High-pitched or low-pitched refer to the frequency of a sound wave. You've heard the word "frequency" before, perhaps in sentences such as: "How frequently do you eat ice cream?" or the dentist might ask you, "How frequently do you brush your teeth?" Your answer is how often you do these activities in a given amount of time. Your responses could be, "I eat ice cream once a week", or "I brush my teeth twice a day."
Because sound travels in a wave, we can relate the characteristics that scientists use to describe sound, such as frequency, to a picture of a wave.
If you were to trace your finger across the wave in the diagram above, you would notice that your finger repeats its motion. A wave has a repeating pattern. One such repetition is known as a wave cycle. When talking about sound, frequency means the number of cycles of sound in a second. The following diagram shows a high frequency wave (top) and a low frequency wave (bottom), plotted as pressure versus time. The high frequency wave has completed twelve cycles over the time shown. The low frequency wave has completed only three cycles over the same time.
The unit used to measure frequency is named Hertz, which is defined to be the number of cycles in one second. (This unit is named after Heinrich Hertz, a famous 19th century physicist.) If you increase the frequency of sound (there are more cycles in a second), you get a higher pitched sound. When you decrease the frequency, you get a lower pitched sound. Listen to the difference between frequencies in the series of sounds below.