地面反射和衰减效应对声源频谱的影响

噪声源辐射的声波在传播过程中,由于地面的存在会引起声波的反射和衰减,并产生地表面波使得声源频谱特性发生显著的变化。为利用观察点处实测声频谱来预估实际声源自由场频谱特性,必须适当地计及地面反射和衰减效应的影响。本文采用Chien—Soroka地面反射衰减效应理论模型以及Delany—Bazley地面导纳函数,在局部反应地面假设基础上,给出了存在地面反射和衰减时实测声源频谱与实际声源自由场频谱之间的换算公式,并编制了相应的计算程序。文中给出的算例与文献3]7]中给出的测量结果进行了比较,两者基本吻合。最后还对地面流阻和接收点高度对地面反射和衰减效应的影响进行了数值计算,结果表明,随地面流阻增加和接收点高度的降低都会使得第一个声波干涉低谷向高频移动。因此,实际测量小,若地面流阻未知,可采用紧贴地面设置传声器的方法测量声源频谱,此时地面影响的修正量,在第一声波干涉低谷以下,均为6dB。

Influence of the Ground Reflection and Attenuation on Source Spectrum

In the process of the sound propagation, the presence of the earth's surface causes a significant change in the source spectrum due to the reflection and attenuation of the sound waves and the creation of surface waves. The ground reflection and attenuation must be properly accounted for in order to use the source spectrum measured at the site of the observer to predict the actual source spectrum in the free field. The ground-effect model chosen in this paper is based on the Chien-Soroka theory. This theory, with the Delany-Bazley ground admittance, assumes that the ground is a locally reacting uniform plane and the source is a point source. The conversion equation is given between the source spectrum measured and the actual source spectrum in the free field. The calculations given in this paper approximately coincide with the measurements presented in Refs.(3) and (7). In the end, the effects of the flow resistivity of the ground and the height of the receiving point on ground effects are calculated. The numerical results indicate that the first sound interference dip moves to the high frequency end with the flow resistivity of the ground and the height of the receiving point decreasing. Therefore, in the actual measurement, if the flow resistivity of the ground is an unknown quantity, flush-mounted microphone method is suggested in measuring the source spectrum. In this case, the amendment quantity of the ground effects is 6dB at frequencies below the first sound interference dip.