Courses:PHYS341/2021/Project21
The Sound of Bottles
Oftentimes while drinking out of a bottle, I found myself blowing over the top of it and making a particular sound. The more I drank out of the bottle, the lower the pitch of the sound emitted. I was curious as to what the reason for this was and what the pitch spectrum of this sound would be.
How does it work?
So it turns out that the process of generating sound from a bottle through blowing is somewhat different to more common instruments that are also played with a similar technique. When air is pushed through the neck of a bottle, it acts in a similar way to a weight on a spring with the air coming through the neck of the bottle acting as the weight and the air inside the body of the bottle acting as the spring. This is known as a Helmholtz Resonator, which is any bottle or container with a thin neck and wider body.
When air is blown into the resonator, the air inside the body is compressed by the incoming air flow which increases the air pressure inside. This leads to the pressure inside the bottle being higher than the air pressure outside, causing the situation to reverse as the airflow changes direction. The higher pressure inside the bottle causes the air to flow outside until the pressure inside is lower than the outside air pressure, reversing the direction of airflow once again.
It is in this way that the air inside the bottle acts as a spring, compressing and extending at a particular rate that translates into the sound frequency that is emitted from the bottle, its resonant frequency.
Calculating the frequency of a bottle
In order to calculate the frequency you can use the following equation :
v = Speed of sound
A = Area of the opening of the neck l = length of the neck V = volume of the bottle
Experimentation
I decided to try the calculations out with a bottle I had and used a 1 liter wine bottle for the experiment.
My calculations were as follows:
An analysis of the plot spectrum of the sound emitted from the bottle showed that my calculations were not too far off of the recorded frequency.
Comparing bottles to other instruments
I recorded the sound of a smaller beer bottle and attempted to recreate the pitch of the sound I heard with both guitar and voice. The calculations of the resonant frequency of the beer bottle with the Helmholtz resonator equation were less accurate due to its "bomber" bottle shape[1] with an irregular neck. The pitch spectrum of the beer bottle is presented in the following picture.
The voice and guitar spectrums are presented below:
How to play
I found that the embouchure of playing a bottle is similar to that of certain woodwind instruments such as the flute or the piccolo. To produce a clear sound it is important to place the top of the bottle around the middle of the bottom lip and blow downwards over the top of the bottle.
Bottle Music
Playing the bottles can be challenging as it is only possible to get the fundamental frequency at a given time and the only way to change to a different pitch would be to increase or reduce the volume of the bottle via adding or removing liquid. It is more common to have a series of bottles tuned to different pitches that the player(s) would be able to play in a certain sequence.
This is demonstrated in the following short musical piece:
- ↑ "Beer Bottles". Wikipedia.