The number of sound wave cycles per second, expressed in Hertz (Hz). A higher number means a higher pitch. The frequency of speech lies primarily between 125 and 8,000 Hz. Humans can hear sounds between 20 and 20,000 Hz.The number of sound wave cycles per second, expressed in Hertz (Hz). A higher number means a higher pitch. The frequency of speech lies primarily between 125 and 8,000 Hz. Humans can hear sounds between 20 and 20,000 Hz.