Hey, it’s been a while since the previous LSNED story. I hope you’ve been learning some interesting facts on your own, but here we meet again to scratch our collective foreheads and raise a glorious chorus of “Huh, neat.” The topic du jour, for no particular reason, is resonant frequency.
The classic image of the concept of resonance is when the big, fat, braided opera singer hits the high note causing wine glasses to shatter. It really can happen. If the singing frequency matches the crystal glass resonance frequency, the vibrations can be strong enough to overwhelm the strength of the glass. So what is this special frequency?
Well it’s different for every object, but it is the frequency of vibration at which the object is naturally prone to movement. Picture a swingset in the playground. As the swing moves back and forth it is, technically, vibrating. (in really slow motion, on a large scale) Each complete movement from back to forth is called an oscillation. The period of time between each oscillation is the frequency. Frequency is measured in hertz. (cycles per second)
Here’s the interesting fact: Whether you are casually rocking on that swing, or going full blast, the frequency will remain the same. The time it takes for you to move back and forth (to oscillate) is the same even if the distance covered is vastly different. (you may be swinging further, but you’ll be moving faster… it all evens out)
I just put this to the test by hanging a pair of scissors from the ceiling on a thread. (a basic pendulum, long enough to move fairly slow) One oscillation took about 3 seconds, no matter how much I pulled the scissors back. So, my pendulum had a resonant frequency of about 0.3 hertz.
The special thing about hitting this resonant stride which causes wine glasses to break is that there is nothing to dampen the vibrations. Near 100% of the energy of that sound wave would be converted into movement. At non-resonant frequencies, the vibrations would fight against the crystals natural tendencies and fizzle out. (so it’s not that resonance can amplify waves, so much as accumulate when incoming energy is “in phase“)
The other important ingredient of resonance is the quality factor (Q factor). This is the difference between the ringing of a crystal wine glass versus a regular drinking glass. The crystal has a higher Q factor that lets the resonance do its thing without dampening. An expensive violin would have a higher Q factor than a cheap student fiddle.
Resonance is why different things have different sounds. If you drop a handful of change, the quarters sound different than the nickels because different metals resonate at different frequencies. Guitar strings will change their tune when stretched tight because it changes the resonant frequency. When you’re driving down the highway, your car might start to shake more at a certain speed because you found a resonant frequency.
One famous resonant disaster occurred in 1940 when the Tacoma Narrows bridge in Washington state shook itself to total collapse on a windy day. (the whole thing was caught on film) It wasn’t terribly windy, just 42 mph, but it was enough to get the solid steel girders resonating, and the Q factor wasn’t low enough to dampen it.
- Source: Resonance – Wikipedia
