Have you ever heard of the lovely opera singer whose voice causes a glass to shatter? Or about the wobbly Tacoma Narrows Bridge collapsing? Do you loathe (or maybe, love?) that weird flippy-floppy noise when driving down the highway with one window slightly rolled down? These are all examples of resonant frequency events.

Have you ever been to a concert and could feel the music in your chest? Now, imagine you feel that same feeling in your chest, that same “let’s dance” feeling in your legs, that heart pumping and thumping. However, you are in a quiet room, a quiet lab, a quiet house. Is my house haunted? Is there a specter in my lab? Why do I feel so woozy in this room?

What is resonant frequency? A resonant frequency is when an external force on an object exactly matches the maximum amplitude frequency of that object, and then the object begins to vibrate more rapidly. (Now, Tacoma Narrows Bridge disaster was caused by resonant frequency and aero-elastic torsion, but we’re not teaching a class on solid dynamics, nor fluid dynamics.) However, there is a special class of frequencies in a range that is from 0.01Hz up to 20Hz, or the infrasound range. Humans can hear from 20Hz up to 20kHz (20,000Hz), and can “feel” frequencies from 20Hz to 150Hz. Though frequencies in the 20s and 10s are so deep to our ears that they are usually felt more than heard. There is another special range of frequencies called “chest slam” or “chest kick”, between 60Hz and 80Hz. Lots of metal music, electro-dance music, rap music, and even disco fusion music. These frequencies, combined with gigantic subwoofers pointed in the crowd, makes it feel like your torso is moving while your feet are planted on the ground. It’s hard to stay still. Your brain tells your legs to move like your torso is moving

It’s all fun and games, until there is a wall-mounted air conditioner oscillating at a low frequency and making people in the room feel woozy. Though different parts of the body have different resonant frequencies, the resonant frequency for the human body is between 3-10Hz, and for the stomach is 4-8Hz. There are documented instances of people feeling that some supernatural force was in play, or that everyone on their team was subjected to some type of harmful chemical or poison, and the source was some machinery which vibrated at an infrasonic range which also was resonant frequency for the human body or some body part.

Whether you’re dancing at a concert, studying in the library, or having a dinner party at home, resonant frequency events occur often.

Sources:

Bass Frequencies and Body Physics

Waves and Acoustics – Examples of Resonance

The Higgs Center for Theoretical Physics Presents: Higgs Hallowe’en 2021: The Haunted Frequency

Assessment of Lift Passenger Comfort and the accompanying diagram, Biomechanical human body model showing the resonance frequencies of individual organs (fig 5 from the article)