On July 5th, a 39-story office/shopping center building in Seoul, South Korea started to shake rapidly. For ten minutes vertical tremors violently rocked the building, causing an immediate evacuation of the premises. After the shaking subsided, engineers began a lengthy process to determine the cause of the incident. Eliminating earthquake and windstorms, the culprit they landed on was bizarre to say the least: an aerobics class of 23 people on a mid-level floor. Their Tae Bo workout was apparently twice as intense that day, making their fancy footwork synch up with the building's structural resonance.
Resonance causing structural failure is rare, but when it happens, it tends to affect bridges. The Broughton suspension bridge in the UK and the Angers bridge in France are classic illustrations of mechanicalresonance, both of them brought on by soldiers crossing in lock-step. (The famous collapse of the Tacoma Narrows bridge has often been described as the result of resonance caused by strong wind gusts. But in fact it was brought down by the effect of aeroelastic instability, which caused the energy of the bridge in motion to feed back on itself until it collapsed.)
The Tacoma bridge is falling down
Millennium Bridge, 2000
More recently, London's Millennium bridge was shut down in 2000 after a surge of pedestrians created oscillating waves across the deck of the bridge. Like pushing a child on a swing at the correct moment so that her motion is amplfiied, resonance's effects are caused by the well-timed push of each individual wave. Push the swing at the right moment, and you could send the kid flying; amplify the vibrations in a building's structure enough, and you can push it to failure.
Infrasound and Humans
Resonance in audio waves and glassware is probably the most classic example of waves affecting structure; everyone has seen the fat lady shatter a wine glass with her pitch perfect operatic voice. Yet what may not be as well-known is the phenomenon of infrasound. Such sound waves, lower in frequency than 20Hz, the lower threshold of human hearing, can be used for monitoring earthquakes, charting rock and petroleum formations below the earth, and also in ballistocardiography and seismocardiography. But they can still be detected subconsciously by humans, and can resonate against the human body with terrifying results.
In 2003 Yang Liwei, the first Chinese astronaut, successfully landed his space capsule to mark a groundbreaking achievement in China's space program. However, when attendants opened the door to the capsule, they found the heroic Yang distraught and covered in blood. Aside from intense g-forces, Yang had been the victim of severe resonance.
Due to a design flaw in the capsule, Yang's launch and re-entry were accompanied by infrasonic vibration, which vibrated his body for 25 seconds, leading to internal damage, a bloody schnoz and intimations of imminent death.
Out of all the infrasonic wave effects, the psychological feeling of terror is the most fascinating and unknown. Imagine waves of noise infiltrating our body and stirring around the fluids in our brain that relate to our emotional stress. In 2003 a team of UK scientists experimented with the emotional effects of infrasonic resonance by exposing a group of 700 people to two identical musical pieces, of which one was laced with inaudible 17 Hz sound waves. The audience's response to the laced concert was discernibly different: "anxiety, uneasiness, extreme sorrow, nervous feelings of revulsion or fear, chills down the spine and feelings of pressure on the chest." The emotional disturbance of infrasonic waves are apparently so strong that they are considered the best reasoning behind supposed ghost sightings
Predictably, the effect, which can cause awe and fear in humans, has been weaponized. Military engineers have created prototypes of acoustic bazookas that generate infrasonic waves in a directed form. Apparently a ten second exposure to such waves, which can travel through concrete walls, leads to severe, crippling nausea. The weapon never entered full production.
There are numerous ways to defend against the onslaught of resonance. Power operated construction equipment is often engineered to hum in resonance frequencies well outside of any building material range. In completed projects, devices that are used in to protect against earthquakes double as resonance dampeners. In taller buildings, these are commonly seen as harmonic absorbers or suspended pendulums that counterbalance any latitudinal motion of the building.
But while vibration dampers can help balance out resonance and harmonics in buildings, defending the human body is much harder. While a building is a relatively static structure with a resonance frequency that doesn't much change, the human body is a big gelatinous pile of matter that absorbs vibrations with aplomb. Just like you can't escape the thump of your neighbor's party, you would not be able to escape sonic weapons that can shake the diarrhea out of you.
Correction: An earlier version of this story said that the Tacoma Narrows Bridge collapsed due to resonance. In fact, while the bridge had a tendency to oscillate, the collapse was due to aeroelastic instability.