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Class 11 Physics (India)
Course: Class 11 Physics (India) > Unit 19
Lesson 6: Beats and interference of sound wavesBeats and interference of sound waves review
Review of how to produce beats through the constructive and destructive interference of sound waves.
Key terms
| Term | Meaning | |
|---|---|---|
| Beat | Regularly alternating soft and loud sound heard from two sound waves of different frequencies interfering. | |
| Beat frequency (f, start subscript, B, end subscript) | The magnitude of the difference between the two interfering frequencies. Scalar with SI units of start text, H, z, end text. |
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, start subscript, 1, end subscript (purple) and f, start subscript, 2, end subscript (green). The waves are interfering constructively in the green highlighted regions, leading to a larger amplitude when the waves combine in f, start subscript, B, end subscript. The white highlighted regions are where destructive interference occurs, and the amplitude of f, start subscript, B, end subscript is smaller than the original sound waves. The variation in amplitude results in the alternating volume (loudness) of sound that we call beats.
Learn more
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?(6 votes)
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.(3 votes)
How much beat frequency can we not perceive the beat sound?(4 votes)- The human hearing range is between 20 -20,000 Hz.*
Thus, you should use frequencies between those value if you want to perceive the beats.
20 Hz > f1 and f2 < 20,000 Hz
beat frequency = |f1 - f2|
Moreover, I believe that the frequencies need to be relatively close to one another (eg, 440 Hz and 445 Hz).
*although there is considerable variation between individuals, especially at high frequencies, and a gradual loss of sensitivity to higher frequencies with age is considered normal.(0 votes)
ijev iudwvni nv iuvniwjvn wv(2 votes)