SIMPLIFIED: "5.4: Beats"

Link to article: https://phys.libretexts.org/Bookshelves/Waves_and_Acoustics/Understanding_Sound_(Abbot)/05%3A_Interference/5.04%3A_Beats 

Note: The image above was taken from the original article.

The article linked above is all about beats! You might be wondering, what actually causes a beat? A beat occurs when two sound waves with different frequencies interfere with each other. That might sound a bit complicated, so I will break it down for you. 

Before we begin, I want to introduce you to sound waves really quickly. There are various sound waves. As they get closer to you, the sound gets louder, and as these waves get farther away from you, the sound gets softer. This is why it is more difficult to hear someone talking when you are further away from them; the sound waves from their voice may not make it to you because you are too far away. 

Moving on, it is vital to understand constructive vs. destructive interference when studying waves. Constructive interference means that two waves will come together and have a larger amplitude, therefore causing a louder sound. The two waves essentially reinforce each other. However, destructive interference means that two waves will come together and start canceling parts of each other out, therefore causing the combined wave to have a smaller amplitude and thereby causing a softer sound. 

Now, we get to the idea of beats. When two waves with different frequencies meet, they will combine into one wave. A beat will be produced when these two frequencies are close to each other. To find the beat frequency, the following equation can be used: frequency of a beat = frequency of wave 1 - frequency of wave 2. You just take the absolute value. When these two waves meet, the amplitude of the combined wave keeps changing, which is why you hear a beat. You will ultimately start hearing a louder sound, then a softer sound, then a louder sound, then a softer sound, and so on.