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A previous blog explored some potentially useful properties of resonance. The fact that resonance has the power to convert sound wave pressure into mechanical motion can be very useful. On the other hand, however, the fact that it takes so little effort delivered at the proper frequency to excite resonance also has its “down” side.
Any connection between a resonant object and a source of sound producing the proper frequency will excite the resonant object to vibrate through a principle called “sympathetic vibration.” A popular high school physics class experiment uses two tuning forks tuned to the same frequency and mounted to wooden boxes which act as sounding boards. When one tuning fork is struck, the second, although not struck, starts to vibrate although there is no physical connection other than the air between them. Clearly, then, it takes very little stimulus to cause resonant vibration.
Eliminating or counter-acting unwanted resonances is a science in itself. Among other problems are the fact that once an object starts vibrationg in resonance the motion may continue to build to the point that forces within the object finally exceed the mechanical limits of of the base material. In this case the result is disastrous as the base material fractures. The best way to understand the “not-so-friendly” properties of resonance is through a few every day examples.
Wine Glass –
Most everyone has heard the story about a wine glass being broken by an opera singer’s voice. If you tap a wine glass gently with a spoon, it will “ring” like a bell at its resonant frequency. If the glass is, then, excited by an singer singing loudly at that exact same frequency, the glass will start to vibrate. As the stimulus continues, the vibration in the wine glass builds until the glass finally shatters as the mechanical limits of the glass are exceeded. Just to save you a little effort, going around yelling at glasses is not usually worth the time to prove this point. In the commercial for one brand of recording tape which you may remember, the sound of the singer’s voice was amplified in order to achieve sufficient amplitude to break the glass. However, in a recent episode of “Mythbusters” on the Discovery Channel, a singer did, indeed, break a wine glass using ONLY his unamplified voice. Amazing, but true.
Automobiles –
Some negative effects of resonance are more subtle. An automobile with a “bumpy” tire may start to vibrate as the automobile is driven at a particular speed. Since the bump in the tire provides a mechanical “push” each time the bump hits the road, the vibration created by the bump at the proper frequency is actually exciting of some part of the suspension or steering system of the automobile at it resonant frequency. An automobile’s suspension system often includes a “resonator” which is anti-resonant at frequencies that the suspension would, otherwise, be resonant. 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.
Loud Speakers –
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? The “thud” you hear is really the resonant frequency of the system’s loudspeakers and the vehicle itself (which is no less than a resonant cavity). The occupant of the vehicle may be hearing distinct bass notes but what leaks out into the neighborhood is sound at the resonant frequency. Loudspeakers utilize acoustic technology to provide reproduction of all frequencies at similar amplitude.
The effects of resonance are at the base of the science of controlling and minimizing or maximizing sounds at particular frequencies. Ultrasonic cleaning systems could not exist if it weren’t for resonance – but at the same time, potential resonance of the system itself or the items being cleaned must be understood and controlled to assure against detrimental effects.
– FJF –