The Sounds of Glass Bottle Music

Introduction

Over the years, I have come across many videos on the internet of people making music just out of bottles and have always been curious about the physics behind it. I wanted to know how they were able to produce different pitches from the same bottle by changing the water level and so I plan to explore this.

Background Research

A picture of a Glass Harmonica

Historical Context

One of the earliest documented uses of a glass bottle as a musical instrument comes from the Glass Harmonica. This instrument was invented by Benjamin Franklin in 1761. Inspired by a musical concert of water-tuned wine glasses, he invented the Glass Harmonica. This instrument consisted of a series of glass bowls of varying sizes and thickness organised horizontally on an iron rod that could be turned by a foot pedal. By rubbing the rims of the glass with wet fingers, musicians were able to produce musical sounds. While this is not exactly blowing air into the bottle, this demonstrates the musical potential of glass containers.^[1]

The Physics of the Glass Bottle Music

A glass bottle labelled at each section

To make sound from a bottle, it needs to have a chamber, neck and a hole on the neck. When you blow into a bottle, it forces air into the bottle and the air pressure inside increases. Similar to a weight on a spring, when air is pushed through the neck it pushes inward and builds up the pressure inside the bottle. At some point, energy runs out and the compression stops. Then, the process reverses and the air decompresses. This forces the air to come back out of the neck. This pressure decrease continues and goes even further so that the pressure inside the bottle is slightly less than air pressure. This cycle continues with less air in each compression and decompression and less energy to drive it. Eventually, the air pressure inside is the same as the air pressure outside. This frequency is known as Helmholtz resonance. ^[2]

How to Play Glass Bottle Music

Picture of a guy blowing on glass bottles

Playing glass bottle music is similar to playing other wind instruments like the flute. Place the hole infront of the middle of your bottom lip. When you blow into the hole, you have to make sure that the hole is not covered as you are trying to blow over the top of the bottle. To produce a clear sound, the goal is to blow a focused stream of air downwards into the bottle in order to create a frequency. The more practice with this, the better you'll get as you find the best way to blow air into the bottle. Make sure to do this in a place you are comfortable and take frequent breaks while practicing as it is easy to feel dizzy.

Calculating the Frequency

In order to find the frequency, the following equation can be used:

formula

v = speed of sound in the air

area = cross-sectional area of the neck

volume = volume of chamber

length = length of neck

Experimentation

I decided to try the calculations out with a 370ml drink bottle that I had for this experiment.

My calculations were as follows:

v = 343 m/s^2

Length of the neck was measured to be 0.127metres

Area was found through the formula $\pi r^{2}$

$\pi (0.01143)^{2}$ = 0.00041043305

Volume of chamber was calculated through the formula $\pi r^{2}h$

$\pi (0.0381)^{2}(0.1143)$ = 0.00052125

Through this calculation, we would plug in the values into the equation to determine the frequency.

Frequency Equation

Blowing air into the empty bottle, we can determine the recorded frequency to be:

Frequency of Empty Bottle - 175 Hz

The frequencies are slightly off but this may be due to minor errors in measurements.

How does the Amount of Water in the Bottle influence the Sound Produced?

The pitch of sound produced by blowing across the opening of a bottle depends on the length of the air column inside the bottle. When you blow across the opening of a bottle, you create vibrations in the air column inside. The length of this air column determines the frequency of the sound produced, which is perceived as pitch.

When there is less water in the bottle, the air column inside is longer. This longer column of air vibrates at a lower frequency, resulting in a lower pitch sound. Conversely, when there is more water in the bottle, the air column is shorter, leading to higher frequency vibrations and a higher pitch sound.So, the amount of water in the bottle directly affects the length of the air column inside, which in turn determines the pitch of the sound produced when blowing across the bottle's opening.