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

Singing Rod & Ringing of the (Spoon) Bells

[Shopping List: singing rods; rosin powder; large metal spoons; string]

Singing Rod

1. Rub a small amount of rosin on your thumb and one or two fingers of one hand.
2. Hold the long aluminum rod between the thumb and first finger of the opposite hand (not too tightly), and quickly and firmly rub or strike from the center towards one end with your rosin coated fingers. If you do it right, you should hear a sharp piercing ringing tone. It may take quite a bit of practice to do this the first time, and even more to really get the ear-splitting shriek that is possible if you do it just right. If you look closely you might even see the rod vibrating.
3. Notice that the rod continues to vibrate and make loud noise for a long time after you stop stroking it.
4. Notice also that as you turn the rod while it's singing, the sound waves produced are very highly directional.

Ringing of the (Spoon) Bells

1. Take a piece of string or yarn at least 4 or 5 feet long, and feed it through the hole in the handle of a large metal spoon. Holding one end of the yarn in each hand, wrap the end several times around the first finger of each hand.
2. Dangle the spoon, centered on the yarn, and gently place your wrapped fingers in your ears.
3. While continuing to dangle the spoon, swing it so that it strikes the edge of the table and begins to vibrate.
4. You should hear very loud, deep-pitched ringing in your ears, not at all what you expect the spoon to sound like.
5. As the spoon continues to ring, remove your fingers from your ears. The spoon should sound very different now.

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 (as with the dangling spoons) or simply pushing them. When you put rosin on your fingers it makes them stick to the aluminum rod a little better, helping you push it. When you rub the aluminum rod your fingers 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 rod 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 rod. If you change the effective length of the aluminum rod that is vibrating, such as by holding it in a different position, you will change the resonant frequencies.

The same thing happens with the spoons, but in this case the vibrating resonating spoon will also vibrate the yarn attached, which in turn vibrates your finger and as you press it into your ears, the bones in your skull as well. Thus the sound waves you hear are not traveling through the air, but directly through a spoon-string-bone path to your eardrums, and the frequencies you gear are very different than those of the sound waves from the spoon which travel through air. When you remove your fingers from your ears, you not only break the direct solid material sound path, but now you can hear the sound waves in the air more easily, so it sounds quite different.