Research from the University of Southampton in the UK indicates that the public is being exposed unknowingly to high levels of airborne ultrasound—from sources such as loudspeakers, door sensors and public address systems. The exposure could result in illnesses ranging from migraines to tinnitus.

Using smartphones and tablets equipped with an app that produces a spectrogram of the microphone reading, Professor Tim Leighton, from the university’s Institute of Sound and Vibration Research, collected readings of very high frequency/ultrasonic fields (VHF/US) fields in a number of public buildings during periods when they were heavily occupied.

He found that the public is routinely exposed to VHF/US levels over 20 kHz in areas such as railway stations, museums, libraries, schools and sports stadiums. The findings were calibrated with several independent microphone and audio data systems.

Researcher Tim Leighton monitors a building for airborne ultrasound. Image credit: University of Southampton.“Individuals who are unlikely to be aware of such exposures are complaining, for themselves and their children, of a number of negative conditions," Leighton says. "Recent data suggests that 1-in-20 people aged 40-49 years have hearing thresholds that are at least 20 decibels (dB) more sensitive at 20 kHZ than that of the average 30-39 year old. Moreover, 5% of the 5-19 year age group is reported to have a 20 kHz threshold that is 60 dB more sensitive than the median for the 30-39 year age group,” he says.

A lack of existing research in this area means it is not possible to prove or disprove public health risk or specific discomforts as a result of this exposure. "However,” he says, “it is important that sufferers are able to identify the true cause of their symptoms, whether they result from VHF/US exposure or not.”

Existing UK guidelines pertaining to public ultrasound exposure are insufficient, Leighton says, as most of them refer to occupational exposure, where workers are aware and can be monitored and wear protection. Furthermore, he says, guidelines are typically based on the average response of small groups, often adult males.

Leighton proposes additional research and guideline revisions pertaining to ultrasound exposure, including:

• Guidelines for occupational exposure must not be applied to public or residential exposure, and recognition must be given to exposure of long-term occupants (for example, in schools, hospitals and prisons).
• Research must be undertaken to assess whether current audiological practices, equipment and standards are suitable for the VHF and ultrasonic regimes—and measures identified to rectify shortcomings.
• A survey should be undertaken of modern devices and their source levels using international standard procedures and calibrations traceable back to primary standards.