声学基础课件fundamentalsofacoustics0714

1、Thus far we have developed a set of differential equations that describe the local behavior of sound waves.We become acquainted with some other important types of wave behavior - spherical waves.Our emphasis here is on the waves themselves and in the following chapter we will pursue the interaction

2、of these waves with their sources. 牛牛文库文档分享Thus far we have developed a sSpherical spreading describes the decrease in level when a sound wave propagates away from a source uniformly in all directions. 牛牛文库文档分享Spherical spreading describes Sound levels are therefore constant on spherical surfaces su

3、rrounding the sound source. Sound levels decrease rapidly as sound spreads out from a sphere with a radius of r0 to a larger sphere with a radius r. 牛牛文库文档分享Sound levels are therefore conSpherical waves are represented in complex form byThe wave diminish in amplitude as the distance from the source

4、increase.the particle velocity is not in phase with the pressure. 牛牛文库文档分享Spherical waves are representeThe Acoustic impedance Separating za into real and imaginary parts, we haveThe first term is the acoustic resistance, and the second term is the acoustic reactance. 牛牛文库文档分享The Acoustic impedance

5、SeparatThe acoustic impedance of spherical wave Real , imaginary parts and phase 牛牛文库文档分享The acoustic impedance of spheWe emphasize again that the acoustic impedance depends on the frequency , as well as on the medium, the wave type, and the geometric position.Neither the frequency nor the distance

6、alone is so important as is the dimension combination kr. For kr 1, Za is predominantly resistive, meaning that energy fed into such waves propagates away from the source, never to return. 牛牛文库文档分享We emphasize again that the acThe larger the value of kr, the less curved are the wave fronts and the m

7、ore nearly do these waves act just like plane waves.But for kr 1The diverging waves are 牛牛文库文档分享When kr 1The diverging wavesThe acoustic intensity is 牛牛文库文档分享The acoustic intensity iswww.nThe relative intensity level decrease less rapidly for cylindrical than for spherical spreading.The total power

8、crossing a cylinder surrounding the source equals the intensity times the area of the cylinder: 牛牛文库文档分享The relative intensity level dThe acoustic impedance When kr 1 牛牛文库文档分享The acoustic impedance When krWhen kr 1, the acoustic impedance In this limit the acoustic impedance again approaches p0c0, s

9、ince very large cylindrical wave fronts will be practically indistinguishable from plane waves.We may remark that , like the spherical waves, cylindrical waves take on a much simpler form when kr becomes large.The pressure amplitude of cylindrical waves approach 牛牛文库文档分享When kr 1, the acoustic impHomeworkTextbook P276 3-8, 3-10Spherical acoustic waves of frequency 125Hz are emitted from a small source. At a radial distance of 1.5m from the source, what is the phase