For engineers and facility managers operating ultrasonic welding or cleaning equipment, the question of operator health is a frequent and valid inquiry. While the technology has been a staple in manufacturing for over 40 years, misconceptions often persist regarding the distinction between ultrasonic energy and audible noise.
This technical brief summarizes the nature of ultrasonic energy, its propagation characteristics, and the consensus from leading researchers regarding occupational exposure.
1. The Nature of the Energy Source
To understand the safety profile, one must first classify the energy type. Ultrasonic energy is strictly mechanical energy, distinct from electromagnetic energy (microwave, radio frequency) or ionizing radiation (X-rays).
In industrial applications, the source is the ultrasonic transducer—essentially a linear motor that converts electrical energy into reciprocating mechanical motion. This motion typically occurs at a rate of 20,000 strokes per second (20 kHz) or higher, placing it well above the hearing range of the average person.
2. Propagation and Attenuation
A critical safety factor in ultrasonic operations is the medium of transmission. While ultrasonic energy travels efficiently through metals and plastics (the workpiece), air is an extremely poor transmission medium.
The attenuation of airborne ultrasound follows steep decay laws. When propagated from a point source into three-dimensional space, the energy diminishes at a cubic rate. Specifically, every time the distance from the source is doubled, only 1/8th of the energy remains. Furthermore, soft materials in the manufacturing environment, such as operator clothing or noise abatement foam, readily absorb this airborne energy, reducing it thousands of times relative to the source.
3. Occupational Exposure Consensus
Extensive research has been conducted to determine if airborne ultrasound presents a physiological hazard. The consensus among researchers is that current industrial practices do not constitute a hazard to hearing or internal organs.
- Physiological Effects: Researchers have noted that "ultrasonic sickness," a phenomenon described in early literature (1948–1962), appears to be largely psychosomatic, stemming from apprehension rather than physical causality. There is no clear evidence that airborne ultrasonic vibrations produce specific effects on the nervous system or sense organs.
- Direct Contact: The primary safety rule regarding ultrasonic energy is avoiding direct contact. Direct exposure of body parts to the vibrating horn or workpiece will cause frictional heat and intolerable pain, usually resulting in a reflex to terminate contact before serious injury occurs.
4. The Real Hazard: Audible Subharmonics
While the ultrasonic frequency itself is inaudible and harmless to the human ear, the process often generates secondary frequencies that are very much audible. This is known as subharmonics—noise generated by the workpiece vibrating at audible lower frequencies.
This audible noise can be disturbing and, in extreme cases, cause temporary shifts in the hearing threshold. Therefore, the focus of industrial hygiene should be on the dBA (A-weighted decibels) levels rather than the ultrasonic frequency itself.
Permissible Noise Exposures (OSHA Standards)
Operators should adhere to standard occupational noise exposure limits. The permissible duration of exposure decreases as sound levels increase:
| Duration per day (Hours) | Sound Level (dBA, Slow Response) |
|---|---|
| 8 | 90 |
| 6 | 92 |
| 4 | 95 |
| 3 | 97 |
| 2 | 100 |
| 1 | 105 |
| 0.5 | 110 |
| 0.25 | 115 |
Note: Ultrasonic welding uses intermittent energy. Exposure is calculated by accumulating the individual energy cycles (seconds per weld) over the workday.
5. Electromagnetic Interference and Pacemakers
A distinct consideration is the interaction between industrial equipment and medical devices. While cardiac pacemakers are generally unaffected by airborne ultrasonic energy, they can be sensitive to electromagnetic energy.
Ultrasonic generators, like all heavy industrial electronics, produce radio frequency (RF) emissions. While modern equipment is designed to comply with FCC limits on radiated energy, the sensitivity of various pacemaker models varies. Until specific data on a device is known, it is prudent to restrict operators with pacemakers from working in close proximity to ultrasonic generation systems.
6. Technical Note on Measurement
Accurately measuring sound levels in an ultrasonic environment requires specific instrumentation.
Standard sound level meters are typically calibrated for frequencies below 10 kHz. Using a standard meter to measure noise in an environment rich in high-frequency harmonics may yield erroneous readings. For precise data, facility managers should use instruments specifically designed with high-frequency microphones capable of accurate results above the 10 kHz range.
Summary
The weight of evidence suggests that airborne ultrasonic energy poses no significant physiological hazard to personnel. The operational focus for safety professionals should remain on:
- Preventing direct skin contact with vibrating components.
- Monitoring audible noise (subharmonics) and utilizing enclosures or hearing protection to stay within OSHA dBA limits.
- Managing electromagnetic exposure for personnel with specific medical implants.