Two waves of slightly different frequency create beats when added together.
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Wave Beats

Sound travels in waves. When the waves hit your ear, you hear a sound. Have you ever noticed the waves in the ocean? They go up and down, up and down. Sound waves act the same way. The number of times they go up and down is called the frequency. People measure frequency in units called hertz. One hertz is equal to one complete wave (up and down) in one second. People can hear sounds between 20 and 20,000 hertz.

Another property of a waves is called amplitude. Amplitude is how strong or weak a wave is. In a lake, the amplitude of waves is usually measured in feet or meters. The amplitude of a wave switches between up and down (positive and negative). The waves switch back and forth and are called periodic functions because they repeat in a certain period of time.

When waves run into each other, they usually don't reflect. Instead, they combine. If the amplitudes of two waves point in the same direction (either up or down), then the new wave has a larger amplitude. This is called constructive interference. Sound waves with a higher amplitude sound louder. If the waves had opposite amplitudes (one pointed up and the other pointed down), then the new wave has a smaller amplitude. This is called destructive interference.

Two waves that add together might have different frequencies. That means that the peaks won't always line up the same way because one wave is moving faster than the other. When this happens there are times when the waves interfere constructively and times when they interfere destructively. In music, musicians call this a beat. You can hear beats when two instruments are almost playing the same note, but not quite. Musicians use beats to tune instruments. When you can't hear beats anymore, the instruments are tuned. This is how Doppler could tell that the frequency of the trumpets on the train had changed--he heard the wave beats.

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