the legislative targe...This paper reports results of the authors’ studies on the virtual design method used in the development of low noise intake system of I.C. engine. The resulting high pass-by noise at level above the legislative target at full throttle when engine speed was around 5200 r/min necessitated a BEM-aided redesign task, following the typical process of design and development of an intake system. During the initial design, based on the acoustic theory and the requirements (1. The air flux of the redesigned should equal to or exceed the value of the original flux; 2. The filtering area must not be degraded), and considering the constraint of space in the engine compartment, total volume and rough internal dimensions were determined. During the detailed design, the exact internal dimensions of the air cleaner were determined, and an effective method was applied to improve the acoustic performance at low frequency. The predicted sound power of the intake system indicated that the objective of reducing the overall engine noise by minimizing intake system noise was achieved.展开更多
The multi-dimensional time-domain computational fluid dynamics(CFD) approach is extended to calculate the acoustic attenuation performance of water-filled piping silencers. Transmission loss predictions from the time-...The multi-dimensional time-domain computational fluid dynamics(CFD) approach is extended to calculate the acoustic attenuation performance of water-filled piping silencers. Transmission loss predictions from the time-domain CFD approach and the frequency-domain finite element method(FEM) agree well with each other for the dual expansion chamber silencer, straight-through and cross-flow perforated tube silencers without flow. Then, the time-domain CFD approach is used to investigate the effect of flow on the acoustic attenuation characteristics of perforated tube silencers. The numerical predictions demonstrate that the mean flow increases the transmission loss, especially at higher frequencies, and shifts the transmission loss curve to lower frequencies.展开更多
The modern transportation system is increasingly developed during recent years.It is an effective solution to set the noise barriers to reduce the traffic noise pollution caused by different kinds of transportation sy...The modern transportation system is increasingly developed during recent years.It is an effective solution to set the noise barriers to reduce the traffic noise pollution caused by different kinds of transportation systems.Many deficiencies on concrete noise barriers and metal noise barriers with rivet structure can be eliminated by a new kind of noise barrier with no-riveted structure.The mechanical performance examination and acoustic performance test are conducted on the new-designed noise barrier with no-riveted structure.The results indicate that the maximum stress is 1.74 MPa and the maximum deformation is 1.04 mm with load acting on the unit plate.The noise reduction coefficient of this kind of no-riveted noise barrier unit plate is 0.75 and its noise insulation is 40 dB,which were conform to or superior to the standard requirements.Therefore,this new designed noise barrier meets the field application requirements of mechanical and acoustic performance,which demonstrates the noise barriers can be widely promoted.展开更多
The performance of oil palm trunk wastes from Banjarbaru of South Kalimantan was improved with the help of chemical modification in a two-step treatment.The first was formalization with formaldehyde solution with vary...The performance of oil palm trunk wastes from Banjarbaru of South Kalimantan was improved with the help of chemical modification in a two-step treatment.The first was formalization with formaldehyde solution with varying pH,and the second was impregnation with melamine-formaldehyde resin under 5 bar pressure for an hour.In these processes,the samples were cured at 120℃ for 10 min and then dried in an oven at(103±2)℃ in order to attain a moisture content of less than 6%.These treatments improved the physical properties(density,moisture content,and volume swelling),mechanical resistance,dynamic-mechanical and acoustic performance of the woods.The combination of impregnation and formalization changed the structure and the morphology of the woods such that the surface became flatter and denser.This was confirmed by results from FTIR,SEM,and DMA.Samples with alkaline modification displayed the best results for dimensional stability,storage modulus,and damping factor in varied frequencies.The treatments in this study also heightened acoustic performances as evidenced by the resulting characteristics of sound absorption coefficient and acoustic impedance.展开更多
A cavity viscoelastic structure has a good sound absorption performance and is often used as a reflective baffle or sound absorption cover in underwater acoustic structures.The acoustic performance field has become a ...A cavity viscoelastic structure has a good sound absorption performance and is often used as a reflective baffle or sound absorption cover in underwater acoustic structures.The acoustic performance field has become a key research direction worldwide.Because of the time-consuming shortcomings of the traditional numerical analysis method and the high cost of the experimental method for measuring the reflection coefficient to evaluate the acoustic performance of coatings,this innovative study predicted the reflection coefficient of a viscoelastic coating containing a cylindrical cavity based on an artificial neural network(ANN).First,themapping relationship between the input characteristics and reflection coefficient was analysed.When the elastic modulus and loss factor value were smaller,the characteristics of the reflection coefficient curve were more complicated.These key parameters affected the acoustic performance of