Understanding Ultrasonic Noise in Modern Workplaces

Ultrasonic noise, often generated by industrial equipment such as ultrasonic cleaners, welders, and medical devices, operates at frequencies above the typical range of human hearing. While these sounds are inaudible to most people, they can still interact with the auditory system and potentially affect overall well-being. Recent laboratory research has begun to shed light on how even short-term exposure might lead to measurable changes in hearing sensitivity and mental performance.

Workplaces in manufacturing, healthcare, and research laboratories frequently encounter these high-frequency sounds. Engineers, technicians, and laboratory staff may spend hours near devices operating between 18 and 40 kilohertz. Traditional noise regulations have focused primarily on audible frequencies, leaving gaps in guidance for this emerging occupational concern.

Background on Hearing Threshold Shifts and Noise Exposure

A temporary hearing threshold shift, commonly abbreviated as TTS, refers to a reversible reduction in the ear's ability to detect sounds at certain frequencies following noise exposure. Unlike permanent hearing loss, TTS typically resolves within hours or days after the exposure ends, provided the individual receives adequate rest from loud environments. This phenomenon serves as an early warning sign that the auditory system has been stressed.

Researchers have long studied TTS in relation to audible noise, but data on ultrasonic frequencies remain limited. High-frequency sounds can bypass some of the protective mechanisms of the outer and middle ear, potentially reaching the inner ear structures more directly. This has prompted renewed interest in how airborne ultrasound might contribute to subtle yet significant auditory changes.

The Landmark Study by Jan Radosz

Jan Radosz, a researcher at the Central Institute for Labour Protection—National Research Institute in Poland, led a controlled laboratory investigation into these effects. The study simulated realistic workplace conditions to measure both auditory and cognitive responses to ultrasonic noise. Participants underwent carefully calibrated exposures while researchers monitored their hearing and performed standardized cognitive assessments.

The experimental design included two exposure levels: one matching current occupational permissible limits at 20 kilohertz and another set 5 decibels lower. Each session lasted one hour, allowing researchers to observe immediate post-exposure changes. This approach provided clear comparisons between the two conditions and helped isolate the role of ultrasonic noise intensity.

Key Findings on Temporary Hearing Threshold Shifts

The results showed statistically significant temporary threshold shifts at higher frequencies when participants were exposed to the full permissible limit. Specifically, mean shifts reached approximately 3.8 decibels at 8 kilohertz and 5.8 decibels at 16 kilohertz in both ears. These changes were not observed at the reduced exposure level, suggesting a dose-dependent relationship.

Importantly, the shifts occurred in the extended high-frequency range, which is often the first area affected by noise-related stress. Although the magnitude of the shifts was relatively small, their statistical significance indicates that current limits may not fully protect against even temporary auditory fatigue. Recovery patterns were not detailed in the primary analysis, but the findings underscore the value of monitoring high-frequency hearing in exposed workers.

Cognitive Performance Impacts During Exposure

Beyond auditory effects, the study incorporated cognitive testing using the Abilitest battery during the noise exposure periods. Compared with established normative data, participants exhibited longer reaction times in simple response tasks—approximately 20 percent slower—and in selective attention tasks, where times increased by about 13 percent.

These cognitive changes suggest that ultrasonic noise may impose an additional mental load, even when the sound remains largely inaudible. Tasks requiring quick decision-making or focused attention could be subtly impaired, raising questions about productivity and safety in precision-oriented work environments. The findings align with broader research on how various noise types can divert cognitive resources away from primary tasks.

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Implications for Occupational Exposure Standards

Current international guidelines for airborne ultrasound vary, with many focusing on preventing subjective discomfort rather than documented physiological effects. The new evidence from this laboratory work supports calls to review and potentially tighten these limits, particularly for frequencies around 20 kilohertz where many industrial devices operate.

Stakeholders in occupational health, including safety officers and regulatory bodies, may need to incorporate high-frequency audiometry into routine screening programs. Engineering controls, such as improved enclosure of ultrasonic sources or the use of barriers, could become standard recommendations alongside traditional hearing protection.

Read the full study on the MDPI platform for complete methodological details and statistical analyses.

Real-World Contexts and Industry Applications

Ultrasonic equipment appears across multiple sectors. In healthcare settings, ultrasonic scalers and cleaners are common in dental and surgical suites. Manufacturing facilities use ultrasonic welding for plastics and metals, while research laboratories rely on ultrasonic baths for sample preparation and cleaning.

Workers in these environments may experience cumulative exposure across an entire shift. The study’s one-hour exposure model provides a foundation for understanding shorter bursts, but longer or repeated exposures could compound the observed effects. Employers are encouraged to conduct site-specific assessments to identify ultrasonic sources and implement targeted mitigation strategies.

Broader Research Context and Related Evidence

Earlier epidemiological observations have noted poorer high-frequency hearing among operators of low-frequency ultrasonic devices compared with workers exposed only to audible noise. Complementary studies have explored measurement techniques for ultrasonic noise in real workplaces, highlighting challenges in accurate assessment due to the directional nature of these sounds.

International reviews of airborne ultrasound exposure limits emphasize the need for consistent standards that account for both auditory and non-auditory effects. The Polish research contributes valuable controlled data that complements field observations and strengthens the case for evidence-based updates to guidelines.

Access additional publications by the research team to explore related pilot studies and conference presentations.

Practical Recommendations for Workplaces

Organizations can take several proactive steps based on emerging evidence. First, inventory all equipment capable of generating ultrasonic noise and measure sound pressure levels in the relevant frequency bands. Second, prioritize engineering solutions such as acoustic enclosures or process modifications to reduce exposure at the source.

Administrative controls, including rotating staff assignments and scheduling high-exposure tasks during periods with natural recovery time, offer additional layers of protection. Regular high-frequency hearing tests can help detect early shifts before they become more pronounced. Personal protective equipment designed for high frequencies may also play a supportive role when other measures are insufficient.

Future Directions and Ongoing Investigations

Longer-term studies examining recovery timelines, cumulative effects over multiple days, and potential interactions with other workplace stressors would build on the current findings. Researchers are also interested in expanding cognitive assessments to include memory, executive function, and real-world task simulations.

Collaboration between acoustic engineers, occupational physicians, and cognitive scientists promises a more holistic understanding of ultrasonic noise impacts. As technology evolves and new ultrasonic applications emerge in fields such as additive manufacturing and advanced medical imaging, proactive research will remain essential.

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Conclusion and Path Forward

The laboratory investigation led by Jan Radosz provides compelling evidence that exposure to ultrasonic noise at current permissible levels can induce small but measurable temporary hearing threshold shifts and modest cognitive performance changes. These results highlight an important area for continued attention in occupational health research and practice.

By integrating these insights into workplace policies, employers and regulators can better safeguard worker well-being while supporting productive environments. Continued dialogue among researchers, industry leaders, and health professionals will help translate these findings into practical improvements that benefit everyone exposed to airborne ultrasound.