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The relationship between the height of water level in a glass

and the frequency of the emitted sound

Introduction, aim and background information

On my last trip to Budapest, Hungary, I watched a street musician play an instrument

called the glass xylophone. A glass xylophone is a musical instrument made of upright

glasses that are filled with water to different heights. There are many ways to make a

glass xylophone, simpler instruments consist of just a few glasses, while more complex

ones are carefully designed and can include over fifteen glasses. Observing the street

musician play the glass xylophone made me curious about the Physics behind this

instrument, so for my internal assessment I decided to investigate how a glass

xylophone works, more specifically, why different glasses produce different pitches

depending on the height of the water in the glass. Pitch in music simply means how high

or low a sound is ("Pitch"). When studying waves, I remember learning that higher pitch

means that the sound wave has a higher frequency, while a lower pitch means lower

frequency. Based on what I know so far, I decided on the following research question

for my investigation:

How does the height of the water level in a glass affect the frequency of the sound

emitted by the glass when the side of the glass is hit with a spoon?

When I started researching glass xylophones, I found many more articles about singing

glasses, also called glass harps. In contrast to a glass xylophone, when playing a glass

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harp one has to rub the glass rim with a finger to produce sound (Nose 4). I also

discovered that it does not matter whether we rub our wet fingers on the rim of the glass

or hit the side of the glass with a metal spoon, because both methods create vibrations

in the glass and produce the same frequency, so in my work I will also refer to articles

about singing glasses ("Singing Glasses"). When energy is transferred to the glass by

rubbing its rim or by hitting its side with a spoon, the glass starts to vibrate. In turn, this

causes the air molecules to vibrate with the same frequency, producing the sound that

we hear (Nose 10). As we add more water, it becomes more difficult for the glass wall to

vibrate due to the added mass and as a result, vibrates at lower frequencies, producing

a lower pitch (Lee 1). Moreover, I also found that the relationship between the height of

the water level and the frequency of the sound is not linear (Lee 3, Nose 11). Hence my

hypothesis is that as the water level in the glass increases, the frequency of the sound

produced by the vibrating glass will decrease at an increasing rate, producing a

nonlinear curve.

Planning

I. Variables

1. Independent variable: Height of the water level in the glass (cm)

2. Dependent variable: Frequency of the emitted sound wave (Hz)

3. Control variables

i. Glass used

● Reason: glass shape and size influence the vibrations

produced by the glass wall, hence affect the frequency.

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● Method: I will use the same glass throughout the experiment.

ii. Glass cross-sectional area

● Reason: the volume of the water might influence the

frequency. If the glass has varying cross-sectional area,

every time we add water, the volume of the water added will

be different.

● Method: I will use a glass that has the same cross-sectional

area along its entire height.

iii. Liquid used in the glasses

● Reason: different liquids have different densities. Larger

density makes it more difficult for the glass to vibrate, hence

affects frequency.

● Method: I will use water throughout the experiment.

iv. Air temperature

● Reason: the speed of sound depends on the temperature of

the medium it travels in. Since frequency is related to wave

speed, temperature should be the same throughout the

experiment

● Method: I will carry out the experiment indoors so that

temperature can be kept constant.

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