共查询到20条相似文献,搜索用时 15 毫秒
1.
Kang J 《The Journal of the Acoustical Society of America》2000,107(3):1394-1404
This paper systematically compares the sound fields in street canyons with diffusely and geometrically reflecting boundaries. For diffuse boundaries, a radiosity-based theoretical/computer model has been developed. For geometrical boundaries, the image source method has been used. Computations using the models show that there are considerable differences between the sound fields resulting from the two kinds of boundaries. By replacing diffuse boundaries with geometrical boundaries, the sound attenuation along the length becomes significantly less; the RT30 is considerably longer; and the extra attenuation caused by air or vegetation absorption is reduced. There are also some similarities between the sound fields under the two boundary conditions. For example, in both cases the sound attenuation along the length with a given amount of absorption is the highest if the absorbers are arranged on one boundary and the lowest if they are evenly distributed on all boundaries. Overall, the results suggest that, from the viewpoint of urban noise reduction, it is better to design the street boundaries as diffusely reflective rather than acoustically smooth. 相似文献
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The propagation of sound through a large number of scatterers (i.e., trees) is treated in a similar way to a classical diffusion problem. A general differential equation governing the sound intensity is derived which is valid under certain conditions, notably that the depth of the belt of vegetation is large, and absorption small. The predictions of this theory are compared with results derived from a small scale model study, and with some field measurements. They are also compared with published field data. The implications of some of the conclusions reached for the practical achievement of effective sound attenuation are pointed out. In general, it would appear that significant noise reductions may be achieved for a predominantly high frequency source if the existing ground cover is acoustically hard, or if there is no “ground effect” attenuation between source and receiver for some other reason. In other cases, the noise reduction will be much lower and may be negative. 相似文献
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Maurice J.M Martens 《Applied Acoustics》1981,14(3):167-189
Three types of vegetation can be discerned with regard to the acoustic climate investigated in 4 different plantations, 2 plant communities, and a grass field. In the first type of vegetation, i.e. beech and ashtree forests, the excess attenuation was at least 10 dB/100 m with the receiver at the same height as the source, 1·2 m, and at least 5 dB/100 m with the receiver at 3·9 m and even more in most -octave bands studied. In the second type of vegetation, i.e. the mixed poplar forest and the Stellario carpinetum, a so-called ‘sound window’ could be detected around 2 kHz and the ground effect was more extended towards the high frequency range compared with the first type of vegetation. In the third type of vegetation consisting of evergreen sprucefir, best excess attenuation was found in the closed forest: at least 10 dB/100 m with the receiver at 1·2 m, and 7 dB/100 m with the receiver at 3·9 m; in the belts the excess attenuation was at least 7 dB/100 m and 4 dB/100 m respectively. Best attenuation was, therefore, found in the closed forest, and not in the belts. It was concluded that the sound attenuating capacity of planted vegetations can be used to abate noise pollution in town and landscape planning if the plantations are at least 12 m wide. To obtain the best effect the rows of trees have to be planted perpendicular to the direction of the sound field. 相似文献
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A model for the determination of the variation of intensity of sound, due to a point source on a convex boundary surface, propagating over the surface is described. It is found that attenuation rates depend upon the surface compliant properties and are much higher for a pressure release surface than for a rigid one. The possibility of a surface giving still higher attenuation rates is explored. However, preliminary indications are that such a surface would have to be active. 相似文献
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This paper presents the results of field experiments done for the measurement of attenuation constant and speed of sound in the snow medium. The field experiments were conducted at two locations corresponding to relatively dry and wet snow. The main objective of our method is to overcome the potential limitations of the other methods. There are two major concerns: one is sound signal leakage and the other is the measurement need to be done within a same layer of snowpack. The ref lections from the boundaries will affect the measurements. In our experiments the transducers are kept far from the snowpack boundaries, so that there will be no likelihood of strong ref lected signals from the boundaries. These issues have not been addressed by the other researchers. This work adds to the measurement results of the attenuation constant and speed of sound in snow that are available in the research literature. It is found that sound signal attenuation greatly depends on the frequency of sound signal and wetness of snow. 相似文献
