Ultrasonic generally refers to acoustic frequencies above twenty thousand hertz, which is the upper limit of human hearing range. These frequencies are however audible to certain animals such as mice, bats, dolphins, and dogs that have a higher hearing range as compared to humans. The human inability to hear ultrasonic sound waves is caused by the middle ear that acts as a low-pass filter.

Humans would have been able to perceive ultrasonic sound waves if these waves could somehow be directed into the skull bone to reach the cochlea without passing through the middle ear. Deep-sea divers who are exposed to high frequency sonar signals above fifty thousand hertz sometimes experience this effect known as ultrasound hearing.

Ultrasonic sound waves form a part of many devices used in industrial and medical sectors. The most commonly used device is an ultrasound machine that is used in the treatment of benign and malignant tumors via a process known as ultrasound surgery (FUS) or high intensity focused ultrasound (HIFU). Medical research conducted to study the effects of ultrasonic waves on the human body suggests that the waves have the ability to stimulate bone-growth and has the potential to disrupt the blood-brain barrier for drug delivery.

Industrial devices also use ultrasonic sound waves to locate microscopic flaws in materials such as metal parts and plastics. Ultrasonic frequencies used in these devices usually range from two to ten mega hertz. Ultrasonic sound waves used in cleaning devices are usually in the range of twenty to forty kilohertz. These are generally used for cleaning jewelry, optical parts, watches, dental instruments, and surgical instruments.

Advanced research work is currently underway to develop a system for producing a phenomenon called sonoluminescence that has the potential to enable nuclear fusion reaction. This will provide a safe and cheap means of producing electricity and will help in solving energy problems faced by countries worldwide.