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8 The Effect of Mass on Sound

Mass is a measure of the amount of matter in an object. For example, some drinking glasses are thicker than others, thus they are more massive. In reference to sound, the more massive a vibrating object, the lower the frequency, and hence the pitch. The reverse is true if the mass decreases with a result of a higher frequency and higher pitch.

In this activity, water will be added to glasses to change their mass. Tapping on a glass with water will produce a sound with a certain frequency. It will be seen that increasing the mass of a glass and then tapping on it will produce a sound with a lower frequency. Altering the amount of water in the glass can produce different musical notes.

See for Yourself

Materials

 6 drinking glasses of equal size and shape (the thinner the glass walls, the better)

 water

 food coloring

 pencil

What to Do

1 Pour different amounts of water in five of the glasses. Leave one glass empty.

2 Using food coloring, make the water in each glass a different color. Note: The coloring doesn't change the sound that will be produced. It merely helps to associate each glass with the sound it produces.

3 Gently tap each glass with the wooden end of a pencil.

4 Compare the sounds made by each glass with water to the sound produced by the empty glass. Determine which produces the highest and the lowest frequency.

FIG 1

An experimental challenge is to alter the amount of water in each glass to produce the notes to play a simple song.

What Happened?

The sound produced by tapping each glass was different depending on the amount of water it contained. The sound heard also depended on the sound produced by the glass; thus, an empty glass was used that vibrated the fastest when tapped, so it had the highest frequency and the note was high. A glass with water in it vibrated slower when tapped because the glass and water together had a greater mass. The greater the mass, the slower the vibrations of the glass when tapped. Slower vibrations mean a lower frequency of sound waves. The high and low musical notes produced are related to frequency. High notes are produced by high frequencies and low notes by low frequencies. Adjusting the level of water in a glass can result in notes that go up or down the musical scale.