Singing Rods

This demonstration will show how a simple metal rod can be made to vibrate or sing with the aid of some wax paper.

Do it yourself or watch the video (QuickTime, 720 KB)

How does it work?

A simple metal rod can be made to sing using the same principle that causes violin strings to vibrate.

It is important to hold the rod exactly in the center so that your grip does not damp out any sound waves in the rod.

Like a violin bow needs to be coated with rosin, or wax, the metal rod also needs a wax coating.

Now when you rub your fingers along the rod, like a violin bow rubs against the strings, your fingers will stick and slip causing vibrations in the rod. The more you rub the rod, the more energy you add and the louder the singing. The loudness or energy of the sound corresponds to the amplitude of the sound wave.

Explore Further

Frequency: How do you think the pitch will change if the rod is shorter?
Find out if you are right by watching the video of a shorter rod! (QuickTime, 735 KB)

The shorter rod is exactly half the length of the longer rod. Use the graphs below to calculate their frequencies. Remember frequency = waves/second.

Long Rod	Short Rod
Click on the graphs for a larger view in a new window. These graphs were produced using a Vernier LabPro with the microphone probe.

Notice that the label on the y axis is "Sound Presure." Sound is a pressure wave that moves through air, metals, you and anything else with mass.

We can see the relative frequencies better by superimposing one wave over the other. In the graph below, the black wave is from the short rod and the red wave is from the long rod.

What is the relationship between rod length and frequency? Is it direct or inverse?
If you were to use a rod twice as long as the long rod, what do you think the frequency of its singing would be?

Amplitude: Which of the sound waves above has the most energy? Remember that the energy of a wave corresponds to its amplitude.

Wavelength: The length of the long rod is 1.83 meters and the short rod is 0.914 meters. Below is a diagram of the wave (green) as it vibrates through the rod (black). What are the wavelengths of the waves that travel through each rod?

Hint: Here are some pictures of waves that might help you out.

One full wave

Two waves

 

Speed of Sound: Using this and the frequency of the waves you can calculate the speed of sound in aluminum! Remember speed of a wave = frequency x wavelength.

Do It Yourself!

Materials

• Metal Rod - solid or hollow, 3 to 6 feet long, aluminum is good because it is light but other metals will work. You can find these at most hardware stores.
• Wax Paper

Make it Sing!

• Tear off a piece of wax paper and rub both sides of the paper along half the rod. You need to use both sides because it is difficult to tell which side the wax is on.
• Carefully balance the rod in the center by resting it on your index fingers and slowly moving them together. The point where they meet is the center of the rod.
• Hold the rod at its center with one hand.
• With your other hand use your thumb and index finger to firmly rub the side of the rod you rubbed the wax paper on. As your fingers stick and slip along the rod you will hear a ringing sound. Each time you rub the rod the singing should get louder.

It may take some practice to get the pressure between your fingers and rod right. If you are still having trouble trying using the wax paper to add more wax.

For more singing rod ideas and video visit http://www.physics.rutgers.edu/~croft/srod.htm