Without knowing it, we are being exposed to airborne ultrasound in public places, a researcher at the University of Southampton reports, yet existing guidelines are insufficient to protect us, and long-term effects of such exposure need further studying.

“Ultrasound” is commonly associated with a pregnancy test, but it actually refers to any sound registering at an acoustic frequency above the limit of human hearing (between 16 kHz and 1 GHz). Though inaudible, ultrasonic frequencies can cause nausea, dizziness, headaches, fatigue, tinnitus (a ringing in the ears), and a sensation of pressure in the ears.

For more than 40 years, scientists have been aware that occupational machines for cleaning and drilling generate ultrasound, yet these frequencies are also generated by door sensors, PA systems, and loudspeakers. Recently, levels of these unheard noises have been increasing in public locations, including schools, libraries, train stations, and sports stadiums, the report found.

Conscious of ultrasound and its effects, workers often wear protective gear, but the general public remains unaware — and unprotected.

Out-of-Date Guidelines

To explore public exposure to ultrasound, Professor Tim Leighton, at the University of Southampton's Institute of Sound and Vibration Research, used readily available equipment — smart phones and tablets equipped with an app to produce a spectrogram. A spectrogram makes speech visible by plotting frequency on a vertical axis and time on a horizontal axis, with sound energy (amplitude) displayed by colors.

Then, Leighton traveled through a number of public buildings, including one unnamed “world renowned museum,” a food concourse in a railway station, and toilets near a public swimming pool, where he collected readings. At the time of his readings, all of these locations were occupied by hundreds of people, including infants.

Leighton discovered public exposures were at levels above the limits defined by current guidelines. Leighton says these existing recommendations are insufficient: They were written to protect workers, and so are based on the average response of a very small group of people, usually adult males, and do not take into account possible health effects on children.

“We still know very little about human responses (both auditory and non-auditory) to ultrasound in air,” wrote Leighton. He added that the symptoms reported by people — migraine, nausea, tinnitus, headaches, fatigue, dizziness, feelings of ‘pressure’ — are so common and non-specific that most doctors are not likely to attribute them to a physical cause.

Symptoms are often dismissed as psychosomatic, when they're actually being caused by very real, if unheard, noise. Leighton is now calling for further research, as well as a new set of guidelines.

Source: Leighton TG. Are some people suffering as a result of increasing mass exposure of the public to ultrasound in air? Proceedings of the Royal Society A. 2016.