The room acoustic rendering equation

An integral equation generalizing a variety of known geometrical room acoustics modeling algorithms is presented. The formulation of the room acoustic rendering equation is adopted from computer graphics. Based on the room acoustic rendering equation, an acoustic radiance transfer method, which can handle both diffuse and nondiffuse reflections, is derived. In a case study, the method is used to predict several acoustic parameters of a room model. The results are compared to measured data of the actual room and to the results given by other acoustics prediction software. It is concluded that the method can predict most acoustic parameters reliably and provides results as accurate as current commercial room acoustic prediction software. Although the presented acoustic radiance transfer method relies on geometrical acoustics, it can be extended to model diffraction and transmission through materials in future.     

ODEON仿真与实际测量语言传输指数实验对比

近年来音质仿真技术的快速发展为语言传输指数STI的预测提供了一个潜在的解决方案。但这种方法的有效性如何,则是在使用该技术之前应该考虑的问题。本文对3个房间内音质仿真与实际测量STI进行实验对比,研究表明:在仿真模型与实际空间的声学等效较为准确的情况下,使用音质仿真软件ODEON计算得到的STI误差较小;混响时间的变化在背景噪声较高时可能会对仿真STI的准确性带来显著影响,随着混响时间的增加,仿真与实际测量STI的差值可能变大;信噪比的变化并不会给仿真STI的准确性带来显著影响;仿真脉冲响应与实际测量脉冲响应的频谱有一定差别,时域上的反射声序列也不相同,但这些差别对仿真STI的影响并不大;仿真过程中比较容易产生的信噪比误差对仿真STI产生了较大影响。由于影响音质仿真结果的因素较多,仿真模型与实际空间的声学等效也比较复杂,尤其是对于没有实际参照校准的房间来说,想要获得准确的STI预测结果是比较困难的。     

Virtual models for the prediction of acoustic fields of Manuel de Falla Auditorium in Granada,Spain

This paper presents the results of the acoustic simulation of the Manuel de Falla Auditorium (MFA), Granada, Spain, carried out in CATT-Acoustic and CATT–TUCT. The 3D geometrical acoustic model created for this purpose is tuned by adjusting the absorption and scattering coefficients of the convex reflector ceiling of the auditorium through an iterative process aimed at rendering differences between simulated and measured values for reverberation time in each octave frequency band lower than 5%. The model is validated by comparing simulated and measured figures for the most significant monaural and binaural room acoustic parameters assessed at multiple locations both on the stage and within the audience areas, then by performing both spectral and spatial analyses of their behaviour, and ultimately studying dissimilarities between the two sets of data in terms of just noticeable difference (JND). The influence of the degree of detail of the geometric model on the outcome is also considered by incorporating the 44 iconic lamps specifically designed for this enclosure, into the model. Likewise, the acoustic effect of an already executed seating replacement in the performance space is analyzed in relation to the degree of occupancy of the room. The simulated results confirm the appropriateness of the choice of the new seating for the refurbishment of the concert hall.     

Predicting the sound absorption of natural materials: Best-fit inverse laws for the acoustic impedance and the propagation constant

Natural materials are becoming a valid option for sound absorption treatments. In particular, among them, natural fibers have received increasing attention given their good thermal insulation properties, lack of harmful effects on health, and availability in large quantities. This paper discusses an inverse method to predict the acoustical properties of nine natural fibers. Six vegetative fibers: kenaf, wood, hemp, coconut, straw, and cane; one animal fiber, sheep wool; recycled cardboard; and granular cork are investigated. The absorption coefficient and the flow resistance for samples of different thickness have been measured. Moving from the Delany-Bazley model, this study compares the impedance tube results with the theoretically predicted ones. Then, using a least-square fit procedure based on the Nelder-Mead method, the coefficients that best predict both the acoustic impedance and the propagation constant laws are calculated. The inverse approach used in this paper allows to determine different physical parameters and to obtain formulas to include the investigated natural fibers in software modelling for room acoustics applications.     

An improved model to predict energy-based acoustic parameters in Apulian-Romanesque churches

In this paper an improved model to predict energy relations in churches is proposed. A detailed acoustic survey was carried out of nine Romanesque churches having volumes ranging from 33 000 to 1500 m³. The measured sound level and early/late ratio showed significant correlation with the source-receiver distance, but the comparison with values predicted using theoretical models initially gave unsatisfactory results. The main difference was due to the early energy which was underestimated at points near the source and overestimated at distant points. Barron's revised theory proved to be the most reliable among the analysed models, so, in order to improve its prediction accuracy, a modified early reflected energy component was added to the direct and reverberant sound. The improved model was finally validated and the comparison between predicted and measured values gave good results.     

