Surprising sound frequency from a bottle

[Pereskia]

Two of my pupils in secondary high school is doing a project on the sound generated by striking glass bottles drumsticks.

They fill the bottle partially with water and strike the bottle with a drumstick and record the sound.

Preliminary results:
As long as the water level is in the cylindrical part of the bottle the frequency gets lower with more water. (Rules out Helmholtz and standing wave in the air above)
Filling water over the cylindrical part does not change frequency any more.
The frequency does not depend on where the bottle is hit. The resonance seems however better if the bottle is stuck below water level.

Any ideas what the resonator is. Is there a standing wave in the water? Or is the water causing a standing wave in the glass below the water level?

 

[A.T.]

... strike the bottle with a drumstick ...Or is the water causing a standing wave in the glass below the water level?

You are hitting the glass, so why blame the water for the glass vibration? The water could act as a damper that affects the frequency.

 

[Pereskia]

Yes, one idea is that the water stabilizes the glass and make a standing wave in the glass possible. But in that case, how does the water do that? Why does the water affect the standing wave in the glass? The pupils will try see if the frequencies match that of a standing wave and what speed that standing wave has in that case.

 

[A.T.]

Yes, one idea is that the water stabilizes the glass and make a standing wave in the glass possible.

Why do you think the standing wave in the glass is not possible without the water? What happens if the bottle is empty? What is the frequency range of your analysis?

 

[Pereskia]

There is of course a sound from the empty bottle as well. There has to be some influence from the water on the standing wave and since the frequency falls with rising water level I guess that the standing wave is mainly in the part of the glass that is below water level. What I can't understand is how the water affect the sound in the glass.

 

[A.T.]

What I can't understand is how the water affect the sound in the glass.

When the glass oscillates and changes shape, the water must as well. The additional inertia from the water increases the oscillation period. The water also filters the high frequencies that pass though it, which affects the sound propagating inside the bottle.

 

[DrClaude]

Not exactly the same situation, but this video might be helpful:

 

[Pereskia]

I can see how the filtering is happening but I can't see how that could affect the base frequency of the note.

The pupils have measured the base frequency for a number of water levels and will try to calculate the wave speed that a standing wave (from the bottom up to the water surface) would have in the glass and see if it matches the speed of sound in glass. They will have to do this in at least two different ways since we do not know how the sound reflects at the top. Maybe this can shed some light on if this is what's going on.

 

[Pereskia]

Thank you. There is a possibility of vibration in "the cup way" also, then maybe the water acts somewhat as a mass in a spring (or a handle of a cup). It could be harder to make the calculations to see if this is the case but it seems lika a resonable possibility as well.

 

[nasu]

The vibration modes belong to the glass wall not to the space inside the glass bottle. Unfortunately they are not so easy to calculate as for standing waves in a tube. They are similar to the modes of a church bell. Maybe you should ask them to excite waves in the air inside the bottle, by blowing air over the mouth of the bottle. These are closer to the simple theory exposed in introductory physics.

 

[Pereskia]

The vibration modes belong to the glass wall not to the space inside the glass bottle. Unfortunately they are not so easy to calculate as for standing waves in a tube. They are similar to the modes of a church bell. Maybe you should ask them to excite waves in the air inside the bottle, by blowing air over the mouth of the bottle. These are closer to the simple theory exposed in introductory physics.

This is the pupils' final project. They choose the subject themself and they do not need to have a final answer to pass. I more see this as an opportunity for them and me to learn as much as possible. It is not every day a pupil comes up with something that is new for me.