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Ultrasound — sound at frequencies above 20,000 Hz — is used extensively in manufacturing for welding, cleaning, cutting, and inspection. While the primary ultrasonic frequencies are inaudible to humans, industrial ultrasound equipment generates both subharmonic frequencies in the audible range and high-intensity sound at the upper end of human hearing (10,000–20,000 Hz) that can cause hearing loss, tinnitus, and other health effects with prolonged exposure. OSHA does not have a dedicated ultrasound standard, but audible byproducts of industrial ultrasound sources are evaluated under 1910.95 using standard noise dosimetry methods. This guide explains what workplace ultrasound is, which sources create audible hazards, what OSHA and NIOSH say about exposure limits, and what employers must do when workers are near ultrasonic equipment.
Soundtrace provides noise monitoring and audiometric testing for employers in industries using industrial ultrasound, with professional supervisor review that recognizes the high-frequency audiometric patterns associated with ultrasound-adjacent noise exposure.
True ultrasound above 20 kHz is inaudible — but it’s not silent. Industrial ultrasound equipment generates audible subharmonics and high-frequency byproducts that fall within OSHA’s noise standard. Workers near operating ultrasonic welders, cleaners, and cutters may be exposed to levels requiring HCP enrollment under 1910.95.
Ultrasound refers to acoustic energy at frequencies above approximately 20,000 Hz (20 kHz) — above the nominal upper limit of human hearing. Industrial ultrasound systems use these high frequencies to achieve useful mechanical effects: cleaning by cavitation, welding by frictional heating, cutting by vibration, or inspection by wave propagation through materials.
The frequency range of industrial ultrasound systems varies by application. Ultrasonic cleaners typically operate at 20–100 kHz. Ultrasonic welders for plastics operate at 20–40 kHz. Ultrasonic inspection (NDT) systems operate at a wide range of frequencies from 20 kHz to several MHz. In all cases, the primary operating frequency is above human hearing.
Industrial ultrasound equipment creates audible noise hazards through two primary mechanisms:
When an ultrasonic transducer operates at, for example, 40 kHz, it generates acoustic energy not only at 40 kHz but also at fractional multiples of that frequency — 20 kHz, 13.3 kHz, 10 kHz, and so on. These subharmonic frequencies fall within or adjacent to the upper audible range and can be quite high-intensity. The 20 kHz subharmonic of a 40 kHz system is at the threshold of human hearing and can be hazardous at high intensity. The 10 kHz subharmonic is well within the audiometric range and is audible to most workers.
The mechanical vibration induced in the workpiece, fixture, and structure of the ultrasonic equipment generates broadband acoustic radiation across a wide frequency range including the audible spectrum. An ultrasonic welder pressing against a plastic part induces vibration in the part, fixture, and support structure that radiates sound at many frequencies, not just the operating frequency. This mechanical radiation is often the primary source of audible noise from industrial ultrasound systems and is what a standard dosimeter captures as the operator’s noise dose.
Standard A-weighted noise dosimeters used for OSHA 1910.95 compliance monitoring measure sound in the frequency range from approximately 20 Hz to 20,000 Hz, weighted to account for human hearing sensitivity. The audible byproducts of industrial ultrasound equipment — including subharmonics and structural radiation — fall within this measurement range and are captured by standard dosimetry. Employers do not need specialized ultrasound measurement equipment to determine whether audible byproducts exceed the OSHA action level.
| Equipment Type | Operating Frequency | Typical Audible Byproduct (at operator) | Industries |
|---|---|---|---|
| Ultrasonic plastic welder | 20–40 kHz | 85–100 dBA at operator position | Automotive, packaging, consumer goods, medical devices |
| Ultrasonic cleaning tank | 20–100 kHz | 80–95 dBA in tank area | Aerospace, automotive, electronics, medical |
| Ultrasonic food cutting | 20–40 kHz | 85–95 dBA at cutting station | Food processing |
| Ultrasonic sealing | 20–40 kHz | 80–90 dBA at operator | Packaging, textiles |
| Ultrasonic NDT inspection | Wide range | Variable; often below action level | Aerospace, manufacturing, construction |
| Ultrasonic flow meters | Variable | Generally below action level for workers | Oil & gas, utilities, industrial process |
OSHA does not have a specific permissible exposure limit (PEL) for ultrasound. The agency acknowledges ultrasound as a potential occupational hazard but has not promulgated a dedicated standard. Instead, OSHA evaluates ultrasound hazards through two frameworks:
OSHA 1910.95 for audible byproducts: Any audible noise generated by ultrasound equipment — including subharmonics and structural radiation — is evaluated under the standard noise dosimetry and HCP requirements of 1910.95. Workers whose A-weighted TWA exposure from audible ultrasound byproducts equals or exceeds 85 dBA must be enrolled in the HCP.
General Duty Clause for pure ultrasound effects: For health effects attributable to the ultrasonic frequencies themselves (above 20 kHz), OSHA may use the General Duty Clause to require abatement when a recognized hazard exists and feasible controls are available. NIOSH and ACGIH guidance documents serve as the evidence of recognition for General Duty purposes. In practice, most enforcement actions related to industrial ultrasound focus on the audible byproduct noise under 1910.95 rather than the pure ultrasonic exposure.
