Interesting Bit of the Day : Resonant Frequencies - Tesla, Art Rock, and Sonic Screams

Resonant frequencies are all around us
August 12th, 2009  by Blaine Garrett

Today's interesting bit of the day is about resonant frequencies. In layman terms, something's resonant frequency is the point at which, when you "vibrate" it, will "shake" more than at other points. These "shakes" can be caused by wind, sound, earthquakes, or people dancing to House of Pain's hit song "Jump Around". The concept of resonance was discovered by Galileo in the early 17th century, but as a physical phenomenon, has been around since the beginning of time. Some historians suggest that resonant frequencies may have even  exploited by early civilizations in war fare. Lets ponder this a bit.

According to Wikipedia:

In physics, resonance is the tendency of a system to oscillate at larger amplitude at some frequencies than at others. These are known as the system's resonance frequencies (or resonant frequencies). At a resonant frequency the frequency of oscillation does not change with changing amplitude. Therefore, at these frequencies, even small periodic driving forces can produce large amplitude vibrations, because the system stores vibrational energy.

Firstly, some classic instances of resonant frequencies involve things collapsing. The Tacoma Narrows Bridge Collapse was an indent in 1940 (4 months after the bridge was built) where high winds caused the bridge the rock back and forth and ultimately collapse.  The wind caused the bridge to shake at its resonant frequency such that the road bed appeared rock back and forth like it was made out of rubber. The shaking was so consistent and prolonged that the bridge finally shook apart. The footage is surreal.

This incident was studied heavily by civil engineers to build more wind resistant bridges. Bridges, such as the Mackinac Bridge connecting Upper and Lower Michigan, have sections of road bed that are simply steal grates to allow the wind to better pass through. While this prevents the bridge from shaking apart, it does create a scary view of the water hundreds of feet below if you look out the car window.

Wind is not the only thing that can destroy bridges using resonant frequencies- people can as well. An often told story introducing waves in highschool trigonometry is  of generals leading their soldiers to their death while marching in step across a bridge. The regularity of the marching shakes the bridge similarly to the wind in the Tacoma Bridge incident and causes its collapse. One such case was of the Broughton Bridge near Manchester England that collapsed in  1831 when troops marched in step over the bridge causing it to sway and rattle apart. Since then, troops simply walk across bridges rather than march. However, even in modern civil engineering, bridges have failed because of pedestrian traffic. One such case is the Millenium Bridge in London that received so much traffic when it opened that the bridge began to shake and had to be closed. The bridge was designed to support that much weight, but even though not marching, the vistors seemed to walk enough in step to hit the ressonant frequency of the bridge. This footage is scary as well. Perhaps this is a commentary on the "hive mentallity" of people or perhaps the state of civil engineering in the London. They do have their own Nursery Rhyme about it. 

As we can see, resonant frequencies can be reached somewhat by accident. However, what if we wanted to utilize this on purpose. Let us first look at the destructive aspects as well as everyone's favorite physicist - Nikola Tesla. Tesla is credited for inventing the radio, alternating current electricity, improving many existing electrical devices and systems, and numerous other inventions. Tesla was a bit of an oddball and images of he with his various wireless electricity experiments are often the archetype of the  "mad scientist". It didn't help that he kept a lot of his ideas secret after a long life of Edison stealing them, lost patent claims, and the government supposedly trying to prevent enemy nations from getting to them first. One of his more interesting experiments  was the Tesla Oscillator, aka  "the earth quake machine". Supposedly, Tesla invented a device that, when tuned to the resonant frequency of the object it was attached to, would cause the object to violently shake as if it was experiencing an earthquake. He even filed a patent for a similar device called the "Reciprocating Engine".

The famous tale is that "[he] established a laboratory on Houston Street in New York at 46 E. There, at one point while experimenting with mechanical oscillators, he allegedly generated a resonance of several buildings causing complaints to the police. As the speed grew he hit the resonance frequency of his own building and belatedly realizing the danger he was forced to apply a sledge hammer to terminate the experiment, just as the astonished police arrived." (from Wikipedia). Based on some proposed designs, Mythbusters "debunked" the idea of the "Earthquake machine" in episode 60. However, to Tesla's credit, the Mythbusters were able to shake a bridge (albeit not to pieces). Personally, I don't know that a stunt guy and and a TV scientist can really produce conclusive evidence that debunks the claims of one of the greatest physicists of all time. Since the machine was destroyed and only the patent docs for a similar machine exist, perhaps we will never know if the earthquake machine lived up to its name. That said, there are obvious military applications to being able to destory buildings and bridges using mechanical ressonance.  There are other types of ressonance that miliaries have been known to experiment-including acoustic ressonance. If you were ever in band, acoustic ressonance can easily be observed by notes being played that shake the response head of the snare drums causing the snares to bounce and make noise. This phenomnon has probably been observed for as long as horns and drums have been used near eachother, even though ancient people may have not understood ressonance. However, some theorize they did. It is thought that the story of the destruction of the Walls of Jericho from the Bible could have been caused by finding the walls' ressonant frequency and playing that note on the horns of the invading Israelites.

Gallilo gave the phenomon a name thousands of years later, but perhaps ancient people embraced ressonance. Other military applications include producing "non-lethal" weapons that disable victims using sound. This site has a nice list of the effects of sound at various levels. At one frequency, your organs rupture. At another, your eyes start to malfunction (see ghosts?). At another, you lose your equilibrium. All are just a matter of frequency and amplitude, the fundamentals of any sound.

Another interesting resonance bit is the shattering of glasses using frequencies. This is often demonstrated when discussing resonance in Physics classes. Usually, however, this is demonstrated using speakers to produce a pure frequency (video). In a local 12th grade Physics class at the Minnehaha Acedemy,  professor Sam Terfa brought in a choir student who was able to do this with his voice alone. Epic. Ressont frequencies are also the stuff of artists. Art metal group Sunn O))) dabbles in low frequency and ressonant music that reportedly knocked down ceiling tiles at their performance at the Walker Art Center. One of my old favorite art dork musical groups Merzbow also dabbles in ressonant frequencies which also caused me to stop listening to their music (long story). Personally, I am often reminded of band practice in the basement of my parents' house where there was wood panelling. One of our bassplayer Josh's favorite things to do was find the ressonant frequency of the wood panneling covering the basement walls and, to my parents dismay, play this note over and over ratting the house. Anywho,  resonant frequencies can destroy bridges, shatter glasses, and sell records. Aren't they great?

Well, I hope you enjoyed this installment of interesting bit of the day. I have been compiling links for the last few weeks for this one. Please comment and let me know what you think of the series so far. I know it has not been daily, but I am trying. Related Links