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Innovative Physical Science Lesson Plan: Properties & Propagation of Sound Waves

By thethinktank

Here's an interesting way to help your students understand the concept of sound waves. By asking the right questions, the teacher will lead the students to discover first hand the thought process involved in actually developing a theory.

This lesson plan should be started after introducing your students to the properties of waves. It is aimed at stimulating the students' thought process by asking questions.

Present these questions, one by one to your class.

Q. What is sound? Is it some kind of energy or is it of a particle (matter) nature?

Ask students to vote for either of the two possibilities by asking for a raise of hands. You may first side with the assumption that sound is of particle nature and present the following hypothesis:

The Particle Hypothesis

HYPOTHESIS: "When I speak, I emit a stream of particles that reaches your ear and then you hear sound."

Testing this hypothesis:

Now pose the next question to both of the groups, asking the students who support the theory to explain it and the rest of the students to counter the explanation.

Q. Can we explain the properties of sound by assuming particle behavior; that is, sound is a stream of particles emitted?

A few observations with sound are listed here. Help your students come up with possible explanations for each one of them. Help the other group to counter the explanation by giving hints.

Properties of Sound:

  • Loudness decreases with distance

Possible explanation: Assume particle velocity decreases with distance and the sound cannot be heard after it reaches a minimum threshold velocity.

Possible counter explanation: Loudness of sound does not vary in fixed quantum steps, instead, it has a continuous variation.

  • Different people have voices with varying pitch

Possible explanation: Different people emit particles at different speeds. Assume pitch varies according to speed with which the particle is emitted, then the property is satisfied.

Counter explanation: Higher speed is related to higher momentum and higher energy. Hence, this theory assumes that the pitch is dependent upon the energy of the particle. This would also imply that a tired person would speak at a lower pitch, which does not match with observations.

  • The speed is different in different media

Possible Explanation: Cannot be explained.

Counter explanation: A particle theory would mean that sound travels the slowest in solids because solids have the most packed arrangement. This is contrary to the fact that actually, sound travels fastest in solids.

  • Varying amplitude

Possible Explanation: Relate the varying amplitude to number of particles emitted in the stream or the particle density of the stream.

Counter Explanation: The hearing will be affected by the probability of particle hitting. This would mean that as the sound level decreases, the probability of detecting the sound decreases. This is contrary to the observed fact that the sound will still be always detected, provided the sound detector is normal.

Another explanation that counters the particle theory: No quantum effects have been observed in the analysis of sound, unlike light.

Conclusion: This hypothesis is unable to be explained by all of the properties of sound and hence, is incorrect. Sound cannot be something discreet like particles, instead it has to be something continuous.

The Real Picture: Sound is a Wave

Now assume that sound is a wave and in the light of the properties of a wave, help your students explain each of the properties one by one.

  • Loudness decreases with distance

Sound is a pressure wave. It travels in air as a pressure disturbance that travels away from the source of sound and hence, the pressure decreases with distance.

  • Different people have voices with varying pitch

Different people speak at different frequencies. The frequency of the emitted wave corresponds to the pitch of the wave.

  • The speed is different in different media

Since sound travels as a disturbance, it can be transmitted more easily in more tightly packed matter. Hence, sound travels fastest in solids.

  • Varying loudness

The loudness of sound is associated with the the emitted pressure wave. Greater amplitude means louder sound. Amplitude of sound wave also corresponds to the energy of the wave.


Sum up the concepts learned as follows:

  1. Sound is a wave or disturbance.
  2. It needs a material medium to travel and it travels as a pressure wave by forming compressions and rarefactions in the material of travel.
  3. The three fundamental characteristics of sound are:
  • Loudness - depends upon the the amplitude of the wave
  • Speed - a property of the medium of travel
  • Pitch - depends on the frequency of the wave