It is time for some fun and a brief lesson on the physics of vibrations. This post is about vibrations of objects that appear rigid. In particular, the vibrations of solid metal surfaces and extremely tight drum heads. The vibrations are barely visible to the naked eye. With the right circumstances and equipment, they can be made obvious and to appear as things of beauty. Here is an wonderful example.
This movie was made for Otona no Kagaku Magazine, a magazine with projects for adults and kids. This shows the art performance by Mr. Kenichi Kanazawa who illustrated the method of making sounds visible. The technique was invented by Ernst Chladni — a physicist of 18-19 century. After placing 4 colored sand piles on a steel pan, Mr. Kanazawa rubs the surface of the metal pan with a rubber ball attached to the tip of metal stick to generate the vibrations. The ball doesn’t smoothly slide. Instead, it slips and sticks, slips and sticks, due to the friction, creating a strong vibration of particular frequency. A different ball surface makes a different frequency and pattern. It is sort of like that squeek you get with a very clean dish in the sink, or a wine glass when you run your finger around the rim.
I hope you found that sand table video fascinating. There are a few more examples of this vibration phenomenon below, and some explanations for those who are interested. But, if this is all you want to see, then thank you for taking time to check it out.
Back in the Old Days
Many years ago at the start of my physics teaching career, we would clamp a metal plate solidly to a demonstration table with a bolt through the center. You then sprinkled salt or sand on top. The vibration of the plate was caused by drawing a rosined bass bow firmly and steadily up and down along the edge. It was a frustrating demonstration because the results were extremely hard to produce and replicate. It took lots of practice. Even then, the results didn’t impress anyone except for the most hard-core nerds in the class.
To form patterns, you had to hold your fingers firmly on the nodes. Nodes were where the sand or salt piled up near the edge. Bowing was on antinodes, spaces where no sand or salt piled up. A well bowed pattern gave a pure tone of sound. The vibrations of the plate caused the sand or salt to pile up in patterns. Varying the force and speed and location of bowing varied the patterns.
Today, Chladni patterns are formed by using circular or rectangular metal plates on a mechanical driver controlled by an electronic signal generator. This method means you don’t need to be a bowing expert. The driver frequency can be controlled precisely over a very wide range. Results are nearly guaranteed for class demonstrations.
Here is an additional video from Sixty Symbols. If you have never explored this site, take some time one day to do that. It is a treasure trove of interesting stuff in physics and astronomy. This video also explains method used to vibrate the plate and the behavior of the particles on the top.
Drums and Cymbals
This web url illustrates some of the modes of vibration for circular drum heads. There are several small animated gifs used to illustrate the various simple and complex modes. Take a minute and have a look. You won’t be disappointed. Most people don’t think of the drum head moving is such complicated ways. Drummers have a better understanding of that phenomenon. For the reader wishing a deeper understanding, there are notes of explanation provided with each animation.
As a demonstration of the drum head movement, this video by drdanku places a rubber membrane over a length of PVC pipe. A loudspeaker to the right is forcing back and forth air vibrations over the top of the membrane as the driving mechanism to get the movement. In the background is a frequency generator value of 82.2 vibrations per second (Hz). The video was made at a high frame rate so it could be slowed down. Increased frequencies produce more complex patterns.
The TV show Time Warp did episodes with high speed cameras to show events normally not seen with the naked eye. The web url for their show has an online collection of episode. Here is a short clip they made of drum heads and cymbals.
Vibrations and Resonance and Musical Instruments
Apply force with a blow to some objects, they will vibrate. Apply a continued force at just the right frequency, they will vibrate and reinforce the previous vibrations causing them to grow in amplitude. We call this resonance. Soldiers marching in-step over a bridge can cause such a resonance and collapse the bridge. Ping the rim of a fine quality wine glass and hear it ring. Wet your finger and run it around the rim just right. It will resonate and ring loudly. The stick and slip of your fingertip creates the steadily applied force to the rim. There are many examples of resonances in nature. Mexico City suffered a major disaster from an earthquake and resonance of the ground in the mid ’80s. Power lines can oscillate with large vertical amplitudes in the wind.
Musical instruments are constructed to respond to vibrations of air, or a string, or a reed. In them you have resonance put to beauty and art. Thank you for reading and viewing.