the viscoelastic coating.Second,a dataset of the acoustic performance of the viscoelastic coating containing a cylindrical cavity was generated based on the finite elementmethod(FEM),which avoided a large number of repeated experiments.The minmax normalization method was used to preprocess the input characteristics of the viscoelastic coating,and the reflection coefficient was used as the dataset label.The grid search method was used to fine-tune the ANNparameters,and the prediction error was studied based on a 10-fold cross-validation.Finally,the error distributions were analysed.The average root means square error(RMSE)and the mean absolute percentage error(MAPE)predicted by the improved ANN model were 0.298%and 1.711%,respectively,and the Pearson correlation coefficient(PCC)was 0.995,indicating that the improved ANN model accurately predicted the acoustic performance of the viscoelastic coating containing a cylindrical cavity.In practical engineering applications,by expanding the database of the material range,cavity size and backing of the coating,the reflection coefficient of more sound-absorbing layers was evaluated,which is useful for efficiently predicting the acoustic performance of coatings in a specific frequency range and has great application value.展开更多
In marine engine exhaust silencing systems, the presence of exhaust gas flow influences the sound propagation inside the systems and the acoustic attenuation performance of silencers. In order to investigate the effec...In marine engine exhaust silencing systems, the presence of exhaust gas flow influences the sound propagation inside the systems and the acoustic attenuation performance of silencers. In order to investigate the effects of three-dimensional gas flow and acoustic damping on the acoustic attenuation characteristics of marine engine exhaust silencers, a dual reciprocity boundary element method (DRBEM) was developed. The acoustic governing equation in three-dimensional potential flow was derived first, and then the DRBEM numerical procedure is given. Compared to the conventional boundary element method (CBEM), the DRBEM considers the second order terms of flow Mach number in the acoustic governing equation, so it is suitable for the cases with higher Mach number subsonic flow. For complex exhaust silencers, it is difficult to apply the single-domain boundary element method, so a substructure approach based on the dual reciprocity boundary element method is presented. The experiments for measuring transmission loss of silencers are conducted, and the experimental setup and measurements are explained. The transmission loss of a single expansion chamber silencer with extended inlet and outlet were predicted by DRBEM and compared with the measurements. The good agreements between predictions and measurements are observed, which demonstrated that the derived acoustic governing equation and the DRBEM numerical procedure in the present study are correct.展开更多
The acoustic performance for the nanoporous frustule of the diatom is studied based on the computational fluid dynamics theory and acoustic theory involved.Representative Coscinodiscus sp.frustule is observed through ...The acoustic performance for the nanoporous frustule of the diatom is studied based on the computational fluid dynamics theory and acoustic theory involved.Representative Coscinodiscus sp.frustule is observed through the scanning electron microscope and modeled by the commercial software Solidworks.Further,the acoustic performance for the Coscinodiscus sp.frustule is studied at the varied depth,diameter or interval of the pore,as well as the film thickness of the fluid surrounding the Coscinodiscus sp.frustule.The numerical results show that,when the upper and lower pore diameters are separately 200 and 300 nm,the upper and lower pore depths are separately 200 and 250 nm,and both the pore interval and fluid film thickness are 500 nm,the elaborate nanoporous structure of Coscinodiscus sp.frustule can lower its acoustic power level by 17.49%,compared with that without porous structure.Meanwhile,the double-layer pore of Coscinodiscus sp.frustule can decrease its acoustic power level by 12.69%,compared with its single-layer pore structures.展开更多
The effect of coaxial, offset and extended inlet/outlet on the acoustic attenuation characteristics of circular expansion chambers are studied by the three-dimensional finite dement method. The numerical results of tr...The effect of coaxial, offset and extended inlet/outlet on the acoustic attenuation characteristics of circular expansion chambers are studied by the three-dimensional finite dement method. The numerical results of transmission loss are compared with experiment results to verify the necessary of using three-dimensional methods. Maps of acoustic pressure level distribution inside of chambers and inlet/outlet ducts are given at a frequency to demonstrate the difference of acoustic wave propagation behavior caused by locations of inlet/outlet ducts. For the chambers of the same length, the chamber with extended inlet/outlet duct has higher attenuation ability than coaxial and offset inlet/outlet duct over middle frequencies.展开更多
