Hands-on Science Carnival 2010 Activity Stations: Sound & Waves

Wine Glass Resonance & Spouting Bowl

[Shopping List: several wine glasses; Chinese Spouting Bowl; non-stick mat or bubble wrap sheet; bucket of water with pouring pitcher; towels]

Wine Glass Resonance

1. Dip your finger in water and gently but firmly rub it across the top of a wine glass at a fairly slow and constant rate to produce a loud continuous tone. It might take a little practice.
2. Partially fill the glass with water and repeat. The tone produced should be lower in pitch.
3. Try different amounts of water; the pitch should decrease as the water level increases.
4. With the glass nearly full of water, observe the water surface as you rub the rim of the glass. You should see tiny waves, and perhaps even enough splashing for your face to get wet.

Chinese Spouting Bowl

1. Place the large spouting bowl on a non-slip rubber mat or bubble-wrap sheet and fill about half full with water. Be sure the handles are clean and free of oil or grease.
2. Wash your hands with soap and rinse them completely with clean water.
3. Dip your hands into the water in the bowl to get them completely wet.
4. Gently rub your palms on the top of each handle in a slow and deliberate motion, pressing down on the handles. You can move both hands in the same direction, or in opposite directions.
5. Watch the water surface in the bowl. You should begin to see small wave patterns develop. Notice that the waves are always largest at specific locations along the edge of the bowl, while at other places there are never any waves.
6. When you can see waves on the surface, gradually increase the force with which you are rubbing the handles, but not the speed. It's better if you go slowly rather than fast. The waves should increase in amplitude, eventually splashing water several inches into the air while the vibrations also make a loud noise.

What's Happening: Sound waves are produced when air molecules vibrate, and air molecules can be made to vibrate when various objects vibrate (and conversely sound waves in the air, if they have enough energy, can make other objects vibrate as well). Objects can be made to vibrate by forcing them to move back and forth quickly, which can be done by hitting them or simply pushing them. Dry fingers will stick to a wine glass pretty well, but when you wet them a little, they will slide much easier. Either way, when you rub the rim of the wine glass, or the handles of the spouting bowl your fingers (or hands) will stick for a short time, then slide a little, then stick again, then slide again, etc. This is called "stick-slip" friction, and in a sense is like banging on the surface of the object very quickly and forcing it to vibrate at various frequencies as you force it to move at different speeds. These vibrations will make sound waves, but in most cases not particularly loud ones.

Any solid object, however, has a set of special frequencies at which it naturally prefers to vibrate, called its natural or resonant frequencies. At these resonant frequencies it takes only a very small amount of input motion or energy to produce very large vibrations and thus output energies (and thus loud sound). As you start the wine glass or spouting bowl vibrating with your stick-slip motion, these resonant frequencies are also excited, but since they require only a small input energy to produce large output vibrations, they quickly dominate the motion and last much longer, thus the loud, continuous pure (not strictly true) tones you hear. This is resonance, and we say the object is resonating. The sound you hear is produced by the resonant vibration of the glass or bowl. If you change the level of water in the glass, you change the rate at which it resonates (essentially by changing its mass), and thus the pitch of the tone you hear. Finally, since the wine glass or spouting bowl is actually moving in order to push the air and produce sound waves, it should be no surprise that water will also move and produce splashing water waves.

Variations: Vary the depth of water in the wine glasses and notice how it changes the frequency or pitch of the sound. The easiest way to explain this is to think about running in water versus running in air. Obviously you can run much easier- and faster- in air than water with the same effort. The vibrating glass is similar. It can vibrate much easier in air, but the vibrations slow down when there’s water in the glass, and the more water there is, the more they slow down. Faster vibrations mean higher frequencies, slower vibrations mean lower frequencies. The vibration amplitude is also smaller with water in the glass, which is why the sound is not as loud.