2. 外语
  3. The range in frequencies of musical sounds is approximately 20-20,000 cycles per second (cy/sec). Some people can hear higher fr

The range in frequencies of musical sounds is approximately 20-20,000 cycles per second (cy/sec). Some people can hear higher fr

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问题     The range in frequencies of musical sounds is approximately 20-20,000 cycles per second (cy/sec). Some people can hear higher frequencies khan others. Longitudinal waves whose frequencies are higher than those within the audible range are called ultrasonic frequencies. Ultrasonic frequencies are used in sonar for such purposes as submarine detection and depth finding. Ultrasonic frequencies are also being tried for sterilizing food since these frequencies kill some bacteria. Sound waves of all frequencies in the audible range travel at the same speed in the same medium. In the audible range, the higher the frequency of the sound the higher is the pitch. The term supersonic refers to speed greater than sound. An airplane traveling at supersonic speed is moving at a speed greater than the speed of sound in air at that temperature. Mach 1 means a speed equal to that of sound. Mach 2 means a speed equal to twice that of sound, etc.
    Musical sounds have three basic characteristics: pitch, loudness, and quality or timbre. As was indicated above, pitch is determined largely by the frequency of the wave reaching the ear. The higher the frequency the higher is the pitch. Loudness depends on the amplitude of the wave reaching the ear. For a given frequency, the greater the amplitude of the wave the louder the sound. To discuss quality of sound we need to clarify the concept of overtones. Sounds are produced by vibrating objects. If these objects are given a gentle push, they usually vibrate at one definite frequency producing a pure tone. This is the way a tuning fork is usually used. When objects vibrate freely after a force is momentarily applied, they are said to produce their natural frequency. Some objects, like strings and air columns, can vibrate naturally at more than one frequency at a time. The lowest frequency which an object can produce when vibrating freely is known as the object’s fundamental frequency. Other frequencies that the object can produce are known as its overtones. The quality of a sound depends on the number and relative amplitude of the overtones present in the wave reaching the ear.
Ultrasonic frequencies are ______.
选项 A、inaudible
B、excessively fast
C、characterized by a great amplitude
D、death rays
答案A
解析 超声频率是听不见的。答案可以在第一段找到。作者说，其频率远远大于可以听见的音域的纵波被称为超声频率。这说明，超声繁率是听不见的。
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