There is no accurate analytical approach for the acoustic performance prediction of Helmholtz resonator with conical neck,which has broad band acoustic attenuation performance in the low frequency range.To predict the...There is no accurate analytical approach for the acoustic performance prediction of Helmholtz resonator with conical neck,which has broad band acoustic attenuation performance in the low frequency range.To predict the acoustic performance of the resonator accurately,a general theory model based on the one-dimensional analysis approach with acoustic length corrections is developed.The segmentation method is used to calculate the acoustic parameters for sound propagation in conical tubes.And then,an approximate formula is deduced to give accurate correction lengths for conical tubes with difierent geometries.The deviations of the resonance frequency between the transmission loss results obtained by the general theory with acoustic lengths correction and the results from the finite element method and experiments are less than 2 Hz,which is much better than the results from one-dimensional approach without corrections.The results show that the method of acoustic length correction for the conical neck greatly improved the accuracy of the one-dimensional analysis approach,and it will be quick and accurate to predict the sound attenuation property of Helmholtz resonator with conical neck.展开更多
A finite elemcnt-Helmholtz integral hybrid method has been developed to calculate the electro-acoustic performance of a finite length cylindrical transducer immersed in water. The numerical study concerned with electr...A finite elemcnt-Helmholtz integral hybrid method has been developed to calculate the electro-acoustic performance of a finite length cylindrical transducer immersed in water. The numerical study concerned with electrical impedances and beam patterns of the transducer is presented. Experimental data are shown to support the numerical study.展开更多
A semi-analytical/numerical model based on the multiple scattering （MS） method has been established for analyzing the effect of acoustic performance on main energy attenua- tion mechanism in viscoelastic coating con...A semi-analytical/numerical model based on the multiple scattering （MS） method has been established for analyzing the effect of acoustic performance on main energy attenua- tion mechanism in viscoelastic coating containing axisymmetric cavities. The basic functions of stress and displacement of the axisymmetric cavity surface are derived in the system of spheri- cal coordinates. The transition matrix between the incident wave and the scattering wave are obtained by the numerical integral of the basic functions of the cavity surface. The reflection, transmission and absorption performance of viscoelastic materials containing periodic cavities are calculated using the MS method and the wave propagating theory of the multi-layered medium. The results indicate that low frequency energy is mainly attenuated through cavity resonance. The resonant properties are found to be very sensitive to the boundary conditions. The coupling of the double-cavity is capable of extending the absorption to even lower fre- quencies. The absorption performance of the viscoelastic coating in the high frequency range is independent of the backing material. Its energy attenuation depends mainly on acoustic properties of cavity scattering and mode conversion.展开更多
Based on the commonly used indicators for speech intelligibility, this work acoustically evaluates the two largest auditoria in the Faculty of Engineering, Helwan University, Cairo, Egypt, using experimental and digit...Based on the commonly used indicators for speech intelligibility, this work acoustically evaluates the two largest auditoria in the Faculty of Engineering, Helwan University, Cairo, Egypt, using experimental and digital simulation techniques. Design treatments were also suggested to improve the acoustic performance of the auditoria, where the impact of these treatments was checked using the simutation as well. The models that were analysed using the CATT-software were first validated utilizing the results of the fietd work in the unoccupied rooms. The results showed that the acoustic quality of the two auditoria are far from the optimal conditions due to their improper acoustic characteristics and the high noise revers as weft. The results of improvement proposals showed that altering the ceiling shape and adding efficient absorptive materials to the rear surfaces successfully reduced the excessive reverberation time to the optimal values, increased the earty reflections and eliminated the shadow zones. In addition, decreasing the noise Levels by 20 dB due to improving the window insulation noticeably improved the speech intelligibitity at all receivers.展开更多
基金supported by National Hi-tech Research and Development Program of China (863 Program, Grant No. 2009AA045103 )Tianjin Provincial Natural Science Foundation of China (Grant No. 05YFJMJC10700)
文摘Diesel powered vehicles, in compliance with the more strict exhaust emission standards such as Euro V, is likely to require a diesel particulate filter （DPF）. A DPF used on a vehicle will affect the acoustic emission of the diesel engine, so it is important to investigate the sound propagation rule in DPF and further to propose the optimum DPF design. However, due to the geometrical complexity of the DPF, the traditional analysis method, such as analytical method, can not assess the acoustic performance of DPF accurately in medium and high frequency band. In this paper, a combined approach of finite element analysis and viscosity correction is proposed to predict acoustic performance of DPF. A simplified model of the full DPF is established and is used to analyze the sound propagation characteristic of the DPF. The distribution of the sound pressure and velocity, the transmission matrix of the DPF are obtained using the finite element method. In addition, the method of the viscosity correction is used in the transmission matrix of the DPF to evaluate the acoustic performance of DPF. Based on the FEM computation and the viscosity correction, the transmission losses under the rated load and idle condition of a diesel engine are calculated. The calculation results show that DPF can effectively attenuate exhaust noise, and sound attenuation increase with the rise of the frequency. Sound attenuation is better under rated condition than idle condition of diesel engine, particularly in frequency above 1 000 Hz.
