中科院声所声学基础试题.doc

2006声学基础 一、名词解释： 受迫振动：系统遭周期性的外力持续作用下发生的振动。 变量器： 反射系数： 偶极子： 群速度：波包的传播速度 可听域：人类能够听到的不同的频率对应的声压的范围 凋落波 声阻抗： 互易原理： 品质因数： 二、振动系统的固有频率为，加一木块后变为，求系统质量。 解： 三、初始时对该系统加一力，放开后，固有频率为，振动模式 解：列出振动方程（设位移，位移） 令，代入上述方程 取， 振动模式 四、弦振动的基频能量和振幅 解：设弦长为l，线密度为，弦张力为T，且两端固定 则基频角频率，波数 振幅为 五、 ，，， 整理 六、脉动球源的声压级、声功率 解：脉动球源的声波方程 则p的一般解为 上式的第一项代表向外辐射 取 七、二列同向运动的波p1 p2 求能量密度、平均声能密度、平均声功率。 解： 设 八：解利用多普勒频移 中心频率 带宽 九：亥姆霍兹共鸣器定义及共振频率的推导 定义：由面积为S的截面长为的短管容积为的腔体组成的最基本的最简单的声学系统； 且满足：1共鸣器的线度远远小于波长，即、、 2短管的体积远远小于腔体的容积，即 3腔体内媒介压缩和膨胀时，腔壁是刚性的，不会把腔内的媒介的疏密过程传递出去。 共振频率的推导 设短管中媒介的质量，管壁对媒介的阻尼系数是，媒质的密度是 设短管中媒质位移为时，腔内压强由变为 得又 所以 短管中的空气柱运动方程：设外界的声压为 得： the same direction of rotation 90 ° or 180 ° for; after each disc rotor when measuring position alignment of the two half coupling constant, should also be in terms of accuracy and consistency. Surface deviation measurement, must in every 180 degrees corresponding to the two radii are equal to each other in order to eliminate errors caused by movement of the rotor. Only a support bearing of the rotor, WA couplings, such as the use of temporary leave the Center, should pay attention to keep the rotor in the measuring process in the center of the bearing oil retaining the nest does not change. Steam oil block, there should be sufficient clearance between the rotor and, in rotors make sure rotor is not on the block or seal oil. At the time of measurement, connections are not allowed between two rotors, each in a free State. Not allowed in case of assembled sleeve, Center work on the toothed coupling. Coupling tool should have adequate stiffness, installation must be secure and reliable. Coupling Center using a dial indicator, the table should be put in prison and avoid touching, to ensure correct measurement. Coupling disc after a week to return to the original position, circumference of dial gauge reading should be able to go back to th