Resonant Frequencies

Every object has one or more frequencies it vibrates at. Wikipedia says:

“In physics, resonance is the tendency of a system to oscillate with greater amplitude at some frequencies than at others. Frequencies at which the response amplitude is a relative maximum are known as the system’s resonant frequencies, or resonance frequencies. At these frequencies, even small periodic driving forces can produce large amplitude oscillations, because the system stores vibrational energy.”

A tuning fork is a small example of this. You strike it and it starts vibrating at a certain pitch, it’s resonant frequency. In the human body we have several resonant frequencies. The eyes resonate at about 20hz, the digestive track at about 7hz, the brain can go down to 1hz. If a 7hz frequency is played loud and long enough it can kill you. This is because as the digestive system starts oscillating at it’s resonant frequency, and that frequency continues to supply more energy,  the oscillation becomes and stronger and stronger until the thing that is vibrating breaks and falls apart. This can happen with buildings and other large structures too. Its called a resonance disaster. A famous example of this is the Tacoma Bridge which resonated itself to death in 1940:

I wanted to see if I could find the resonant frequency of an object. I didn’t want to destroy anything, but just figure out how to do it. Maybe I code use the information creatively? I started with this PVC pipe:


After researching online methods to do this, I tried passing a sine wave through speakers into the pipe. I had a microphone on the other side attached to an oscilloscope, the theory being that if the pipe resonates at a certain frequency the waveform on the oscilloscope should jump.


This didn’t work out so well so I went old school and decided to use my body as the instrument… i.e.,  I put my hand on the tube and felt it, and my ear to the end of it and listened to it.


I could definitely hear and feel the resonance get stronger at certain places. These places turned out to be the overtone series with a fundamental note of B. The overtone series (harmonic series) is a base of multiples over a fundamental pitch. The sound we hear from a piano playing one note is actually that note with its overtone series on top of it, each upper partial being weaker and weaker, so that we usually just hear the fundamental.

I made a recording with my phone of what ambient sound coming through the tube sounds like: 

Hear that pitch everything is tinted with? That’s a B.


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