On November 12th, 2015 we learned about

Natural resonant frequencies for first graders

Our downstairs neighbors sometimes sound like they’re playing soccer in their living room. It’s hard to say for sure, since only the low bumps and thuds of the ball make it through our floor to our ears. Unless they’re shouting about the rules, the kids’ voices can’t be heard the same way, but why is that? Why do some sounds actually get louder, to the point of destruction while others, thankfully, get dampened entirely?

Matching the sound to the material

In the case of my athletic neighbors, this uneven sound spectrum is thanks to the natural resonant frequency of the wood, carpet and plaster between us. Every material has a natural frequency, which is the way its structure will oscillate without outside influences. These vibrations are influenced by the components of the material, its shape, and size, basically anything that affects how flexible it may or may not be.

When outside vibrations, such as a loud sound, hit a material, that sound energy is transmitted to the material, like my floor. Most likely, the sound will be absorbed and dampened. However, if the sound happens to match the natural resonant frequency of the floor, it will instead be transmitted (and possibly amplified) through the floor, being audible in the room above.

Natural resonance on the playground

At this point in the explanation, my first grader is just staring at me, so let’s use a relatable analogy. Assume that my daughter on a playground swing set is our material. The length of the swing’s chain can change the natural rhythm of her swing— shorter chains mean faster oscillations (and higher sounds), and vice versa. That swinging rhythm is then the natural frequency of this material.

If I come along and push her out of sync with her swinging, I’ll either miss her entirely and push the air, or maybe push forward while she swings back, interfering with her movement and slowing her down. As an outside source of sound energy, I’m fighting her natural frequency, and the result is dampened movement, or less sound.

If I push in sync with the swinging, right as she crests to swing forward, I can add to her movement. Each push puts her higher and higher, adding more energy to her swing. As a material, she’d be vibrating more and more strongly, transmitting the sound more loudly with each push. This is a case of natural resonance, which can lead to all kinds of outcomes more interesting than thumps from my neighbors.

Too much of a good thing

When a material is resonating, it can lead to music, buzzing or even destruction. On the constructive side, musical instruments rely on this concept to amplify the desired sounds we want to listen to. On the destructive side, if a material vibrates too strongly it can break apart. Many people erroneously reference the Tacoma Narrows Bridge as an example of resonance causing an object to tear itself apart, although that collapse was actually due to aeroelastic flutter. (Basically, the bridge acted too much like a sail rather than allowing wind to pass through it.)

A more accurate example is the shattering of a wine glass by an opera singer. While quite unusual, it is possible that a human voice could hit a resonant frequency with enough energy to cause the glass to vibrate and shatter. It’s highly unlikely, and the person would need to produce a sound close to 105 decibels to pump the glass’s natural vibrations up enough for a break, but it is possible. With that in mind, I hope my neighbors’ soccer games don’t get any louder, since we already know the thumps are a resonant frequency for my floor.

Source: Fact or Fiction?: An Opera Singer's Piercing Voice Can Shatter Glass by Karen Schrock , Fact or Fiction

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