Wine Glass Acoustic Resonance

An acoustic wave is an oscillation of pressure (longitudinal waves) that travels through a solid, liquid, or gas in a wave pattern. Longitudinal waves are the ones which have the same direction of vibration as their direction of travel. Important quantities for describing acoustic waves are sound pressure, particle velocity, particle displacement and sound intensity. Acoustic waves travel with the speed of sound which depends on the medium they are passing through.

If a periodic force is applied to such a system, the amplitude of the resulting motion is the greatest, when the frequency of the applied force is equal to one of the natural frequencies of the system. This is often referred as resonance. The resonance frequency is essential in this experiment, because the sound the wine glass makes when its rim is rubbed by a moistened finger, is the resonance frequency sound of the glass with the specific amount of water.

In this Instructable I will show you how to set up a simple resonance experiment, which you can do at home. I have also used this demonstration to support physics lessons, and students seemed to really enjoy it.

- Two wine glasses (preferably the same)

- Plastic straw

- Pipette (alternatively spoon)

- Tap water

Tools:

- Scissors

- Smartphone

First step is to download and install free spectrum analyser app from the Play-store. I'm using Android phones since the system came out, but I know there are spectrum analyser apps available for Apple phones too.

Open the Play-store app and type in the search box 'spectrum analyser'. You will get few hits, so look for the icon with microphone and some coloured waves in the background (see image above). Download and install the app and then open it to see if everything works. The app will analyse sound spectrum and show you breakdown of various sound frequencies (see second picture above).

The feature that we will use to set up our experiment is the 'peak frequency'. The app will show you the peak frequency of the generated sound and it will display the exact value at the bottom (measured in Hz).

Now comes the tricky part. You have to set up both wine glasses to generate exactly the same frequency. In principle, if you pour the same amount of water into both glasses it should work, but it's not that straight forward unfortunately.

Even if the glasses look the same and came from same manufacturer, there always will be some inconsistencies in crystal structure, glass thickness etc.

Start with pouring exactly the same amount of water into both glasses and then measure the peak frequency of first glass using spectrum analyser app.

Another tricky part is to make a sound by moving your finger around the rim of the glass. At first you should wet your index finger and then slowly, with constant speed, move your finger around the rim of the glass. You will have to apply some pressure to make a sound. The amount of pressure and speed depends on the glass, but also on your finger, so this requires a little bit of practice if you have never done it before.

Here is a short video of me making sound and I hope you will find it helpful:

After you know the peak frequency of your first glass, the second glass have to make the sound at exactly the same frequency (+/- 1 Hz). To do that, use a pipette (or spoon) to add or remove water from the second glass.

Every time you make adjustments, make a sound with your finger and measure the peak frequency to see if you are getting closer to your target frequency (frequency of first glass).

When you have both glasses to vibrate at exactly the same frequency, you are ready to do the acoustic resonance experiment.

Place both glasses on the carpet or rubber base about 1cm - 2cm apart to prevent the vibration travelling through the substrate.

Cut the straw, so it's few centimetres longer than the diameter of the glass, and place it on top of second glass. Make sure that the straw and rim of the glass are dry, otherwise the straw could stick to the glass and not move.

Now wet your finger and start generating sound using first glass and, if you set up everything correctly, the straw on second glass should move.

Here is another video I made a while ago with a ping pong ball on a string:

This experiment is quite interesting in my opinion. When I do the demonstration for the students, I always have third glass with frequency slightly different to the main two, to show them that it won't work if the generated frequency isn't exactly the same as the natural frequency of the system (glass in this case).

In addition as you know by now, when you make the sound you can see one clear spike (peak frequency) which means that entire glass crystal structure is vibrating with same frequency. This is because you are moving your finger with constant speed generating standing wave.

If you hit the glass gently, with metal spoon for example, it will generate sound too, but it will be different. You will observe not one peak frequency but few! (see second picture above). This is happening, because you hit the glass in one specific place and the wave is propagating through glass, not causing the vibration of entire structure.

In addition you can observe the standing wave which will appear on surface of the water inside the glass. If you look closely, you will also notice that the wave maximum is following your finger.

Lastly, if you pour more water, about 80% of the glass volume, and manage to create sound (which is much more difficult) you should be able to see your regular peak frequency, but also second harmonic frequency and maybe third, if you lucky. The 2nd and 3rd harmonic frequency should be and integer multiple of the fundamental frequency.

Happy experimenting :)