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# What Is Resonance?

Everyday Einstein discusses how physicists, chemists, and writers use the word “resonance” differently. Plus, how to shatter a glass with just the sound of your voice.

By
Lee Falin, PhD,
April 4, 2014
Episode #095

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Sometimes when speaking about a book or movie that they really enjoyed, people will say something like, “Wow, that really resonated with me.” They usually mean that the message of the book or film caused them to feel some emotional response. The amazing writing teacher David Farland even wrote a book about this phenomenon called Drawing on the Power of Resonance in Writing to help other authors understand this response..

But just like other words we commonly use, resonance can mean something slightly different to scientists than it does to everyone else. Even more confusing is the fact that resonance can mean something different to different scientists, depending on what branch of science they study. So let’s see if we can shed some light on this multifaceted word.

## Resonance in Physics

When we discussed sound waves a couple of episodes back, I said that if you imagine sound as a series of waves traveling through the air, the frequency would be how many waves pass a fixed point in a given amount of time. With sound, we usually measure frequency in Hertz, which is the number of waves passing a fixed point every second. The more waves passing per second, the higher the frequency.

If you pull a string on a guitar, it starts to vibrate at a certain frequency called its natural frequency. The natural frequency of an object depends upon its physical and chemical properties. That is, its shape and length as well as what it is made of (steel, nylon, sheep intestines, etc.)

Recall also from our lesson on sound that the vibrating guitar string pushes against the surrounding air molecules, causing them to vibrate. Those molecules push against the ones next to them, and so on, until the vibrating molecules reach your ear drum. These poor air molecules are vibrating at the same frequency as the guitar string.

Now let’s say you have another object sitting close by that has the same natural frequency as the guitar string. Once the vibrating air molecules (which are again, vibrating with the same frequency as the string) strike this second object, that object will also start to vibrate. This induced vibration is called resonance.

You might have heard of people being such talented (or awful) singers that they can shatter a wine glass with nothing but their voice. This phenomenon is also caused by resonance. If you run your finger around the top of a wine glass, the sound you hear is the natural frequency of that glass. If a singer can produce a strong enough sound wave at the same frequency, they can induce vibration in the glass. If the vibrations are strong enough, that glass will shatter.

Another area where resonance is often mentioned in physics is the infamous Tacoma Narrow bridge collapse. This bridge is often used as the textbook example of how not to build bridges. According to many sources, strong winds hit the bridge with the same natural frequency as the bridge itself, causing resonance which build in strength until the bridge collapsed. It’s a nice story, but most physicists don’t believe it had anything to do with resonance. Instead they blame something called aerostatic elasticity

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