Course:PHYS341/2022/Project27
Glass Harp
Glass harps (also known by a multitude of other names such as musical glasses, singing glasses, angelic organ, or ghost fiddle) are a musical instrument made from many upright wine glasses filled with varying amounts of water.
History
Richard Pockrich, considered the first virtuoso of the glass harp, created the first glass harp in 1742. His name for the instrument was “angelic organ” and originally it was played by hitting the glasses with sticks instead of running fingers around the rim [5]. However, there has also been documentation of a similar kind of instrument as far back as 14th century Persia and 12th century China. Later, in 1761, Benjamin Franklin invented the glass harmonica. This had the same concept and physics as the wine glasses, but instead used specially shaped bowls, pre-tuned to certain pitches, which were nested in each other to conserve space [3]. The bowls were mounted onto a giant spindle with a foot pedal, much like a sewing machine. This allowed the player to play the bowls without having to move their hands around too much. Despite it's popularity during the classical era of music, where composers like Mozart, Naumann, Beethoven, and many others were writing music for the glass harp, by around 1835 the glass harp all but disappeared from the European music scene. The revival of the glass harp in the 20th century is largely due to the efforts of Bruno Hoffmann, and his innovation of using specially cast glasses optimized and tuned for ease of playing.
Tuning & Playing
The amount of water in the glass determines its pitch, in addition to other factors like the glass’s shape or size. An alternative way of tuning involves grinding (as F. Hopkinson-Smith did in the 1830s) or casting (like in Bruno Hoffmann’s specially designed glass harp) each individual glass to produce a specific pitch [3]. This allows you to have a consistent and fixed pitch per glass and does not require water for tuning.
The instrument is played by running a wet finger around the rim, and it produces a clear “crystal-like” sound, like a very high-pitched pure tone. There is a specific pressure/speed you have to use in order to produce the clearest, most consistent sound, which is related to the resonant frequency of the glass you are using.
Physics
The glasses produce sounds due to the slip-stick phenomenon, where the friction between your wet finger and the glass makes the glass vibrate, much like a bow and a string. As you go around, your finger sticks to and displaces the rim of the glass in a very slight elliptical shape. As your finger pushes the rim in the elliptical shape, there is more force, which overcomes the initial fiction between your finger and the glass, which causes the glass to slip off your finger. As it slips off, it tried to return to it's normal shape, but the backwards velocity causes friction, which causes it to stick to your finger again [2]. This cycle then continues for as long as you continually run your finger around the rim. It is necessary that your finger is wet, as the water reduces the friction between your finger and the glass just enough to allow the slip-stick to work; otherwise, there would be too much friction. The vibrations in the glass will then cause the surrounding air to vibrate. Wine glasses have a resonant frequency which is typically within the range of human hearing (20-20,000 Hz), so the vibrations will produce a sound that we can hear and perceive as a musical note [1]. Filling the glass with water lowers the resonant frequency since the water molecules are heavier than air molecules, thus slowing down some of the vibrations. More water means lower frequency, and it is based on the percentage of the volume of the glass that is filled, as opposed to a specific amount of water.
The exact timbre of the sound is affected by the shape and thickness of the glass, and the exact resonance of the glass is actually fairly complicated to calculate, likely involving the density of the glass, the density of the liquid inside, and other similar properties.
Example
Total volume of glass: ~140 mL
F0 (no water): 1716Hz (A6)
F0 (50% full; 70mL water): 1623Hz (G#6)
F0 (75% full; 105mL water): 1391Hz (F6)
F0 (~58% full, ~81mL water): 1559Hz (G6)
F0 (~65% full, ~91mL water): 1438Hz (F#6)
*these notes are generally about 20-40Hz more flat than the actual frequency for the pure tone of the respective note, except for F6, which is only about 5Hz flat [4]. This means that we would need slightly less water to produce the proper pure tone.
From these results, we can see that the lower you tune, the less water you need to add in order to hear a difference. The difference from an empty glass to a 50% full glass was about one semitone. However, we only needed an additional 8% more of the volume filled to produce a note that was another semitone lower.
We can also see from the spectra that the wine glass plays a pretty pure tone. The fundamental frequency is the strongest, and while there are harmonics, they are much lower in amplitude, so we mostly only hear the fundamental.
Example glass being played:
References
- Buddies, S. (2015, November 26). Singing glasses. Scientific American. Retrieved April 2, 2022, from https://www.scientificamerican.com/article/singing-glasses1/
- Dhanjee, N. (2021, December 31). Investigation in acoustics of Wine Glasses. UK Essays. Retrieved April 2, 2022, from https://www.ukessays.com/essays/physics/investigation-acoustics-wine-glasses-3750.php
- Szafraniec, A. (n.d.). History of the glass harp. Glass Harp Musicians. Retrieved April 2, 2022, from https://glassduo.com/en/history-of-the-glass-harp
- Tuning. Frequencies of Musical Notes, A4 = 440 Hz. (n.d.). Retrieved April 2, 2022, from https://pages.mtu.edu/~suits/notefreqs.html
- Wikimedia Foundation. (2022, March 28). Glass Harp. Wikipedia. Retrieved April 2, 2022, from https://en.wikipedia.org/wiki/Glass_harp