Anyone who has been to a loud concert knows about the connection between sound waves and the feeling of touch. A low bass note doesn’t only reach the concert goers' ears — it also vibrates their bodies. Subramanian says the experience of feeling such low notes inspired him to investigate sound waves.The human body detects sound and touch in similar ways. Cells in the skin have nerve endings, called mechanoreceptors (Meh-KAN-oh-ree-SEP-terz). They detect pressure, which triggers the release of signals to the brain. The inner ear also has mechanoreceptors. Called hair cells, they convert sound into electrical signals that travel along nerves to the brain.
Whether a sound is high or low depends on how many waves pass a single point during a given time. This measurement is called frequency. The higher the rate, the higher the frequency. Sound waves that make high notes have a higher frequency than do those that make low notes. An average person can hear sounds up to about 20,000 hertz, meaning 20,000 vibrations per second. (As people age, that upper limit drops. So children and teens generally can hear higher pitches than can older folks.) Ultrasonic waves are frequencies higher than those that the human ear can hear.
Many devices use ultrasonic frequencies. Some cars have parking sensors that send out ultrasonic waves and detect those that bounce back to identify obstacles. Medical ultrasound devices emit high-pitched sound waves to peer inside the body and “see” things, such as a growing fetus.