MnSTEP Teaching Activity Collection > MnSTEP Activities > Investigating Sound: Using Open Tubes To Demonstrate Beat Notes

Investigating Sound: Using Open Tubes To Demonstrate Beat Notes

Gary Loontjer, Mayer Lutheran High School, Mayer, MN, using tubes similar to those distributed by Flinn Scientific under the name, Singing Tubes
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In this interactive demonstration, students will observe while the teacher creates two sound frequencies using open tube resonators of nearly identical length. The nearness in length produces two slightly different frequencies which interfere with each other to produce a beat note
Beat notes are used by musicians to tune instruments. Frequencies that differ by less than 7 Hz can be detected by the human ear. The frequency of the beat note is equal to the difference between the frequencies of the two sound waves.

Learning Goals

1. This activity is designed for students to experience beat notes.
2. The students will observe the generation of sound using an open tube resonator.
3. Upon determining the frequency of the sounds produced, the students will calculate the beat frequency.
Key Concepts
1. Production of sound in open-tube resonators.
2. Production of beat notes by instruments slightly "out of tune".
Vocabulary Words
Open-tube resonator
Beat note

Context for Use

This interactive demonstration could be used in grades 6-12. The demonstration and discussion could require an entire 45 minute class period. A set of steel tubes "Singing Tubes" will be needed as well as two Bunsen burners. The students should already have knowledge of sound, waves, and interference prior to this demonstration. Adaptations of this demonstration could be made to include larger tubes, electronic equipment to measure frequencies, and student participation is always a possibility with proper safety precautions.

Subject: Physics:Oscillations & Waves:Acoustics
Resource Type: Activities:Classroom Activity:Short Activity:Demonstration, Activities:Lab Activity
Grade Level: Middle (6-8), High School (9-12)

Description and Teaching Materials

The lesson is perhaps best introduced by using students in the class that are members of a musical group (band) to describe the "tuning" process for their individual instrument. Besides using an electronic tuner, how does a director check for instruments that are not in tune?

The materials for this demonstration are: two metal tubes, two Bunsen burners, and heat resistant gloves.

Students are to quietly listen to the two separate sounds produced by the individual tubes and then listen to the sound produced when both tubes produce sound at the same time. With older students and with available electronic equipment, the demonstration can be extended into the measurement of the two sound frequencies and the mathematical calculation of the beat frequency. Older students might be allowed to perform the activity themselves with proper safety precautions and supervision.

For closure, music students will be asked to report back to class any experiences in the coming days of out-of-tune instances. Review of interference of sound waves using the marker board would also be beneficial.

This activity is an adaptation of Singing Tubes as produced by Flinn Scientific Inc, Batavia Illinois. Physical Science FAX, Publication No. 6305

Teaching Notes and Tips

With practice, the production of sound from the tubes becomes easy. Use the suggested wire screens to increase the heated surface area. Turning the tube horizontal stops the sound and could be used as an introductory "magic trick" to get the students totally involved in the demo! Prior to the use of sound tubes, I have attempted to show beat patterns by playing musical instruments or with the use of adjusting tuning forks. I did not have resonating chambers for the tuning forks and the amplitude of the sound produced was quite small. The sound tubes make the demonstation much easier to hear by all.


Informal assessment of student understanding will be made using targeted questions of the class. Formal assessment of the concepts will follow the close of the unit of study.


6.II.C.5 Describing waves in terms of speed, wavelength and frequency
6.II.C.6 Waves move at different speeds

References and Resources