• Though inaudible to the human ear, ultrasound may still affect hearing.

Ultrasound is high-frequency sound that is inaudible to, or cannot be heard by, the human ear. However, it may still affect hearing and produce other health effects. Factors to consider regarding ultrasonics include:

• The upper frequency of audibility of the human ear is approximately 15–20 kilohertz.
  • This is not a set limit and some individuals may have higher or lower (usually lower) limits.
  • The frequency limit normally declines with age.
• Most of the audible noise associated with ultrasonic sources, such as ultrasonic welders or ultrasonic cleaners, consists of subharmonics of the machine’s major ultrasonic frequencies.
  • Example: Many ultrasonic welders have a fundamental operating frequency of 20 kilohertz, a sound that is at the upper frequency of audibility of the human ear. However, a good deal of noise may be present at 10 kilohertz, the first subharmonic frequency of the 20 kilohertz operating frequency, and is therefore audible to most persons.