147th ASA Meeting, New York, NY

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Acoustics of Glass Harmonicas

Thomas D. Rossing -
Physics Department
Northern Illinois University
DeKalb, IL 60115

Popular version of paper 4pMU11
Presented Thursday afternoon, May 27, 2004
147th ASA Meeting, New York, NY

Glass musical instruments are probably as old as glassmaking. At least as early as the 17th century it was discovered that wine glasses, when rubbed with a wet finger, produced a musical tone. A collection of glasses played in this manner is called a glass harp. Another type of glass harmonica, called the armonica by its inventor Benjamin Franklin, employs glass bowls or cups turned by a horizontal axle, so the performer need only touch the rim of the bowls as they rotate to set them into vibration. We have observed the modes of vibration of both types of glass harmonica, and also the different sounds that are emitted by rubbing, tapping, or bowing them.

Vibrational modes are accurately shown by means of holographic interferometry, which displays a contour map of the vibration. Several modes of vibration of a wine glass are shown in the photo below. Points of maximum motion, which occur around the rim, appear as "bull's eyes." The vibrational amplitude changes by half a wavelength of light (316 nm) in moving from one bright fringe to the next one. The fundamental mode, designated as the (2,0) mode, has four such regions, with the glass moving in alternate directions as it vibrates.

Rubbing with a wet finger tends to excite only the (2,0) mode and its harmonics through a "stick-slip" process, while tapping the glass excites the other modes as well. The mode patterns in the photo rotate around the glass following the player's finger, resulting in asound that pulsates with about 4 to 8 beats per second, depending upon the speed of the player's finger.
Fine tuning the glasses is done by grinding the glass. Grinding down the rim raises the frequency, whereas thinning the glass bowl near the base lowers the frequency. Fine tuning can also be accomplished by adding water (or wine), but the tuning range is small.

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