REFLECTION OF SOUND PULSE BY PLANE INTERFACE BETWEEN AIR AND POROUS MEDIUM

This paper presents a theoretical analysis of the reflection of a sound pulse by the plane interfacebetween air and ideal porous medium.A general expression of impulse response for a pulsed pointsource is derived.The detailed analysis is presented for three special cases:1)source and receiverlocated on a line perpendicular to the interface,semi-infinite media;2)a layer of porous medium;3)semi-infinite media,grazing incidence.In the second case,the problem is solved by consideringthe multi-reflections in the layer of porous medium.Experimental results are compared with the-oretical calculations for a kind of porous material.     

SHAPE AND ACOUSTICAL CHARACTERISTICS OF ANCIENT BELLS

In addition to the Great Bell of the Yongle period,there are more than thirty ancient bells at the Templeof Great Bell in the west outskrit of Beijing.Most of the them were cast in the Ming Dynasty and Qing Dynasty.Only a few were cast in the Song and Yuan Dynasties.The heights of the bells from 79cm to 210cm andthe corresponding weights from 114kg to 3108kg.The largest and the samllest opening radius are 84cm and31.5cm respectively. In this paper the dynamic spectra of bell sound,beat frequencies and sound decay characteristics havebeen studied,The relationshps between fundamental frequeney and bell shape are discussed.For bells withsimilar shape,fundamental frequency is approximately proportional to T_0/R_0~2,where R_0 is the openingradius of bell and T_0 is the thickness of bell rim.The subjective appraisal of the sound quality of bell sound ispresented.     

A STUDY OF THE SOUND REFLECTION FROM POROUS MATERIAL BY IMPULSE METHOD

This paper presents a study of the reflection of sound pulse from porous material when the im-pulse sound source and receiver are located on a line perpendicular to the surface of the material.The transfer functions of reflected sound waves were calculated from the experimental incident and thereflected sound pulses by use of DFT.The impulse response of reflected sound waves can be calculatedtheoretically under the assumption of idealized porous material.Both local reacting and extendedreacting materials were taken into account in the calculations. comparison of the calculationsfor spherical waves and plane waves provides a measure of the effect of spherical waves on thetransfer function of reflected sound waves.For a layer of porous material the reflections in thematerial are studied.     

用脉冲法研究多孔性材料的声反射

本文提出当脉冲声源和接收点在同一条垂直于材料表面的直线上时,声脉冲从多孔材料反射的研究。由实验入射和反射声脉冲利用DFT计算了反射声波的传递函数。在把材料假设为理想多孔性材料的前提下,理论上可以计算反射声波的脉冲响应。计算中考虑了局部反应和延伸反应材料。通过球面波和平面波计算的比较,提供球面波对反射声波传递函数的影响量。对一多孔材料层,也研究了材料中的反射。     

声脉冲在多孔性媒质界面上的反射

本文在理论上分析球面脉冲声波在空气和理想的多孔性媒质界面上的反射问题。在声源为脉冲点源的情形下,给出反射脉冲声波的一般表示式。对以下三种情况做了较详细的分析:（1）声源和接收点在垂直于界面的同一直线上,半无限厚多孔性媒质;（2）声源和接收点在垂直于界面的同一直线上,多孔性媒质层;（3）掠射,半无限厚多孔性媒质。脉冲声波在媒质层上的反射是以声脉冲在媒质层内多次反射和透射来处理的。文中还给出实验结果和理论计算的比较。     

礼堂音质——测试和模拟的比较

用一台微处理机模拟了一个普通礼堂的音质,主要是混响时间和声场起伏。混响时间的计算、测试与模拟结果之间的比较表明,在正确选用观众吸声量后,模拟得到的混响时间比按公式计算得到的混响时间更接近实验结果,在125Hz到4000Hz的频率范围内,平均相对偏差小于10％,而声场起伏的测试值与模拟值大致相符。文中还讨论了观众吸收及模拟中的一些细节问题。     

脉冲法在现场声反射测量中的应用

本文运用电火花声源在一厅堂内对胶合板条和钢板网组成的顶棚以及胶合板贴在混凝土墙面上构成的反射板进行了声反射特性的实验研究。在测试人射和反射声脉冲而利用FFT计算声反射传递函数时,讨论了反射脉冲序列截取的长短对该材料反射传递函数的影响。文中还分析了有肋吸声结构声反射传递函数的变化。提供0°、30°、60°反射传递函数的比较。     

FINITE ELEMENT ANALYSIS OF THE YONGLE BELL

The Great Bell of Yongle cast in the reign of Yongle of the Ming Dynasty is a bell for prayer.By use ofthe finite element method for the calculation of truncated conical shell,natural frequencies and vibrationalmodes of the Yongle Bell have been calculated.Theoretical results are consistent with those obtained frommeasurements of sound.Within the range of calculation,the law of distribution of nodal circles for eachvibrational mode is presented.Through comparison between the Yongle Bell and assumed bells,theinfluences of waist and lip thickness changes on the bell timbre are discussed.The tuning for A_3 and E_3partial tones is achieved by changing the thickness at waist and lip.These partials play a harmonious part inthe bell sound.     

永乐大钟振动的有限元分析

永乐大钟是明朝永乐年间铸造的祈事大钟。本文通过旋转壳体的有限元方法,计算了永乐大钟的主要固有频率和振型。得到与永乐大钟钟声特性分析较一致的结果,在运算范围内还得出大钟各振型周向节圆分布的规律。通过永乐大钟与假设钟体的对比近似计算,基本上明确了钟腰及钟唇厚度变化对永乐大钟音色的影响。钟腰和钟唇厚度变化分别调整出音高为A₃和E₃的分音,而这两个分音起到增加钟声谐和性的作用。     

二胡的声学特性

本文给出二胡共鸣器的特性。与京胡相比,二胡琴筒内空气和膜互相作用的共振频率比琴筒四分之一波长共振频率低得相当多。膜的最低固有频率比这频率约高二倍。共鸣器的频率响应主要在这频率及其4.6倍的频率之间。文中给出琴弦在脉冲激励和拉弓情形下的声频谱。综合演奏七声音阶的频谱可得出拉里、外弦的二胡声频谱包络作为二胡声的频率特性。