Tue November 08, 2011 By: Pratibha Kamath

1)Why are infra sounds not used in detecting cracks in a metal ? 2) Why does a vibrating pendulum not make noise ?

Expert Reply
Wed November 09, 2011
1) Ultrasounds can be used to detect cracks and flaws in metal blocks. Ultrasonic waves are allowed to pass through the metal block and detectors are used to detect the transmitted waves. If there is even a small defect, the ultrasound gets reflected back indicating the presence of the flaw or defect. But, infrasound have low frequecy and they can't penetrate into the metal to detect the cracks.
2) A pendulum bob vibrating back and forth about its equilibrium position. A pendulum does not produce a sound when it oscillates . A pendulum consisting of a longer string vibrates with a longer period and thus a lower frequency (less than a natural frequecy). Once more, there is an inverse relationship between the length of the vibrating object and the natural frequency at which the object vibrates
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