Cost-free signal generator or function generator apps are available for download to your smartphone. Have two members of your group download an app that has the capability of performing a frequency sweep. Set one of the phones to play a continuous sine wave of frequency 4 000 Hz. Set the other to begin a downward sine wave sweep from 3 800 Hz to 3 000 Hz. Start both phones playing at the same time. Have each member of the group listen carefully to the combined sound. While you will clearly hear the frequency of the second phone going down, some members of the group may also hear a sound going up in frequency. It will be different in nature; it will sound as if it is coming from inside your ear rather than from the smartphones. Rotate your head back and forth, which may help you hear it. What is causing this sound?

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