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Properties of sound attenuation around a two-dimensional underwater vehicle with a large cavitation number 下载免费PDF全文
《中国物理 B》2015,(6)
Due to the high speed of underwater vehicles,cavitation is generated inevitably along with the sound attenuation when the sound signal traverses through the cavity region around the underwater vehicle.The linear wave propagation is studied to obtain the influence of bubbly liquid on the acoustic wave propagation in the cavity region.The sound attenuation coefficient and the sound speed formula of the bubbly liquid are presented.Based on the sound attenuation coefficients with various vapor volume fractions,the attenuation of sound intensity is calculated under large cavitation number conditions.The result shows that the sound intensity attenuation is fairly small in a certain condition.Consequently,the intensity attenuation can be neglected in engineering. 相似文献
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V. T. Faerman 《Acoustical Physics》2016,62(1):22-26
Experimental measurements are presented for sound wave attenuation in foam without additives (standing wave method) and in foam with added particles (pulse method). A setup is developed that makes it possible to obtain a standing sound wave in stable foam and estimate the attenuation coefficient. A comparison is made of the coefficients of sound attenuation in foam in the sonic and ultrasonic frequency ranges, which have been published in a number of works. It is shown that the introduction of particles into foam leads to an increase in sound wave attenuation and may be the result of the viscous mechanism of sound wave energy loss. 相似文献
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Nelson BS 《The Journal of the Acoustical Society of America》2003,113(5):2901-2911
Attenuation over distance in natural habitat is often difficult to predict when measured without respect to sound frequency. The physical-acoustic structure of Florida scrub habitat is described and both attenuation and reliability of attenuation are measured as a function of sound frequency, over several distances, speaker elevations, and microphone elevations. The spatial context of sound propagation in Florida scrub habitat is discussed and a model designed to describe contributions to overall attenuation from individual factors is presented. Sound frequencies below approximately 3.5 kHz attenuate more reliably than higher sound frequencies, suggesting that animals should pay greatest attention to relatively low sound frequencies when they assess attenuation or estimate sound-pressure level. 相似文献
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Scheichl S 《The Journal of the Acoustical Society of America》2004,115(2):534-555
The present paper deals with the classical problem of linear sound propagation in tubes with isothermal walls. The perturbation technique of the method of multiple scales in combination with matched asymptotic expansions is applied to derive the first-order solutions and, in addition, the second-order solutions representing the correction due to boundary layer attenuation. The propagation length is assumed to be so large that in order to obtain asymptotic solutions which extend over the whole spatial range the first-order corrections to the classical attenuation rates of the different modes come into play as well. Starting with the case of the characteristic wavelength being large compared to the characteristic dimension of the duct, the analysis is then extended to the case where both of these quantities are of the same order of magnitude. Furthermore, the transmission line parameters and the transfer functions relating the sound pressures at the ends of the duct to the axial velocities are calculated. 相似文献
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A method (tentatively called ‘the method of equivalent sound source’) of predicting the sound attenuation achieved by multiple barriers with knife edges and/or right-angled wedges is proposed. This paper shows that the sound pressure level in the shadow region behind the multiple barriers can be obtained by successively setting imaginary sound sources for respective edges and/or wedges. The locations of the imaginary sound sources are determined by means of the traditional solution for sound attenuation by a single edge. The calculated results for several kinds of small-scale model, obtained by ‘the method of equivalent sound source’, are compared with experimental results. 相似文献
14.
