The Science House

Hoot Tubes

This demonstration will show how hot air rising through an open tube (stove pipe) vibrates to create a sound.

Do it yourself or watch the video (QuickTime, 525 KB)

How does it work?

There are two ways to make your tube hoot. In the video we use a Meeker burner. This is different from a Bunsen burner in that it burns hotter and has a metal grid over the flame that causes the flame and heat from the burner to vibrate as the propane burns. For a better idea, watch a short video of the burner (QuickTime, 155 KB).

The hot, vibrating air rises through tube setting up a convection current. A convection current is simply the movement of air caused by different densities. Hot air is less dense than cold air and will move upward while the cool air sinks.

When the vibrating air moves through the tube, it begins to resonate or vibrate at the natural frequency of the tube. Since our tube is open on both ends, the wave that resonates will have a wavelength 1/2 the length of the tube.

If we double the length of the tube, what happens to the resonant wavelength? How does this affect the pitch we hear? Find out by watching this video of a longer hoot tube (QuickTime, 415 KB).

Pipe organs, foghorns, the recorder and other musical instruments use this same concept to produce sounds, only the air is pushed through the pipe by other means than a hot burner.

If you have a bunsen burner, you can replicate this effect with a twist by placing a metal (copper works well) inside the tube about 6 inches from the end. You will not hear a hooting until after the metal mesh heats up and you remove the tube from the burner. As the mesh cools, the hot air will rise and cause the tube to hoot.

Do It Yourself!

Materials

4 inch diameter stove pipe, 3 or more feet long, available at most hardware stores
propane gas source (we use cannisters for camp stoves) with tube and connector
hot pad, gloves or other hand protection
Bunsen burner or Meeker burner
copper mesh (for Bunsen burner only)

Make the tube hoot!

SAFETY WARNING: This demonstration involves open flame. Be sure to have an adult present when performing this demonstration. Wear gloves or some type of hand protection as the metal tube gets very hot.

There are two ways to make yout tube hoot, depending on what type of burner you have access to.

If you have a Meeker burner connect the burner to a small propane tank, open the valve slightly and light the burner. You want to be sure to have a nice blue flame. Place the tube down over the burner so that the top of the burner is inside the tube. In a couple of seconds you should hear a hooting sound.

If you have a Bunsen burner, you will need to insert some copper mesh about 6 inches into the tube so that it covers the opening. Connect the burner to a small propane tank, open the valve slightly and light the burner. You want to be sure to have a nice blue flame. Place the tube down over the burner so that the top of the burner is inside the tube and the flame is heating the copper mesh. Keep the tube over the flame until the mesh glows red. You will probably be able to see it reflected on your burner. Remove the tube from the burner. In a couple of seconds you should hear a hooting sound. Turn the tube horizontally and the hooting will stop. Right the tube and it will hoot again until the mesh cools.

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	Hoot Tubes This demonstration will show how hot air rising through an open tube (stove pipe) vibrates to create a sound. Do it yourself or watch the video (QuickTime, 525 KB) How does it work? There are two ways to make your tube hoot. In the video we use a Meeker burner. This is different from a Bunsen burner in that it burns hotter and has a metal grid over the flame that causes the flame and heat from the burner to vibrate as the propane burns. For a better idea, watch a short video of the burner (QuickTime, 155 KB). The hot, vibrating air rises through tube setting up a convection current. A convection current is simply the movement of air caused by different densities. Hot air is less dense than cold air and will move upward while the cool air sinks. When the vibrating air moves through the tube, it begins to resonate or vibrate at the natural frequency of the tube. Since our tube is open on both ends, the wave that resonates will have a wavelength 1/2 the length of the tube. If we double the length of the tube, what happens to the resonant wavelength? How does this affect the pitch we hear? Find out by watching this video of a longer hoot tube (QuickTime, 415 KB). Pipe organs, foghorns, the recorder and other musical instruments use this same concept to produce sounds, only the air is pushed through the pipe by other means than a hot burner. If you have a bunsen burner, you can replicate this effect with a twist by placing a metal (copper works well) inside the tube about 6 inches from the end. You will not hear a hooting until after the metal mesh heats up and you remove the tube from the burner. As the mesh cools, the hot air will rise and cause the tube to hoot. Do It Yourself! Materials 4 inch diameter stove pipe, 3 or more feet long, available at most hardware stores propane gas source (we use cannisters for camp stoves) with tube and connector hot pad, gloves or other hand protection Bunsen burner or Meeker burner copper mesh (for Bunsen burner only) Make the tube hoot! SAFETY WARNING: This demonstration involves open flame. Be sure to have an adult present when performing this demonstration. Wear gloves or some type of hand protection as the metal tube gets very hot. There are two ways to make yout tube hoot, depending on what type of burner you have access to. If you have a Meeker burner connect the burner to a small propane tank, open the valve slightly and light the burner. You want to be sure to have a nice blue flame. Place the tube down over the burner so that the top of the burner is inside the tube. In a couple of seconds you should hear a hooting sound. If you have a Bunsen burner, you will need to insert some copper mesh about 6 inches into the tube so that it covers the opening. Connect the burner to a small propane tank, open the valve slightly and light the burner. You want to be sure to have a nice blue flame. Place the tube down over the burner so that the top of the burner is inside the tube and the flame is heating the copper mesh. Keep the tube over the flame until the mesh glows red. You will probably be able to see it reflected on your burner. Remove the tube from the burner. In a couple of seconds you should hear a hooting sound. Turn the tube horizontally and the hooting will stop. Right the tube and it will hoot again until the mesh cools. Return to the top

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