Soda Bottle Symphony

Developer:

Charles Scaife
Department of Chemistry
Union College
Schenectady, NY 12308

Grade Levels:

K through 12

Discipline:

Physical Science/Physics

Goals:

  1. Partially fill soda bottles with different heights of water.
  2. Test the sounds produced when the bottles are tapped with a spoon.
  3. Test the sounds produced when air is blown over the open mouth of the bottles.
  4. Recognize that sounds are produced by vibrations and that the vibrating media must be different in these two cases.
  5. Adjust water levels in the bottles so that a tune can be played by each of the methods.

Background:

Plucking a ruler (1) or a meter stick is a simple means of producing sound and showing that some medium must vibrate to make the sound. This present experiment is a bit more challenging and can be used at a variety of levels. Several authors describe one or both of the methods used to produce sound in this very popular experiment (2, 3, 4, 5, 6, 7).

Percussion instruments which musicians strike to make musical sounds are made from wood, metal, or animal hides. Examples include wooden blocks, bells, and drums. These instruments produce a sound when the wood, metal, or animal hides are made to vibrate. Instruments which musicians blow into to make musical sounds are made from wooden or metal pipes. The pipes produce a sound when the air inside them is made to vibrate. If the instrument is made from one very long piece of pipe, the pipe may be curled around in a circle or loop. Instruments such as clarinets or oboes, in which the pipes are made from wood, are called woodwind instruments. Instruments such as trumpets or trombones, in which the pipes are made from metal, are called brass instruments.

Teacher's Notes:

The wind instrument part of this experiment is an excellent example of one for which the teacher must creatively adapt the procedures so that they can be used with more than one student. Blowing over the mouth of a bottle creates a sanitation problem that cannot be tolerated today. Authors do not address this issue. Lips are bound to come in contact with the bottle, and then the bottle must somehow be cleaned thoroughly before the next student uses the same bottle. One person has suggested that bottles be sterilized between students, but most of us do not have a sterilizer in the corner of our classroom and wouldn't have time to use it if we did. Another person has suggested that the bottle be cleaned between students with an alcohol swab. This suggestion is more workable in the classroom, but is still not easy to implement. We have found that having each student blow through his or her own half-straw (even less expensive than a whole straw!) solves the problem. The straw is held at a slight angle so that the student is blowing slightly downward and partially at and mostly over the lip at the mouth of the bottle. It takes several practice attempts and then occurs relatively easily after that.

Materials:

FOR EACH GROUP OF FOUR STUDENTS

4 Identical glass soda bottles

4 Metal spoons (wooden rulers also work but give a less mellow tone)

1 Centimeter ruler

6 Half straws

1 Water container (a 2-L plastic soda bottle works fine)

1 Plastic tennis ball container marked USED STRAWS

Explanation:

The surprise or discrepant event in this experiment occurs when students find that the pitch of sound from a bottle lowers when more water is added to the bottle played as a percussion instrument (tapped with a spoon) whereas the pitch rises when more water is added to the bottle played as a wind instrument (blowing air over the mouth). Obviously, different media must be vibrating to produce the sounds in order for this result to be exhibited. This evidence elicits many ideas and considerable discussion among students. When tapping the bottles, both the bottle and contents are vibrating, and the larger the mass of bottle and contents, the lower the rate of vibration, and the lower the pitch. When blowing over the mouth of the bottle, only the air column within the bottle is vibrating, and the longer the air column, the lower the rate of vibration, and the lower the pitch.

Questions:

1. What happens to the pitch when water is added to a bottle tapped with a spoon and played as a percussion instrument?

The pitch lowers as more water is added.

2. What happens to the pitch when water is added to a bottle as you blow over the mouth and play as a wind instrument?

The pitch rises as more water is added.

3. Why are the differences in Questions 1 and 2 observed?

Different media must be vibrating in the two cases. When tapping the bottles with a spoon, both the bottle and contents are vibrating. As more water is added to the bottle, the mass of bottle contents increases. A larger mass vibrates at a lower rate of vibration, and the pitch is lower. When blowing over the mouth of the bottle, only the air column within the bottle is vibrating. As more water is added to the bottle, the air column becomes shorter, there is less mass of air vibrating, at a higher rate of vibration, and the pitch is higher.

References:

  1. Janice Pratt VanCleave, Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound, Wiley, New York, NY, 1991, pp. 222-223; Ray Broekel, Sound Experiments, from A New True Book Series, Childrens Press, Chicago, IL, 1983, pp. 7-12.
  2. Janice Pratt VanCleave, Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound, Wiley, New York, NY, 1991, pp 216-217.
  3. WonderScience: Fun Physical Science Activities for Children and Adults to Do Together, volume 6, number 1, January, 1992, American Chemical Society/American Institute of Physics, Washington, D.C., pages 2-3.
  4. Terry Cash, Steve Parker, and Barbara Taylor, 175 More Science Experiments to Amuse and Amaze Your Friends, Random House, New York, NY, 1990, p. 36.
  5. Ray Broekel, Sound Experiments, from A New True Book Series, Childrens Press, Chicago, IL, 1983, pp. 32-35.
  6. Tik L. Liem, Invitations to Science Inquiry, 2nd ed., Science Inquiry Enterprises, Chino Hills, CA, 1987, (12.2) p. 294.
  7. Alfred E. Friedl, Teaching Science to Children: An Integrated Approach, 2nd ed., McGraw-Hill, New York, NY, 1991, pp. 97-98.

Procedure:

USING THE BOTTLES AS PERCUSSION INSTRUMENTS

Be very careful not to spill water during this experiment. Pour different amounts of water into three bottles so that the heights of water in the bottles are 0 centimeters (cm), and approximately 3 cm, 6 cm, and 9 cm. If you gently tap the side of each bottle about three-quarters of the way up the bottle where the rounded portion begins, do you think the sounds produced will be the same or different? How do you think they might differ? Try it by tapping the bottles gently with a spoon.

Arrange the bottles in a row from the bottle with the lowest pitch on the left to the bottle with the highest pitch on the right. What happens to the water level as you move toward higher pitch? Try to adjust the water levels in the bottles to obtain the pitches necessary to play Mary Had a Little Lamb. Work cooperatively on this effort, and talk about your ideas and make predictions before trying them. Each student should play one bottle. Were you able to do it?

USING THE BOTTLES AS WIND INSTRUMENTS

Using the present amounts of water in the bottles, predict what will happen if, instead of tapping each bottle with a spoon, you blow through a half straw across the top of each bottle. Do you think the sounds produced will be the same and in the same order, or different? How do you think they might differ? Try it by blowing through a half straw tipped slightly downward and held so that some air hits the lip of the bottle but most air passes over the mouth of the bottle. You should be able to produce the sound of a horn or whistle. What do you hear? Are you surprised at the result?

Arrange the bottles in a row from the bottle with the lowest pitch on the left to the bottle with the highest pitch on the right. What happens to the water level as you move toward higher pitch? Try to adjust the water levels in the bottles to obtain the pitches necessary to play Mary Had a Little Lamb. Work cooperatively on this effort, and talk about your ideas and make predictions before trying them. Each student should play one bottle. Were you able to do it? Discard your half straw in the waste container when you are finished.

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