Here's a classic science demonstration that is sure to grab your students' attention... and the people down the hallway... and every dog in the neighborhood! When you run your fingers lightly along this 60-cm-long metal rod, an ear-piercing sound is generated. You control how loud the sound gets, from a whisper to an auditorium-filling shriek. Absolutely incredible! This is a great experiment to demonstrate the difference between longitudinal and transverse waves. Each singing rod comes with instructions and a bag of crushed rosin to lightly coat your fingertips. Set of three consists of rods of lengths 60, 80, and 100 cm.
Download the pdf of this lesson!
Download the pdf of this lesson!
When a long metal rod is held in the center with one hand and stroked with the other, a high-pitched sound is produced.
- Crushed Rosin
- Singing Rod, 24-inch (61 cm) aluminum rod: SNG-100
Singing Rod – Three Tone Set - 24, 30, 36-inch (61, 76, 91 cm): SNG-300
- Firmly hold the center of the aluminum rod horizontally using the thumb and forefinger of one hand.
- Pinch and release a small amount of crushed rosin with the thumb and forefinger of the other hand.
- Gently stroke the aluminum rod from the center to the end of the rod using your rosin coated thumb and forefinger. Repeat and slightly increase the pressure of your stroking hand until you hear a high-pitched tone. Too little pressure will not set up vibrations in the rod; too much pressure will dampen the sound. It takes practice!
- Firmly hold the aluminum rod vertically at a point that is ¼ the distance from the upper end.
- Repeat Steps 2 and 3 in Procedure A until you hear a different pitched tone.
Every material has a set of natural vibrations. When you hold the aluminum rod in the center and
stroke it with rosin coated fingers, your fingers slip and stick as they slide along the rod. This causes the rod to start moving with one of its natural frequencies of vibration – a half-wave tone. As you continue to stroke the rod, the vibrations increase and the loudness increases. The node is a place on the object that is not moving. An anti-node is a place on the object with maximum vibration. Touching a node will not dampen the sound; touching an anti-node will.
You can calculate the frequency of the note you are hearing by dividing the velocity of sound in the
rod (ca. 5100 m/s for aluminum) by the wavelength which is twice the length of the rod (remember:
a rod held in the center vibrates at a half-wave frequency). For the 24-inch aluminum rod, the
frequency is about 4200 cycles per sec or Hz.
Frequency = velocity / wavelength = (5100 m/s) / (2 x .61 m) = 4200 Hz
Holding the rod at a point from the end that is a quarter of its length, will produce a higher pitched
sound, a full-wave tone, about 8400 Hz.
While holding the rod at one node, touch the other node and notice that the sound is not dampened.
Things to Try:
- Clamp the singing rod to a table at one of the nodes, allowing the remainder of the rod free
movement. Stroke the rod and notice the difference.
- Hold the rod vertically and drop it a few cm onto a hard surface. Then, while holding the rod
in the center, hit the side near the end with a hammer. Compare the sounds. Which sound is
the same as stroking the rod?
- Try holding the rod at different places while stroking.
- When the rod is 'singing', where is the sound the loudest? At the ends or the side?
- Balance the rod on pencils, placed at the 1/4 and the 3/4 points of the rod. While using fingers
to apply pressure on the rod at the points above the pencils, ask someone to gently hit the end
of the rod with a hard object such as a small hammer. How does the sound change when you
move the position of the pencils?
National Science Education Standards
- K-4 - Employ simple equipment and tools to gather data and extend the senses (Standard A. 1.3).
- K-4 - Sound is produced by vibrating objects. The pitch of the sound can be varied by changing the rate of vibration. (Standard B. 2.4).
- 5-8 - Energy is a property of many substances and is associated with sound (Standard B. 3.1).
- 9-12 - Waves, including sound, have energy and can transfer energy when they interact with matter