Crystal glasses can be used to make sounds in at least two different ways. One is by lightly "pinging" the sides of the glasses, and a second way is by rubbing the glasses' rims. Both methods work because of vibrations and frequency. The rim method is explained here.
Sounds from rubbing the rim of a glass have been studied for centuries. Galileo wrote of the phenomenon in 1638.
Benjamin Franklin invented a musical instrument he called the Glass Armonica. It was a series of crystal bowls of different sizes mounted on a rotating spindle. It was played by lightly touching the rims of the rotating bowls with the fingertips. The sound created was haunting and beautiful. Many composers wrote music for the instrument, including Mozart and Beethoven.
To create the sound, use a crystal wine glass and a wet fingertip. Slowly run your fingertip around the rim. A light touch is required; if no sound is made by four or five times around the rim, alter the pressure on the rim.
The principle at work here is a "slip-stick" form of friction. It is also called "dry-damping." When the fingertip circles the rim of the glass, it excites the glass molecules. They follow the motion of the finger around the glass, in effect "stretching" in the direction of the moving finger. The frictional force is high as the molecules stretch.
The elastic forces within the glass pull the molecules back into place, but because of inertia they overshoot and go too far. At this point, the molecules are moving in the opposite direction of the finger. The frictional force is low. The stretching and returning action of the molecules becomes a cycle. An imbalance in the frictional forces is created, causing the vibration that makes the sound.
This illustrates the fundamental natural frequency of the glass. Different glasses have different frequencies, which depend on factors such as the size of the glass, the position of the glass and the volume of liquid in the glass.
Crystal glassware works best.
Regular glass is "soft" glass. It has high internal friction (molecular) so that sound waves produce a dull sound. Crystal contains lead, which makes it "harder," with very little internal molecular friction: molecules are freer to follow the fingertip. This creates the clearer sound in crystal.
Regular glass can make a rim sound, although with more difficulty, and the tone has less quality.
People used to think that the sounds of Franklin's Armonica would drive listeners mad. The music is often considered ethereal or eerie, but there is no evidence supporting it as a cause of mental instability. One theory is that because the notes were painted on the glasses with lead paint, and players licked their fingers to keep them wet while playing, they got lead poisoning as a result of ingesting lead paint.
Besides being an entertaining parlor trick, this illustrates principles of sound in a concrete way. Some musicians still use adaptations of the Glass Armonica.
- Photo Credit morguefile.com NinoAndonis
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