Whirly – The Twirling Sound Hose
Whirling an ordinary, corrugated, plastic tube over your head becomes a musical experience.
A Sound Hose is a corrugated pipe (with ridges in it) that causes molecular gymnastics to occur when air molecules are moving through it. One air molecule causes its neighbors to vibrate at the same time it does and it becomes a musical party. Of course, there are only a few tones to hear at the party and you really have to work to hit the high ones… but, who cares? It’s all in the name of science!
- Sound Hose (No official Sound Hose? Use a length of corrugated tubing from a hardware store.)
- Lightweight plastic bag (larger garbage bag or shopping bag)
- Rubber band
- Adult supervision
A Sound Hose is a simple device. It’s a piece of flexible, corrugated (with ridges in it) plastic pipe that’s larger at one end but open on both ends. The toy is about 36″ (91 cm) long and 2″ (5 cm) in diameter. There are a couple of ways to make tones with it. Blow some air through it from the larger end and see if you can make a tone. It takes of lot of air! Maybe there’s an easier way to get the air moving through it.
Grab the larger end of the sound hose and – only if it’s safe to do so – quickly whirl the hose in circles over your head to hear some of the tones it produces. Go faster and then slower to change the pitch of the tones.
Notice that fast whirling creates higher pitches and slow whirling creates lower pitches. Change your whirling speed and calculate how many tones you can make in all. The higher tones are a lot of work!
Securely attach the plastic bag to the larger end of the Sound Hose with tape or a rubber band. Keep your mouth a few inches (cm) from the small end and blow into the Sound Hose to inflate the bag. If you keep the space between your mouth and the Sound Hose, the bag will inflate with just a few breaths. (Check out how a Windbag works.)
Once the bag is inflated, whirl your Sound Hose over your head as before. As the “music” plays from the hose, watch the bag deflate. Keep whirling the Sound Hose until the tones stop completely. Keep whirling it a few times to prove to yourself that if air can’t move through the Sound Hose, there will be no tones produced.
How Does It Work
As you twirl the tube, air molecules are pulled from the standing end (by your hand) up and out of the moving end. The difference in speed between the moving end of the tube and the stationary, hand-held end creates a difference in air pressure. A higher pressure is at the fixed end and a lower pressure is at the moving end. This difference pulls air through the tube and the air’s speed changes (making the changes in the tones) with the speed of the spin. The pitch, loudness, and tone of the sound come from the tube’s length and diameter, the distance between each ridge, and the speed the tube spins around, which moves the air faster or slower through the tube changing the tone in steps.
The bag creates a stunning effect. It allows you to see the movement of air through the Sound Hose as you watch the bag deflate when the Sound Hose is “played.” It’s proof positive that air has to move through the Sound Hose for tones to be produced.
Not all plastic tubes sing. The tube must be corrugated on the inside. Why? Aerodynamics researchers in Japan put a whirly in a wind tunnel and used very tiny hot wire anemometers to measure the airflow near the corrugations. As the air flows first over one ridge then over a second it tumbles into a vortex. The faster the air flows through the tube, the higher the frequency of the sound produced by the vortex. When the frequency of the vortex matches one of the natural resonant frequencies of the tube, it is amplified.
Most vacuum cleaner hoses are not corrugated. If they were, your vacuum cleaner would make tones whenever you vacuumed. That could be one way to get out of doing your chores but don’t bet on it.
Take It Further
If you’re limited on safe whirling space, try this technique using the bag. Make sure the bag is securely in place and fully inflated (a larger, stronger bag may be better for this). Wrap your arms around the bag and squeeze it like a player of bagpipes. A soft squeeze on the bag produces lower tones and a vigorous squeeze produces higher tones. It’s a bit difficult but you might get several tones from your Sound Hose before the bag is empty.
Taking this idea even further is something altogether unique. Snag a piece of a larger diameter corrugated tubing from the hardware store. It’ll need to be longer than your Sound Hose, too. On the way home (while someone else is driving) put one end of the tubing out the window and see what tones you make as the speed of the car changes. As other drivers throw you nasty looks, just scream out the window, “It’s all science, people!” Do it on your next road trip.
Information for this activity was gathered from a great website presented by Paul Doherty from the Exploratorium, San Francisco, CA, USA. For more details on the Singing Tube (or Sound Hose), visit Paul’s website.