文摘This paper reports results of the authors’ studies on the virtual design method used in the development of low noise intake system of I.C. engine. The resulting high pass-by noise at level above the legislative target at full throttle when engine speed was around 5200 r/min necessitated a BEM-aided redesign task, following the typical process of design and development of an intake system. During the initial design, based on the acoustic theory and the requirements (1. The air flux of the redesigned should equal to or exceed the value of the original flux; 2. The filtering area must not be degraded), and considering the constraint of space in the engine compartment, total volume and rough internal dimensions were determined. During the detailed design, the exact internal dimensions of the air cleaner were determined, and an effective method was applied to improve the acoustic performance at low frequency. The predicted sound power of the intake system indicated that the objective of reducing the overall engine noise by minimizing intake system noise was achieved.
基金Project(11174065)supported by the National Natural Science Foundation of China
文摘The multi-dimensional time-domain computational fluid dynamics(CFD) approach is extended to calculate the acoustic attenuation performance of water-filled piping silencers. Transmission loss predictions from the time-domain CFD approach and the frequency-domain finite element method(FEM) agree well with each other for the dual expansion chamber silencer, straight-through and cross-flow perforated tube silencers without flow. Then, the time-domain CFD approach is used to investigate the effect of flow on the acoustic attenuation characteristics of perforated tube silencers. The numerical predictions demonstrate that the mean flow increases the transmission loss, especially at higher frequencies, and shifts the transmission loss curve to lower frequencies.
基金supported by the National Natural Science Foundation of China (No. 51768014).
文摘The modern transportation system is increasingly developed during recent years.It is an effective solution to set the noise barriers to reduce the traffic noise pollution caused by different kinds of transportation systems.Many deficiencies on concrete noise barriers and metal noise barriers with rivet structure can be eliminated by a new kind of noise barrier with no-riveted structure.The mechanical performance examination and acoustic performance test are conducted on the new-designed noise barrier with no-riveted structure.The results indicate that the maximum stress is 1.74 MPa and the maximum deformation is 1.04 mm with load acting on the unit plate.The noise reduction coefficient of this kind of no-riveted noise barrier unit plate is 0.75 and its noise insulation is 40 dB,which were conform to or superior to the standard requirements.Therefore,this new designed noise barrier meets the field application requirements of mechanical and acoustic performance,which demonstrates the noise barriers can be widely promoted.
基金The authors would like to acknowledge the Beasiswa Pendidikan Pascasarjana Dalam Negeri(BPPDN)KEMDIKBUD Indonesia who provided doctoral scholarship.
文摘The performance of oil palm trunk wastes from Banjarbaru of South Kalimantan was improved with the help of chemical modification in a two-step treatment.The first was formalization with formaldehyde solution with varying pH,and the second was impregnation with melamine-formaldehyde resin under 5 bar pressure for an hour.In these processes,the samples were cured at 120℃ for 10 min and then dried in an oven at(103±2)℃ in order to attain a moisture content of less than 6%.These treatments improved the physical properties(density,moisture content,and volume swelling),mechanical resistance,dynamic-mechanical and acoustic performance of the woods.The combination of impregnation and formalization changed the structure and the morphology of the woods such that the surface became flatter and denser.This was confirmed by results from FTIR,SEM,and DMA.Samples with alkaline modification displayed the best results for dimensional stability,storage modulus,and damping factor in varied frequencies.The treatments in this study also heightened acoustic performances as evidenced by the resulting characteristics of sound absorption coefficient and acoustic impedance.
