Some sounds, like a car's squealing brakes, are very high-pitched. Others, like a booming bass drum, are very low-pitched. What makes them different is the frequency of the sound waves. If the sound waves follow very rapidly one after another, they are high-frequency and make a high sound. If there are long gaps between each wave, they are low-frequency and make a low sound. A low-frequency sound is about 20 Hz or waves per second. A high-frequency sound is 20,000 Hz or waves per second.