Hands-on Science Carnival 2010 Activity Stations: Sound & Waves

Pop Bottle Organ & Sound Tubes

[Shopping List: glass soda, wine or beer bottles, preferably several that are identical and several others with a variety of sizes and shapes; plastic sound tubes; bucket of water; pitcher for pouring; towels]

Pop Bottle Organ

1. Fill several identical pop bottles with different levels of water and arrange them in order.
2. Lightly rest your lower lip on the top of a bottle and gently blow across the top to produce a sound. Try another bottle. You should find that the pitch or frequency of the tone produced increases with the level of the water (i.e. just the opposite of what you observed with the wine glasses).
3. Try to play a song.
4. Try other bottles of different sizes and shapes. How does the size and shape affect the tone produced?

Sound Tubes

1. Hold one end (either works) of the corrugated plastic sound tubes and swing the other end in a large circle, starting slowly. You should begin to hear a tone.
2. Swing the tube a little faster, and the frequency of the tone should increase (higher pitch). Swing faster still for higher frequency tones. If you're lucky you should be able to produce up to five different frequencies as you change the speed of rotation.

What's Happening: When you blow across the top of a soda or wine bottle, you are creating turbulence (pressure differences) at the top that causes the air inside the bottle to resonate (in much the same way that the stick-slip friction caused the rod to resonate), but in this case it is not the bottle itself which resonates. The natural resonant frequencies for these sound waves depend on the dimensions inside the bottle, and by raising the water level you make the length of the cavity inside shorter and thus the resonant frequency higher (because its wavelength is smaller). A very large bottle (when empty) thus has a very low frequency or pitch (large wavelength), but if it is filled nearly to the top with water can still produce a pitch as high as a much smaller bottle.

Swinging the sound tubes also causes the air inside to resonate due to pressure differences at the two ends. Imagine that the tube was filled with marbles. As you swing it in a circle what would happen to the marbles inside, remembering that the outside end is open? Of course the marbles would shoot out of the end since there is no (centripetal) force to keep them turning in a circle. The air molecules inside the tube act the same way, except that as the ones inside are flung out of the tube, more air molecules rush in through the end you are holding in your hand, which is also open. We say that the air flow inside creates a low pressure region at the end your holding, which sucks more air in to keeps the flow inside going. Once inside, the corrugated ribs of the tube create a turbulent airflow and vibrations, and just as in the previous examples, certain natural or resonant vibrations will be more stable than others, and grow louder. The particular resonant frequency which you hear best will depend on the speed of the airflow, which in turn depends on the speed at which you rotate the tube. The lowest frequency you can make is normally called the fundamental frequency (or sometimes the first harmonic), with the higher frequencies called harmonics. For these tubes however, you just can't spin them slowly enough to produce a sufficiently loud fundamental tone, so the lowest pitch you will hear is actually the second harmonic, and the higher pitches are the 3rd, 4th, 5th and 6th harmonics.

Variations: Try rubbing the top of a glass soda bottle with your wet finger to make sounds. You will find that while you can make sound, it's not very loud or long-lasting. This is because while you are vibrating the bottle, you are not making it resonate. If you have different sizes and shapes of wine glasses, compare the resonant frequencies of each. If you blow very hard across the top of a bottle you may be able to make a higher frequency tone by exciting the second harmonic (with some bottles you might even get the third harmonic also). To demonstrate the low pressure suction power of the sound tubes, tear up some small pieces of tissue paper and place them on a table. While holding one end of a sound tube firmly an inch or two above the tissue paper, rotate the other end in a circular motion as before. With some practice, you should be able to vacuum up the paper bits and shoot them out the opposite end.