Singing Rod A classic science demo that is sure to wake you up... email to friend
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Here’s a classic science demonstration that is sure to wake you up... and the people down the street... and every dog in the neighborhood! With a little practice and some science know-how, you'll turn an ordinary piece of aluminum rod into a singing virtuoso.

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Materials

You'll need a solid aluminum rod - approximately 5/8" in diameter and 24" long. You can experiment with different lengths of rod to produce different sounds. You'll also need some violin rosin... and some patience.

Experiment

Hold the rod in the middle with your first and second fingers on top and the thumb supporting the rod from below. The key is to make as little contact with the rod as possible so as not to dampen the vibrations.

Apply some rosin to the thumb and fingers of your other hand and lightly coat the aluminum rod with rosin.

Use your rosin coated fingers to pinch the metal while sliding your fingers from the middle to the end of the rod. Don’t stop. As soon as your fingers reach the end, repeat this pinch-n-sliding process...and again...and again...maybe upwards of 20 times before the metal bar begins to resonate. Don’t give up.

Because of the sticky nature of rosin, your fingers should stick and slide across the rod causing it to resonate.

The sound of the vibrations will be soft at first but will strengthen with each successive stroke. The resulting high pitch sound will be ear-piercing! Each stroke reinforces the vibrations of the last. Pinch and slide, pinch and slide, pinch and slide...don’t give up!

**Try holding the bar in the middle and tapping on the sides. Compare this sound to the sound produced by hitting the bar directly on the end. How does this sound compare to the sound made by stroking the bar?

How does it work?

“Vibrations...the reason the bar makes the sound is because of vibrations.” Your students might be inclined to offer this simple explanation before you increase their level of understanding by asking these questions:

Why did the pitch of the bar sound different when you tapped the bar instead of stroking the bar?
Why does the metal bar vibrate when you rub it with your fingers? Why is the rosin necessary?
Where is the high pitch sound coming from (middle, sides, or ends of the bar) and why?

In terms of making the bar vibrate, the rosin is responsible for making your fingers stick and slide as they move across the bar. In turn, this repeated stick and slide action sets up vibrations in the bar.

You probably noticed that holding the bar in the middle and tapping slide produced a lower pitch sound and striking the bar on the very end created a higher pitch sound. The same high pitch sound is also made by stroking the bar with your fingers. In either case, the high pitch sound resulted from the formation of compression waves or longitudinal waves throughout the bar. Each successive stoke of the bar reinforces the strength of the previously established longitudinal wave, resulting in a louder sound.

Here’s a way to illustrate a longitudinal wave using a Slinky toy. Picture a Slinky stretched out on the floor with another person holding the Slinky at the other end. Compress a section of the spring and let go. Notice how the energy of the released coil moves up and down the length of the spring. See the illustration below. This is an example of a compression or longitudinal wave. The high pitch sound of the metal rod is the result of a longitudinal wave which travels throughout the entire length of the bar.

It the rod is held in the middle and tapped on the slide with a solid object, a transverse wave is created. These waves have much longer wavelengths and, as a result have a much lower tone or pitch compared to the higher pitched longitudinal wave. A transverse wave is made by moving the Slinky in an up and down motion, creating nodes and antinodes. Notice the formation of standing waves.

Practice hitting the bar with a solid object close to the end with a slight diagonal stroke. By doing so, you can actually create both longitudinal and transverse waves at the same time, and you’ll be able to hear both sounds at once.

Additional Info

Over the years, this activity has been published in a number of demonstration books, and my initial efforts to perform the Singing Rod always resulted in tired fingers and no sound. Success came after seeing Dr. Albert Baez present the demonstration in San Antonio in 1995. The motion of repeatedly sliding your hand down the bar is smooth and graceful. Dr. Baez effortlessly caused the metal bar to resonate a high pitched sound that filled the auditorium to everyone’s amazement. So can you!