Most people cannot heareither infrasound or ultrasound. Until now, it was assumed that these inaudible sound waves were harmless. Scientist Timothy Leighton of the University of Southampton in England has his doubts.

Sounds are produced by vibrations and propagate as sound waves. The intensity of the sound—the sound pressure—can be measured. The displayed measurement is the sound pressure level, which is expressed in decibels. If pressure fluctuations occur, they are perceived by the ear. The number of vibrations per second—the frequency—determines the “pitch.” The higher the frequency, the higher the pitch. Humans can hear sounds in the frequency range between 16 and 19,000 hertz. They perceive sounds best between 2,000 and 5,000 hertz. Conversations, among other things, take place within this pitch range.

Childrenwith healthy hearing can even hear up to 30,000 hertz at birth. However, this ability declines over the course of a lifetime because the hair cells in the ear are damaged by everyday noise exposure. And it is precisely these hair cells that are responsible for converting sound waves into signals that the brain recognizes as sound.

Ultrasonic sounds have a frequency above 19,000 hertz, while infrasonic sounds fall below 16 hertz. Most people cannot hear infrasound or ultrasound because the sounds are too low or too high, respectively. Only a few adults are able to hear frequencies of 20,000 hertz. Even then, the sounds are only perceptible as a high-pitched whine.

Modern daily life has long since become impossible without ultrasound. Countless devices use these waves as signals. At train stations, loudspeakers constantly emit ultrasonic waves, while ultrasonic sensors are responsible for automatically opening doors. Some people seem to be able to hear and even feel ultrasound. They often react to it with dizziness, nausea, tinnitus,or headaches.

Professor Leighton has been researching ultrasonic waves for several decades and is one of the leading experts in this field. In his most recent publication, he warns that ultrasound transmitted through the air can be harmful to health. To prove his theory, he has begun measuring ultrasound waves in public places. Together with his team, he investigated whether ultrasonic waves are present at train stations, in schools, sports stadiums, swimming pools, museums, libraries, and inside cars, and how intense they are.

Ultrasonic waves were detected at each of these locations. The strongest waves emanated from public address systems. Some volunteers from Leighton’s team complained of headaches and dizziness near these devices. Following this successful measurement, the expert studied existing ultrasonic guidelines worldwide, all of which date back to the 1960s and 1970s but have never been revised.

Leighton believes that the current exposure limits have been set too low and that the waves can indeed harm the health of sensitive individuals. Nevertheless, many doctors doubt this. Since the inaudible sounds do not come into contact with the cilia, they cannot be destroyed by the sound. Swiss ear, nose, and throat specialist Andreas Schapowal is also critical of Leighton’s theory, but calls for a more precise definition of ultrasound. Devices that use ultrasound should operate at frequencies significantly higher than 20 kilohertz. That way, they cannot be heard by adults or children.

So far, Timothy Leighton has not been able to prove that ultrasound is actually the cause of headaches, nausea, and other symptoms. That’s why the researcher now wants to collect more data using social media. With a free app, anyone can use their smartphone to measure ultrasound waves in the air. The results are being collected on Instagram under the hashtag #UltrasoundInAir, along with the time and location.