Level: Elementary to High School
Time involvement: 10-15 minutes.
Musical instruments abound in the world. From a cheap tin whistle to a priceless Stradivarius man has fashioned musical instruments from plants to animal parts. Even our human voice is an instrument capable of producing music. Music is sound waves in air. A gently babbling brook is sort of accidental music. The singing wine glasses may not have been made for music but they often can produce a pleasing musical sound.
The trick to making a wine glass sing is to cause it to begin vibrating. Lightly tapping the rim with a spoon will give a brief snapshot of what is possible. Pure sound waves of a single pitch or frequency are like the sounds you hear on a touch-tone phone. Pure, but not very musical. Sort of like a rainbow but only one color. Most sounds are a blend of different pitches with one dominant one. To make your glass sing, pick a good candidate. Thick heavy ones may not work at all. A goblet is a glass on a stem with a foot. Goblets are a good choice. The trick to making the goblet sing a sustained note is to keep simulating it with a small bit of energy.
You may begin with an empty goblet to get the “hang” of the right touch. Wash your hands to remove any oils from your fingers. Dry your hands and then dip one finger in water. Carefully hold the base with one hand and lightly and smoothly slide your dampened finger around the rim. You may have to play with the amount of finger pressure to get it right. The speed of your finger is not too critical but the pressure is. Almost immediately you will hear the “voice” of your goblet. Try adding a bit of water to the empty goblet and you will notice the pitch of the musical note change, usually to a lower tone.
Discussion: If the finger sliding around the rim is dipped in soap or oil the effect will not occur. Sliding friction vs. static friction are seen everywhere in the world. A simple example is created by taking a brick, or similar weight, attach a rubber band to one end and begin attempting to pull it across the floor. As you pull harder and harder nothing happens until suddenly the brick begins to move quiet well. The reason it was hard to get the brick moving is the rough surfaces of the brick and floor interlock until the force breaks the friction. On a much smaller scale your finger and the glass “interlock” until you add enough force to begin movement.
The skin is somewhat like the rubber band: elastic. In effect that small “jerk” creates some movement in the glass. When you tapped the glass to test its “voice” you set up vibrations that lasted for a few seconds. With your finger alternately sliding and stopping you set up continuous vibrations that sustain the natural vibrations of the goblet. The addition of the water changed the natural frequency or resonance point by requiring the glass to slow down a bit to transfer some of the vibration energy to the water. By “playing” with the amount of water you can “tune” your instrument. With enough goblets tuned to a musical scale a song could be played!
Notes: Expensive goblets may give more pleasing sounds but could be a costly lesson if care is not taken.