Both NIOSH and ACGIH have published occupational exposure guidance for ultrasound that goes beyond OSHA’s regulatory framework. While neither constitutes a legally enforceable standard, both are widely recognized as the professional standard of care for industrial ultrasound exposure assessment and control.
ACGIH publishes TLVs for high-frequency sound in the 10,000–20,000 Hz range, recognizing that the upper audible frequency region is potentially vulnerable to damage from industrial sources at levels below the OSHA PEL. The ACGIH TLVs for this frequency range are expressed as ceiling values in one-third octave bands and are more stringent than the OSHA A-weighted PEL at frequencies above 10,000 Hz. These ceiling values account for the fact that the standard A-weighting network substantially underweights frequencies above 10,000 Hz — meaning a noise source concentrated in the 10–20 kHz range may have a relatively low A-weighted level while still delivering significant energy at the frequencies where ACGIH guidance suggests greater caution.
ACGIH also publishes separate TLVs for pure ultrasound in the region above 20 kHz, based on the non-auditory health effects (nausea, fatigue, headache) associated with high-intensity ultrasound exposure. These are sub-harmonic ceiling values for the 20, 25, 31.5, 40, 50, and 63 kHz octave bands.
NIOSH has addressed industrial ultrasound exposure in its occupational noise criteria and recommends that employers characterize both the audible and ultrasonic components of exposure for workers near high-intensity ultrasonic equipment. NIOSH guidance supports using engineering controls as the primary exposure reduction method — enclosures, barriers, and remote operation — and treating ultrasonic welder and cleaner operators as candidates for HCP enrollment pending dosimetry results.
Standard OSHA noise dosimetry uses A-weighting, which applies increasingly heavy attenuation to frequencies above 6,000 Hz. A noise source concentrated in the 12,000–16,000 Hz range may produce a lower A-weighted TWA than its potential for high-frequency cochlear damage would suggest. For workers near high-frequency ultrasound byproducts, octave-band or one-third octave-band analysis alongside A-weighted dosimetry provides a more complete picture of the exposure — and may trigger ACGIH TLV evaluation even where the OSHA action level is not exceeded.
The occupational health literature on industrial ultrasound exposure documents a range of effects that extend beyond audiometric hearing loss. Workers with significant ultrasound exposure have reported:
These non-auditory effects are not captured by standard audiometric testing. Employees reporting these symptoms after ultrasound equipment exposure should be referred for occupational medicine evaluation, and the exposure itself should be assessed using both A-weighted dosimetry and one-third octave-band analysis to characterize the frequency content.
The decision to enroll workers near ultrasonic equipment in the hearing conservation program follows the same framework as any other noise hazard under 1910.95: conduct dosimetry, compare to the 85 dBA action level, and enroll workers whose TWA meets or exceeds the threshold.
For ultrasound-adjacent workers, the following practical approach is recommended:
Many industrial ultrasound processes are intermittent — the welder cycles on and off, the cleaning tank runs during dip-and-hold operations but is idle between parts. TWA dosimetry integrates exposure over the full shift, correctly accounting for these duty cycles. An ultrasonic welder that operates at 95 dBA for 30% of an 8-hour shift contributes less to the worker’s TWA than one operating continuously at 88 dBA. Dosimetry worn for the full shift is the only reliable way to characterize intermittent ultrasound exposure.
The audiometric signature of hearing loss from industrial ultrasound exposure is not always identical to classical occupational NIHL from broadband noise. Because audible ultrasound byproducts are concentrated at high frequencies (8,000–16,000 Hz), the cochlear damage pattern may differ from the classic 4,000 Hz notch associated with broadband noise exposure.
Workers with significant high-frequency ultrasound byproduct exposure may show:
Because OSHA’s STS calculation uses 2,000, 3,000, and 4,000 Hz — the standard speech frequencies — high-frequency loss from ultrasound byproducts may not trigger an STS at these lower frequencies until the damage has extended significantly into the speech range. Workers in ultrasonic welding operations with early high-frequency loss may therefore not show formal STS under OSHA’s 10 dB criterion until the damage is more advanced than comparable broadband noise exposure would indicate.
▶ This is a clinically significant point: PS reviewers evaluating audiograms for ultrasound-exposed workers should examine thresholds at 6,000 and 8,000 Hz — not just the STS frequencies — to detect early high-frequency loss before it progresses into the speech range and triggers formal STS.
OSHA’s preference hierarchy places engineering controls above HPDs. For industrial ultrasound, the most effective controls address exposure at the source:
When engineering controls are infeasible or insufficient, HPDs are required for enrolled workers. Standard foam earplugs provide reasonable attenuation across the frequency range where audible ultrasound byproducts are concentrated and are the standard first choice for workers near ultrasonic welders and cleaners.
Soundtrace provides noise monitoring and HCP management for employers in industries using industrial ultrasound — with professional supervisor audiogram review that examines high-frequency thresholds beyond the standard STS frequencies.
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