1. Beats

2. Acoustical Beat
Periodic change in sound intensity caused by interference between two nearly identical sound waves
They occur when the frequency of the 2 waves is nearly identical
You hear beats as a change in loudness, from soft to loud

4. You can use beats to assess small differences in frequency between 2 waves
When you tune an instrument, by checking the frequency of the beats, you can tell how well the instrument is tuned.

5. Beat frequency
Frequency of the beats produced by the interference of 2 waves with slightly different frequencies.
It is equal to the difference in frequencies of the 2 interfering waves
video

6. How does it work?
Strike a tuning fork and a piano key at the same time
Keep tuning string by adjusting the tension until you don’t hear any beats