“The Note That Can’t Be Sung?”

By Scott Hamilton

I watched a very interesting video on Facebook earlier today. It was a video about a note that cannot be sung, even by a professional singer with years of voice training. If you want to have a little fun with your friends, you can do this experiment for yourself and learn something new about sound waves. If you take a tube, like a section of PVC pipe or even just a paper towel roll, you can make it impossible for someone to sing a certain note. The note that they cannot sing will be different depending on the size of the tube, and I am not sure it will work with a short tube, but you can always try. If you try to sing into a tube and the note you sing matches the resonant frequency of the tube, you cannot sing the note.

So now comes the science part, first you need to understand that a music note is simply a sound wave of a given frequency. If you look closely at a guitar string you will notice that it vibrates; this vibration creates a musical note by vibrating the air around the string at a given frequency. When we sing, our vocal cords do the same thing as the guitar string and vibrate the air at a given frequency. The next thing you need to know about sound waves is that they cause things they come in contact with to vibrate at their frequency.

Have you ever seen an opera singer break a wine glass by singing just the right note? This happens because the singer is able to produce a vibration that matches the resonant frequency of the wine glass. A resonant frequency is the natural frequency at which an object vibrates at the highest amplitude. In the case of a guitar string, it is where the string moves the farthest. When an object is hit by a sound wave that matches the resonant frequency, it will vibrate, making the sound louder and louder until the vibration eventually destroys the object. It even happened with the infamous Tacoma Narrows Bridge in 1940, when the wind blowing around the bridge caused it to vibrate at its resonant frequency, sending the bridge into the river valley below.

This might lead you to think that if you sing into a tube at its resonant frequency, the sound would get louder and eventually shatter the tube. However, this is not the case, as tubes work differently when they reach their resonant frequency. If you try to sing into a tube at its resonant frequency, the tube will begin to vibrate and create a back pressure in the tube, making it impossible to force the sound through the tube. You will not be able to produce a sound that matches the resonant frequency of the tube. As a result you create an environment that makes it impossible to sing a certain note.

I found this so interesting I began to wonder if it could be used in any way. After a little research I found that this method of blocking the resonant frequency of a sound wave with a tube is in fact one of the methods used to deaden the engine noise with a muffler. It is also the primary principle in effect with a didgeridoo, which causes the instrument’s unique sound. The didgeridoo is an Aboriginal instrument usually carved from bamboo and is simply a tube between three and ten feet long.

A skilled player of the didgeridoo uses the resonant frequency feedback, which causes you not to be able to sing the note, and adjusts their vocal tract to act like a resonator and amplify the sound coming back up the pipe into their vocal tract, creating the unique sound of the instrument. It takes years of practice to master the skill of tuning your vocal tract to the instrument, and every didgeridoo has a slightly different resonant frequency, meaning that you really want to use the same instrument throughout your didgeridoo career.

Until next week stay safe and learn something new.

Scott Hamilton is an Expert in Emerging Technologies at ATOS and can be reached with questions and comments via email to shamilton@techshepherd.org or through his website at https://www.techshepherd.org.

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