Tang SK 《The Journal of the Acoustical Society of America》2011,129(5):2830-2840
The sound generated by a vortex propagating across a two-dimensional duct section with flexible walls (membranes) in an infinitely long rigid duct conveying a flow is investigated numerically using the matched asymptotic expansion technique and the potential theory. The effects of the initial vortex position, the mechanical properties of the flexible walls, and the mean flow on the sound generation are examined in detail. Results show that the presence of a vortex inside a uniform mean flow can strengthen or attenuate the sound generation, depending on the phase of the membrane vibration when the vortex starts vigorous interaction with the membranes and the strength of the mean flow. The results tend to imply that there is a higher chance of sound amplification when a vortex stream is moving closer to the lighter membrane under a relatively strong mean flow or when the mean flow is weak. The chances of sound amplification or attenuation are equal otherwise. 相似文献
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海底沉积物作为海洋波导声传播的下边界普遍存在于大洋中,获知其特性对于准确的声传播和混响建模是十分必要的。为了能够快速而准确地测量沉积物中的声速和衰减系数,提出一种基于脉冲压缩技术的测量方法,对接收信号进行压缩来提取透射波,根据不同厚度样品的透射波来计算沉积物中的声速和衰减系数。该方法不仅可以克服实验过程中经常遇到的多途干扰,而且测量过程简单,可以同时获得测量频带内所有频点的声速和衰减系数,即实现了对声速和衰减系数的宽带测量。在实验室环境条件下,90~170kHz的测量频带内,测得沙样品中的声速为1710~1713m/s,衰减系数在56~70dB/m之间。通过窄带和宽带测量结果的比较可以看出,声速的宽带测量结果与窄带测量结果吻合得较好,而衰减系数在频带后半部分存在较大的起伏。 相似文献
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A study has been undertaken of the barrier attenuation produced by 40 different finite garden fences shielding suburban homes from traffic noise. Two alternative techniques of measuring barrier attenuation were used and an empirical relationship between the results is obtained. The attenuation measured in terms of , or generally agrees within ± 1 dB(A) for a given barrier. Similarly constructed fences can produce different attenuations, depending on their surroundings. The effects of unshielded ends, open gates and vegetation are considered. It is postulated that buildings reflect back sound, the effect depending on building size and distance from the fence. A reasonable trend is obtained for brick barriers; however, the wooden fence results may be obscured by sound leakage through the fence. 相似文献
19.
Quantitative ultrasound features such as the attenuation slope, sound speed and scatterer size, have been utilized to evaluate pathological variations in soft tissues such as the liver and breast. However, the impact of variations in the sound speed and backscatter due to underlying fat content or fibrotic changes, on the attenuation slope has not been addressed. Both numerical and acoustically uniform tissue-mimicking experimental phantoms are used to demonstrate the impact of sound speed variations on attenuation slope using clinical real-time ultrasound scanners equipped with linear array transducers. Radiofrequency data at center frequencies of 4 and 5 MHz are acquired for the experimental and numerical phantoms respectively. Numerical phantom sound speeds between 1480 and 1600 m/s in increments of 20 m/s for attenuation coefficients of 0.3, 0.4, 0.5, 0.6, and 0.7 dB/cm/MHz are simulated. Variations in the attenuation slope when the backscatter intensity of the sample is equal, 3 dB higher, and 3 dB lower than the reference is also evaluated. The sound speed for the experimental tissue-mimicking phantoms were 1500, 1540, 1560 and 1580 m/s respectively, with an attenuation coefficient of 0.5 dB/cm/MHz. Radiofrequency data is processed using three different attenuation estimation algorithms, i.e. the reference phantom, centroid downshift, and a hybrid method. In both numerical and experimental phantoms our results indicate a bias in attenuation slope estimates when the reference phantom sound speed is higher (overestimation) or lower (underestimation) than that of the sample. This bias is introduced via a small spectral shift in the normalized power spectra of the reference and sample with different sound speeds. The hybrid method provides the best estimation performance, especially for sample attenuation coefficient values lower than that of the reference phantom. The performance of all the methods deteriorates when the attenuation coefficient of the reference phantom is lower than that of the sample. In addition, the hybrid method is the least sensitive to sample backscatter intensity variations. 相似文献
20.
Traffic noise attenuation at different 1/3-octave frequencies is measured at three vegetation sites and a control site in Delhi, the capital city of India. The study indicates that attenuation generally increases with frequency. At low frequencies, maxima (between 10 and 16 dB) in relative attenuation are observed in the frequency interval between 315 and 400 Hz. Comparatively greater relative attenuation (>20 dB) is observed in the high frequency range between 10 and 12.5 kHz. A significantly higher relative attenuation of more than 24 dB is observed characteristically at 3.15 kHz at all the vegetation sites. The results indicate that vegetation belts could be used as effective barriers for traffic noise control along the roadsides. 相似文献