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DENJIRO: Today, we have an experiment using sound. First, take a listen to this sound.
BOY: It’s the sound of a bell at a temple.
DENJIRO: The bell at a temple has a pulsing sound. Why is that so?
First, we’re going to use these two speakers to produce sounds that have the same frequency of 500 Hz. You don’t hear a pulsing sound like we did with the bell. Now, I’m going to raise the frequency of one of the sounds. Let’s see what it will sound like. I’m raising the frequency to 501 Hz.
Now, let’s move it up to 502 Hz. And 503 Hz.
BOY: I can hear a pulsing sound.
DENJIRO: Sound travels as a wave by vibrating the air. If two sounds with slightly different frequencies overlap, there are places where the waves cancel each other out and places where the waves are strengthened. As a result, the difference in volume creates a pulsing sound. This phenomenon is known as a “beat.”
NARRATOR: The sound of a temple bell sounds like it is pulsing because when the bell is struck, two sounds with slightly different frequencies are produced.
DENJIRO: You can actually see a beat with your eyes as well.
Here we have 100 evenly spaced lines. If we think of this as the frequency of sound, it represents a sound with a frequency of 100 Hz.
Now, let me place a sheet that has 101 lines over it. This represents a sound with 101 Hz. Watch what happens.
BOY: There are light and dark spots.
DENJIRO: We have a pattern of light and dark regions. It is light where the lines overlap, and it is dark where the lines don’t overlap. This is a beat.
If we use a 100 Hz sheet and a 102 Hz sheet, we get two patterns. With 103 Hz, we get three patterns. As you can see, adding 1 Hz adds a pattern of light and dark regions, or a beat.
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