Sound: is a kind of vibration of material molecules (such as air) transmitted in the form of waves. Vibrations with a frequency between 16 Hz and 20,000 Hz are in the audible range, and can form a sensory impression in the human hearing. People have different hearing abilities. Vibrations below 16 Hz are called infrasound, and those above 20,000 Hz are called ultrasound. We can know from the above definition that. ['Few people can hear vibrations below 16 Hz in sound.', 'Vibrations between the infrasound and ultrasonic frequency ranges cannot cause human auditory sensory impressions.', 'There is a type of vibration that is neither called infrasound nor ultrasound.', 'Most animals have a more sophisticated hearing function than humans.']