基金the National Natural Science Foundation of China(Nos.51765008 and 11304050)the High-Level Innovative Talents Project of Guizhou Province(No.20164033)+1 种基金the Science and Technology Project of Guizhou Province(No.2020-1Z048)the Open Project of the Key Laboratory of Modern Manufacturing Technology of the Ministry of Education(No.XDKFJJ[2016]10).
文摘A cavity viscoelastic structure has a good sound absorption performance and is often used as a reflective baffle or sound absorption cover in underwater acoustic structures.The acoustic performance field has become a key research direction worldwide.Because of the time-consuming shortcomings of the traditional numerical analysis method and the high cost of the experimental method for measuring the reflection coefficient to evaluate the acoustic performance of coatings,this innovative study predicted the reflection coefficient of a viscoelastic coating containing a cylindrical cavity based on an artificial neural network(ANN).First,themapping relationship between the input characteristics and reflection coefficient was analysed.When the elastic modulus and loss factor value were smaller,the characteristics of the reflection coefficient curve were more complicated.These key parameters affected the acoustic performance of the viscoelastic coating.Second,a dataset of the acoustic performance of the viscoelastic coating containing a cylindrical cavity was generated based on the finite elementmethod(FEM),which avoided a large number of repeated experiments.The minmax normalization method was used to preprocess the input characteristics of the viscoelastic coating,and the reflection coefficient was used as the dataset label.The grid search method was used to fine-tune the ANNparameters,and the prediction error was studied based on a 10-fold cross-validation.Finally,the error distributions were analysed.The average root means square error(RMSE)and the mean absolute percentage error(MAPE)predicted by the improved ANN model were 0.298%and 1.711%,respectively,and the Pearson correlation coefficient(PCC)was 0.995,indicating that the improved ANN model accurately predicted the acoustic performance of the viscoelastic coating containing a cylindrical cavity.In practical engineering applications,by expanding the database of the material range,cavity size and backing of the coating,the reflection coefficient of more sound-absorbing layers was evaluated,which is useful for efficiently predicting the acoustic performance of coatings in a specific frequency range and has great application value.
基金the National Natural Science Foundation of China under Grant No.10474016.
文摘In marine engine exhaust silencing systems, the presence of exhaust gas flow influences the sound propagation inside the systems and the acoustic attenuation performance of silencers. In order to investigate the effects of three-dimensional gas flow and acoustic damping on the acoustic attenuation characteristics of marine engine exhaust silencers, a dual reciprocity boundary element method (DRBEM) was developed. The acoustic governing equation in three-dimensional potential flow was derived first, and then the DRBEM numerical procedure is given. Compared to the conventional boundary element method (CBEM), the DRBEM considers the second order terms of flow Mach number in the acoustic governing equation, so it is suitable for the cases with higher Mach number subsonic flow. For complex exhaust silencers, it is difficult to apply the single-domain boundary element method, so a substructure approach based on the dual reciprocity boundary element method is presented. The experiments for measuring transmission loss of silencers are conducted, and the experimental setup and measurements are explained. The transmission loss of a single expansion chamber silencer with extended inlet and outlet were predicted by DRBEM and compared with the measurements. The good agreements between predictions and measurements are observed, which demonstrated that the derived acoustic governing equation and the DRBEM numerical procedure in the present study are correct.
基金supported by the National Natural Science Foundation of China (no.52175160).
文摘The acoustic performance for the nanoporous frustule of the diatom is studied based on the computational fluid dynamics theory and acoustic theory involved.Representative Coscinodiscus sp.frustule is observed through the scanning electron microscope and modeled by the commercial software Solidworks.Further,the acoustic performance for the Coscinodiscus sp.frustule is studied at the varied depth,diameter or interval of the pore,as well as the film thickness of the fluid surrounding the Coscinodiscus sp.frustule.The numerical results show that,when the upper and lower pore diameters are separately 200 and 300 nm,the upper and lower pore depths are separately 200 and 250 nm,and both the pore interval and fluid film thickness are 500 nm,the elaborate nanoporous structure of Coscinodiscus sp.frustule can lower its acoustic power level by 17.49%,compared with that without porous structure.Meanwhile,the double-layer pore of Coscinodiscus sp.frustule can decrease its acoustic power level by 12.69%,compared with its single-layer pore structures.
