A Study On The Acoustics Of Long Enclosures

Acoustics is an important concern in many long enclosures, such as traffic tunnels, public corridors, subway stations and subway carriages. Some problems need to be resolved, such as predicting sound variation trend in long enclosures accurately, influence degree of a long enclosure on speech intelligibility, arranging the loudspeakers of sound amplifying system in long enclosures, etc.According to the fact that four boundaries are different in long enclosures, a theoretical model has been developed for the prediction of sound propagation in a long rectangular enclosure with different acoustics boundaries. The coherent model is based on the image-source method, and the effect of interference among the infinite number of image sources generated by multiple reflections is incorporated by coherently summing the contributions from the image sources. The various impedances of the boundaries are considered in the model. In experiments the enclosure walls are lined with different kinds of sound absorption material to simulate different impedance boundaries. It is show that the developed model agrees better with the experimental data than that of the ODEON software. Comparison with experimental results as well as the predictions of the ODEON software is carried out to validate the proposed model.A new coherent model for predicting sound propagation in long enclosures with a high sound-absorbing boundary has been developed. The proposed model starts from formulating reflections of the spherical sound radiation into integrals of plane wave expansion. The effect of the rigid boundaries on the sound field of the receiver is weighted by the improved single reflection coefficient, and then, the integral, representing the influence of a single sound-absorbing boundary, is approximated with the method of steepest descents for the larger parameter. In experiments, the enclosure walls are lined with high density fiberboard and high sound-absorbing glass wool to simulate different boundaries. The existing coherent models only have approximate results because in the application, numerical values exceed the calculation range of software. The predictions of the proposed model show good agreements with experimentsl results. Comparison with the existing coherent model as well as experiments is carried out to validate the proposed model.Based on requirement of sound field nonunifornity, a method has been developed for arranging many loudspeakers of public broadcasting systems in long enclosures. The arranging method for loudspeakers of public broadcasting systems which meet the requirement of certain sound field nonunifornity is based on the sound field attenuation curves at different locations of loudspeakers in long enclosures and some locations which meet requirements of certain attenuations. Arranging loudspeakers in a long enclosure with given parameters is designed by using a model for the sound field in long enclosures. With the design result, simulation and analysis are carried out to validate the proposed method. Comparison with the common method shows the number of loudspeakers from the proposed mothod is much less than that from the common method, and meets the design requirement.Some studies show that speech intelligibility generally improves while spatial separation between target and interference increases, and this effect is known as spatial release from masking. Reverberation and sound reflections in rooms influence speech intelligibility. Spatial release from masking at different locations of long enclosures is investigated by psychoacoustic experiments. Adaptation of Chinese listeners to English corpus has been discussed first. On this basis, effects of interferences at different locations, from different directions and with different signal-to-noise ratio (SNR) in long enclosures on correct rate have been investigated, and compare with experimental results in anechoic environment. The results of experiments show that the spatial separation has an obvious gain effect on English speech intelligibility of Chinese-native listeners, which agrees well with the conclusion of the related researches on other native languages. The correct rate drops with SNR decreasing under most situations. Since reverberation and sound reflections in long enclosure, the correct rate at most locations of long enclosure is lower than that in anechoic environment at the same interference condition. Furthermore, change trends of gains from spatial release from masking with change of SNR are different in long enclosure and anechoic environment, and reflection effects at different locations of long enclosure have different effects on gains from spatial release from masking.