英语翻译Sound waves radiating from a source are reflected or diffracted by objects in their path in much the same way as waves on the surface of water.Diffraction is the process by which sound waves experience a change in their direction of propa

英语翻译Sound waves radiating from a source are reflected or diffracted by objects in their path in much the same way as waves on the surface of water.Diffraction is the process by which sound waves experience a change in their direction of propa
英语翻译
Sound waves radiating from a source are reflected or diffracted by objects in their path in much the same way as waves on the surface of water.Diffraction is the process by which sound waves experience a change in their direction of propagation after encountering an obstacle; it is equivalent to a bending effect.For most general noise control purposes,if one or more major dimensions of an intervening object are of the same order of magnitude or larger than the wavelengths associated with the frequencies of interest,then the object will act as a barrier to the sound.Otherwise,diffraction will occur around the edges of the object as illustrated in figure 2.5.The effect typically means that high-frequency waves tend to be much more directional in character; their path of propagation can often be successfully traced using a geometric ray approximation.Diffraction is particularly important when examining the influence of microphones and barriers on sound fields (see sections 4.1.4 and 6.2.1.1,respectively).
2.3.2 Superposition
The principle of superposition,which applies to waveforms with amplitudes typical of industrial situations,states that the net result of adding two waves is to sum their effects arithmetically.In figure 2.6 this is illustrated for two sets of waves which are identical in all respects except that their phase differences are respectively 0 ° and 180 °.
This result flows from the addition of n linear equations:
p(x,t) = pl(x,t) + p2(x,t) + • ..+ pn(x,t)
2.3.3 Standing Waves or Resonances
When a free progressive wave encounters a sharp change in impedance such as at an air-hardwall boundary,and if the angle of incidence is normal,then the reflected wave travels back along the original path of propagation with a 360 ° change in phase.If there is another boundary parallel to the first one,then the reflection process occurs again,and so on ad infinitum,until all the acoustic energy has dissipated by absorption at the boundaries and by propagation losses in the intervening medium.Standing waves or resonances occur when the distance between the boundaries coincides with an integer multiple of the halfwavelength; they are more likely to appear when the original complex waveform has strong discrete components.
By superposition,two waves 360 ° out of phase add constructively to produce a waveform greater in amplitude than the original constituents.In this unique case,however,because a complete reversal of direction has also taken place,fixed positions are created of zero net pressure amplitude (nodes) and of maximum pressure amplitude (antinodes).At intermediate points,the amplitude varies sinusoidally with a fluctuating range described by the envelope in figure 2.7 at a rate equal to the frequency of the contributing waves.Because of the fixed pattern of amplitudes in space,the resultant waveform is called a standing wave (figure 2.7).

英语翻译Sound waves radiating from a source are reflected or diffracted by objects in their path in much the same way as waves on the surface of water.Diffraction is the process by which sound waves experience a change in their direction of propa
【手工译文】
声波从声源发出后在传播路径上遇到障碍物时会发生反射或衍射,其模式与水面波极其类似.衍射是声波遇到障碍物后改变其传播方向的一种过程,相当于一种弯曲作用.在大多数需要对噪音进行控制的情况下,若障碍物的主要尺寸与声波波长相当,或大于特征频率相关波长,则障碍物将成为声垒.否则,声波将如图2.5所示沿障碍物周边发生衍射.这一结果清楚表明高频波的方向性要好得多,常常可以使用几何射线近似的方法来探求其传播路径.测试声场中扬声器和声垒的影响时,衍射现象尤为重要（相应内容见章节4.1.4和6.2.1.1）.
2.3.2 叠加
叠加原理适用于具有工业环境典型振幅的波形,内容是说两列波叠加的结果就是其各自波形的代数相加.图2.6中演示了两组完全相同的波形进行叠加的结果,只不过一组波形相位差为0°,另一组波形相位差为180°.
可由n阶线性方程相加得出结果：
p(x,t)=pl(x,t)+p2(x,t)+...+pn(x,t)
2.3.3 驻波和共振
当自由行波遇到波阻突变时,比如遇到气壁边界,若入射角度垂直于边界,那么反射波会沿原路径返回,相位变化360°.若另有一个边界与第一个边界平行,则会再次发生反射,之后再次反射……依次类推,直到所有的声波能量都在边界吸收和媒介传播过程中消耗殆尽.当边界间的距离恰巧为半波长的整数倍时就会产生驻波或共振,尤其当原始复杂波形中含有大量离散元素时更容易产生.
这两列相位差为360°的波列相互叠加所产生的新波形振幅大于入射波.不过在这种情况下,由于入射波与反射波方向完全反转,其叠加波形振幅在某些固定点处恒为零（节点）,而在某些固定点处达到最大（波腹）.在其它中间位置,振幅按正弦曲线变化,变化范围如图2.7中包络线所示,其频率等于入射波的频率.由于合成波形的振幅沿空间的分布模式固定不变,所以被称作驻波（图2.7）.