The frequency of sound wave vibration per second is called frequency, and the frequency between 20 Hz and 20 kHz is called sound wave; The frequency greater than 20khz is called ultrasonic wave.

The frequency below 20hz is called infrasound. Ultrasonic waves and infrasound waves are inaudible to human ears. Seismic waves and tsunamis are infrasound waves. Some animals have much more sensitive ears than people. For example, bats can "hear" ultrasonic waves. There are few "pure tones" with single frequency in the world. Most of the sounds we hear are polyphony of various frequencies, such as the periodic polyphony of single tones emitted by musical instruments and the aperiodic polyphony of speech.