HiSET Science: Sound Waves

Sound Waves
The pitch of a sound as it reaches one's ear is based on the frequency of the sound waves. A high-pitched sound has a higher frequency than a low-pitched sound. Like all waves, sound waves transmit energy. The rate at which this energy is transmitted is the sonic power. Loudness, or intensity of sound, is the sonic power received per unit area.
Beats occur when a pair of sound waves, whose frequencies differ only slightly, interfere with one another. This interference causes a periodic variation in sound intensity, whose frequency is equal to the difference between that of the two sound waves.
This is noticeable when tuning two instruments to one another. As the two pitches get closer, the beat frequency will become smaller and smaller until it disappears entirely, indicating that the instruments are in tune.