Sounds with a frequency of 20 kHz and higher are referred to as ultrasound (or ultrasonic sound). High frequency sound is sound of which the frequency lies between 8 and 20 kHz. High frequency sound with a frequency over 16 kHz can hardly be heard, but it is not completely inaudible. High frequency sound and even ultrasound in the lower frequency zone (up to 24 kHz) can be audible if the sound level is high enough. The sound threshold (the sound level where sound can be perceived) rises sharply once the frequency (and therefore, the tone) becomes higher. Younger persons hear high frequency sound better and their hearing range is greater toward the high frequencies.