文摘The effect of coaxial, offset and extended inlet/outlet on the acoustic attenuation characteristics of circular expansion chambers are studied by the three-dimensional finite dement method. The numerical results of transmission loss are compared with experiment results to verify the necessary of using three-dimensional methods. Maps of acoustic pressure level distribution inside of chambers and inlet/outlet ducts are given at a frequency to demonstrate the difference of acoustic wave propagation behavior caused by locations of inlet/outlet ducts. For the chambers of the same length, the chamber with extended inlet/outlet duct has higher attenuation ability than coaxial and offset inlet/outlet duct over middle frequencies.
基金supported by the National Nature Science Foundation of China(51275262,51345007)
文摘There is no accurate analytical approach for the acoustic performance prediction of Helmholtz resonator with conical neck,which has broad band acoustic attenuation performance in the low frequency range.To predict the acoustic performance of the resonator accurately,a general theory model based on the one-dimensional analysis approach with acoustic length corrections is developed.The segmentation method is used to calculate the acoustic parameters for sound propagation in conical tubes.And then,an approximate formula is deduced to give accurate correction lengths for conical tubes with difierent geometries.The deviations of the resonance frequency between the transmission loss results obtained by the general theory with acoustic lengths correction and the results from the finite element method and experiments are less than 2 Hz,which is much better than the results from one-dimensional approach without corrections.The results show that the method of acoustic length correction for the conical neck greatly improved the accuracy of the one-dimensional analysis approach,and it will be quick and accurate to predict the sound attenuation property of Helmholtz resonator with conical neck.
文摘A finite elemcnt-Helmholtz integral hybrid method has been developed to calculate the electro-acoustic performance of a finite length cylindrical transducer immersed in water. The numerical study concerned with electrical impedances and beam patterns of the transducer is presented. Experimental data are shown to support the numerical study.
基金supported by the National Nature Science Foundation of China(11104310)the 973 National key Basic Research Program of China(2013CB632900)the 863 National High Technology Program of China(2011AA11A103)
文摘A semi-analytical/numerical model based on the multiple scattering （MS） method has been established for analyzing the effect of acoustic performance on main energy attenua- tion mechanism in viscoelastic coating containing axisymmetric cavities. The basic functions of stress and displacement of the axisymmetric cavity surface are derived in the system of spheri- cal coordinates. The transition matrix between the incident wave and the scattering wave are obtained by the numerical integral of the basic functions of the cavity surface. The reflection, transmission and absorption performance of viscoelastic materials containing periodic cavities are calculated using the MS method and the wave propagating theory of the multi-layered medium. The results indicate that low frequency energy is mainly attenuated through cavity resonance. The resonant properties are found to be very sensitive to the boundary conditions. The coupling of the double-cavity is capable of extending the absorption to even lower fre- quencies. The absorption performance of the viscoelastic coating in the high frequency range is independent of the backing material. Its energy attenuation depends mainly on acoustic properties of cavity scattering and mode conversion.
文摘Based on the commonly used indicators for speech intelligibility, this work acoustically evaluates the two largest auditoria in the Faculty of Engineering, Helwan University, Cairo, Egypt, using experimental and digital simulation techniques. Design treatments were also suggested to improve the acoustic performance of the auditoria, where the impact of these treatments was checked using the simutation as well. The models that were analysed using the CATT-software were first validated utilizing the results of the fietd work in the unoccupied rooms. The results showed that the acoustic quality of the two auditoria are far from the optimal conditions due to their improper acoustic characteristics and the high noise revers as weft. The results of improvement proposals showed that altering the ceiling shape and adding efficient absorptive materials to the rear surfaces successfully reduced the excessive reverberation time to the optimal values, increased the earty reflections and eliminated the shadow zones. In addition, decreasing the noise Levels by 20 dB due to improving the window insulation noticeably improved the speech intelligibitity at all receivers.
