多孔材料对空腔对吸声有什么影响?

解题思路：（1）物质由液态变成气态的过程叫做汽化，汽化吸热；
（2）物质由气态变成液态的过程叫做液化，液化放热．

（1）热管内的液体在高温下，由液态变成气态，属于汽化过程，汽化吸热；
（2）温度降低时，这些水蒸气又遇冷变成液体，属于液化过程，液化放热．
故选A．

点评：
本题考点： 汽化及汽化吸热的特点；液化及液化现象．

考点点评： 解决此类问题要结合汽化和液化现象及其特点进行分析解答，注意汽化和液化是两个相反的过程．

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多孔材料材质一般都比较松软,特别是纤维材料,强度比较低.气流对多孔材料大致上有两方面的影响.一方面是吹烛材料,当气流速度校高时,容易吧材料吹散,这在选用护面材料时应予充分注意,如采用双层透声织物或用塑料薄膜和金属箔等加以保护、使吸声系数保持稳定.另一方面是在气流速度较大时,也会引起声波波长的变化.声波顺着气流方向传播时,相当于无气流情况下较高频率的吸声系数,同样,声波朝气流相反向传播时,相对于无气流情况下较高频率的吸声系数,这时吸声系数的变化规律和温度变化对多孔材料吸声的影响情况相似.