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### Course: Class 11 Physics (India)>Unit 19

Lesson 6: Beats and interference of sound waves

# Beats and interference of sound waves review

Review of how to produce beats through the constructive and destructive interference of sound waves.

## Key terms

TermMeaning
BeatRegularly alternating soft and loud sound heard from two sound waves of different frequencies interfering.
Beat frequency (${f}_{B}$)The magnitude of the difference between the two interfering frequencies. Scalar with SI units of $\text{Hz}$.

## How interference produces beats

When a musician needs their instrument to have the correct sound, they produce the desired frequency and try to match it with their instrument’s frequency. If the two frequencies don’t match, they interfere to produce alternating loud and soft sounds.
Figure 1 below shows two waves with different frequencies, ${f}_{1}$ (purple) and ${f}_{2}$ (green). The waves are interfering constructively in the green highlighted regions, leading to a larger amplitude when the waves combine in ${f}_{B}$. The white highlighted regions are where destructive interference occurs, and the amplitude of ${f}_{B}$ is smaller than the original sound waves. The variation in amplitude results in the alternating volume (loudness) of sound that we call beats.

For deeper explanations of the beat frequency, see our video about beat frequency.
To check your understanding and work toward mastering these concepts, check out the exercise on analyzing the interference of sound waves.

## Want to join the conversation?

• On the previous practice, I am confused about the pressure graphs. From what I am understanding from the practice, it is saying that when there is more pressure, there is more displacement and thus a larger amplitude/volume, but when I look it up, I find the opposite. Can someone please help me understand what I am missing here?
• https://bit.ly/2OdXTXZ

You may be mixing up the terms loudness and pitch (see image above).

If not, post the link that says higher amplitude = quieter sound.
• How much beat frequency can we not perceive the beat sound?