Perceptual validation of virtual room acoustics: Sound localisation and speech understanding

The reliability of algorithms for room acoustic simulations has often been confirmed on the basis of the verification of predicted room acoustical parameters. This paper presents a complementary perceptual validation procedure consisting of two experiments, respectively dealing with speech intelligibility, and with sound source front–back localisation.The evaluated simulation algorithm, implemented in software ODEON®, is a hybrid method that is based on an image source algorithm for the prediction of early sound reflection and on ray-tracing for the later part, using a stochastic scattering process with secondary sources. The binaural room impulse response (BRIR) is calculated from a simulated room impulse response where information about the arriving time, intensity and spatial direction of each sound reflection is collected and convolved with a measured Head Related Transfer Function (HRTF). The listening stimuli for the speech intelligibility and localisation tests are auralised convolutions of anechoic sound samples with measured and simulated BRIRs.Perception tests were performed with human subjects in two acoustical environments, i.e. an anechoic and reverberant room, by presenting the stimuli to subjects in a natural way, and via headphones by using two non-individualized HRTFs (artificial head and hearing aids placed on the ears of the artificial head) of both a simulated and a real room.Very good correspondence is found between the results obtained with simulated and measured BRIRs, both for speech intelligibility in the presence of noise and for sound source localisation tests. In the anechoic room an increase in speech intelligibility is observed when noise and signal are presented from sources located at different angles. This improvement is not so evident in the reverberant room, with the sound sources at 1-m distance from the listener. Interestingly, the performance of people for front–back localisation is better in the reverberant room than in the anechoic room.The correlation between people’s ability for sound source localisation on one hand, and their ability for recognition of binaurally received speech in reverberation on the other hand, is found to be weak.     

Prediction of the absorption exponent in rectangular enclosures with a single absorbent boundary

It is known that the classical theory of room acoustics cannot be strictly applied to the study of the sound field in a rectangular enclosure with only five boundaries (i.e., with a single absorbent boundary), as the sound field in the enclosure is not diffuse. A theoretical method is developed for the prediction of the absorption exponent in a rectangular enclosure with a single absorbent boundary, and the absorption exponent is used to describe the exponential decay of the sound energy. The method is based on the radiosity-based theoretical/computer model and is used for diffusely reflecting boundaries. The predicted absorption exponent is compared with the Kuttruff values and the simulation results from Monte-Carlo computations. It is found that the predicted absorption exponent of the proposed method shows better agreement with the simulation results from Monte-Carlo computations than the Kuttruff values. With the more accurately predicted absorption exponent, the slope of the energy decay curve and other acoustic parameters in an enclosure with a single absorbent boundary can be obtained accurately.     

室内声学时域有限差分模拟中的边界条件

给出了时域有限差分法用于室内声学问题模拟中的边界条件,结合声波方程的基本差分格式,模拟并分析了高斯脉冲在4m×4m房间中的波动过程和脉冲响应;模拟了一9m×6m×4m房间的简正频率,并与经典理论计算值进行了对比;模拟了一12m×5m×4m水平地面房间中的坐席吸声低谷效应,并与Joe LoVtri的模拟结果进行了对比;模拟并实际测量了一10.6m×5.8m×3.4m房间在几个受声点的脉冲响应和早期衰变时间EDT,将模拟结果与实际测量结果进行了对比分析,计算程序是用Metlab语言编写的。模拟与经典理论、相关研究、实际测量几方面的对比分析,验证了本边界条件的可靠性。     

Practical methods to define scattering coefficients in a room acoustics computer model

To predict acoustics of rooms using computer programs based on geometrical assumptions, it is important that scattering is included in the calculations. Therefore scattering is usually included in terms of scattering coefficients which are assigned to each surface telling the software the ratio between the part of the reflected energy which is not being reflected specularily and the total reflected energy. However the effective scattering coefficient of a surface depends not only on the roughness of the surface material indeed diffraction caused by limited dimensions of the surface as well as edge diffraction also causes scattering. For complex rooms it can be difficult to give a reasonable estimate to the magnitudes of scattering coefficients if these should also include diffraction and even if these frequency dependent coefficients could be obtained in the design phase, the processes of obtaining the data becomes quite time consuming thus increasing the cost of design. In this paper, practical methods to define scattering coefficients, which is based on an approach of modeling surface scattering and scattering caused by limited size of surface as well as edge diffraction are presented. The predicted and measured acoustic parameters in real rooms have been compared in order to verify the practical approaches recommended in the paper.     

Acoustics of the Great Hall of the Moscow State Conservatory after reconstruction in 2010–2011

The Great Hall of the Moscow State Conservatory was built in the early 20th century. For more than 100 years of service, it had a high acoustic reputation both among musicians and audience. By the beginning of the 21st century, the hall was in nearly critical condition. Thus, major renovation was needed. In terms of architectural acoustics, the main task was to keep the good acoustics of the hall. This paper presents the results of acoustic parameter measurements of the hall after Reconstruction in 2010–2011. The parameters of the hall measured before and after reconstruction are also compared. The comparative acoustic characteristics between the Great Hall and world leading concert halls are given.