If you blow across the top of a soda bottle, you produce a sound. The more soda you drink, the deeper the pitch of the sound. This is because the resonant frequency of the bottle increases as the height of the air above the liquid increases. This project does the same thing, except on a much bigger scale. You use longer tubes that produce deeper sounds. You may just want to give the person who is responsible for the building you are in (such as the building principal) advance warning that the sound they will be hearing is not an earthquake, not a water buffalo in labor, and not the propulsion system for a space alien spacecraft.
What You Need
- large Bunsen burner
- cylindrical tubes roughly 0.8–2 meters in length—good candidates for this are cardboard tubes used for carpet rolls or hollow metal sections of old driveway basketball backboard supports
- fire extinguisher and/or bucket of water (you should not need this, but just in case)
- Place the Bunsen burner on the floor.
- Route a hose long enough not to get tangled and connect it to a natural gas outlet (which, at this point, is not turned on).
- Position a cylindrical tube on the floor, so it can easily be placed over the burner.
- Light the burner and adjust it, so the flow is maximum, with the greatest flame.
- Make sure nothing flammable is near the burner, including possible objects on the ceiling and loose papers that might inadvertently be drawn into the flame by convections.
- Lift the cylinder with both hands and position it above the burner. Hold it a few inches above the top of the burner, but not so low that it constricts the air going into the burner. See Figure 70-1.
- Be somewhat careful to avoid charring the edge of the tube. Do not hold the edge of the tube directly over the flame. If held correctly, the tube will not burn. Also exercise similar care to avoid excessively heating a metal tube, which could possibly result in a burning hazard.
- It may take as much as a minute or so until a sufficient stream of exhaust from the Bunsen burner flows through the tube to produce an acoustic resonance.
- Measure (or, if you prefer, observe) the pitch (or frequency) of the sound produced using the pitch gauge or oscilloscope. You can also compare the pitch with a known frequency, such as produced by hitting a tuning fork or sounding a musical instrument.
- Remove the cylinder and don't forget to turn off the burner when you are finished.
With a sufficient flow of heated air, the tube should resonate. The taller tubes produce a much lower pitch than the shorter tubes. The diameter of the tube does not affect the pitch, but it may affect the volume by impacting how much air can flow.
For a given length, the frequency of the tube is given by:
The shorter columns produce tones consistent with the sounds of common concert instruments. The longer columns produce very deep, resonant pitches.
Why It Works
The flowing air, just like a wind instrument, creates a resonant standing wave in the tube. The longer the tube, the longer the wavelength, but the shorter the frequency.
The frequency for an open tube is given by f = v/2L. For a given tube length, this frequency is twice as high as it would be for a closed tube. Refining this to take into account the slight offset of the node from the ends of the tube, the equation is:
where d is the diameter of the tube (in m), L is the length of the tube (in m), and v is the speed of sound (in m/s).
Other Things to Try
If you have more than one tube whose diameters are similar, you can nest several tubes together, one inside the other, as shown in Figure 70-2. This can let you continuously vary the pitch of the tube, like a trombone. You may want to practice before auditioning for the spring musical.
The resonant frequency in an open pipe is lower for a greater length.