How can resonance be useful and harmful

Initially, the body tends to vibrate with its natural frequency. But very soon, the natural vibrations die out and it begins to vibrate with the frequency of the applied periodic force. Characteristics of Forced Vibrations:. The amplitude of the forced vibrations depends on the difference between the natural frequency of the body and the frequency of the applied periodic force. When the difference between the two frequencies is large, the response of the body is poor or the forced vibrations are of small amplitude.

When the frequency difference becomes smaller, the body vibrates more readily or the amplitude of the forced vibrations increases. Finally, when the frequency f of the applied periodic force becomes the same as the natural frequency fo of the body, the amplitude of the forced vibrations becomes maximum and the phenomenon is known as resonance. If anybody is made to vibrate, by an external periodic force, with a frequency which is the same as the natural frequency of the body, the body begins to vibrate with a very large amplitude.

This phenomenon is called resonance. Distinguishing Between Forced Vibrations and Resonance:. Advantages of Resonance:. Disadvantages of Resonance:. Soldiers are asked to break steps when crossing a bridge. It can be explained as follows. Soldiers marching on a bridge take steps with definite frequency and force the bridge to vibrate with the frequency of the steps. If the forced frequency on the bridge is equal to the natural frequency of vibration of the bridge, the bridge is set into resonant vibrations.

Due to the resonance, the bridge vibrates with higher amplitude and due to this, it may collapse. Due to the rhythmic clapping of the audience, the roof of the stadium may collapse. When the audience claps rhythmically they do so with a certain frequency and force the roof of a stadium to vibrate with the frequency of the clap. If the forced frequency on the roof of a stadium is equal to the natural frequency of vibration of the roof of a stadium, the roof of a stadium is set into resonant vibrations.

Due to the resonance, the roof of a stadium vibrate with higher amplitude and due to this, it may collapse. When the speed of an aircraft increases, different parts are forced to vibrate. Typically field modal testing is performed with a calibrated modal impact hammer. The hammer contains a load cell in the tip which provides a direct measurement of the impact force of applied to the system.

A spectrum of this ringdown can be used to determine the natural frequencies of the system. This test is typically performed with the machine turned off, however advanced signal processing can also be used to average out running condition vibration and identify only the free vibration.

If it is determined that resonance is in fact the cause of excessive vibration, what can be done to stop or minimize the effect of a resonant condition? The natural frequency of a system is dependent upon two main factors; stiffness, and mass. Where k is the stiffness and m is the mass. Therefore, in order to change the natural frequency, we need to change either k or m or both.

Typically, the objective is to increase the natural frequency such that it is above any expected vibration frequencies. If the natural frequency is above or significantly far away from any expected vibration frequencies the resonance will likely no be excited. This theory forms the basis for any structural redesigns implemented to avoid resonance.

In practice, the following rules can be used to shift a natural frequency and minimize the vibration response of a system;.

If changing the natural frequency is determined to be the best solution, it is important to fully characterize the system before attempting any structural redesigns. Recently we performed a startup vibration analysis on a small building adjacent to a MW natural gas power turbine. On startup it was noted that there was a large increase in vibration in the building when the turbine went through the rpm range. Just to save you a little effort, going around yelling at glasses is not usually worth the time to prove this point.

Amazing, but true. Some negative effects of resonance are more subtle. Exhaust systems also use devices to counteract resonant effects including the those of the exhaust pipe itself which, otherwise, would make a pretty good wind instrument with a single frequency.

A final good example of unwanted resonance is found in loudspeaker systems. If a loudspeaker system has a resonant frequency, it will reproduce sounds at the resonant frequency much more efficiently than those at other frequencies. In most cases, loudspeakers are designed so that their unavoidable resonant frequency is far below or above the normal range of music the loudspeaker is intended to reproduce.

Have you ever noticed that automobiles with powerful sound systems when heard from outside the vehicle seem to only make a dull thud instead of distinct bass notes?