Distinguish acoustic phenomena
Conventionally, infrasound lies below about 20 Hz and ultrasound above about 20 kHz. These are mechanical waves, not electromagnetic radiation. Human hearing limits are not universal safety boundaries.
INSST’s NTP 205 explains that some ultrasonic sources also generate audible components, which must be considered alongside the specific exposure.
Locate sources and exposed people
Review starts with equipment and processes: ultrasonic cleaning or joining, testing and other devices using acoustic energy. For low frequencies, analyse identified sources and their actual operating conditions.
Observe operation, adjustment, maintenance and nearby workstations. Manufacturer emission information helps planning but is not automatically equivalent to each worker’s exposure during the working day.
Determine how the energy reaches people
For ultrasound, NTP 205 distinguishes airborne and contact transmission. That distinction changes assessment: standing near a source is not equivalent to placing a body part in a medium transmitting ultrasonic energy.
Equipment selection and use must prevent unintended contact with active areas. Respect guards and instructions rather than bypassing protection to accelerate an operation.
Choose appropriate measurement
Instruments and methods must be capable of characterising the frequency range of interest. An overall reading without knowledge of the instrument’s response can conceal relevant information. The assessor must justify the criterion used and its scope.
The BAuA assessment guide explicitly covers infrasound and ultrasound. Its technical references help guide analysis, but do not turn German criteria into Spanish obligations.
Control the source and check the result
The priority is to select suitable equipment and processes, maintain them and reduce exposure through engineering measures compatible with the frequencies and transmission medium. Enclosures, separation or isolation need specific design and verification.
Do not assume every screen or hearing protector performs equally at every frequency. After modification, verify exposure under representative conditions, including tasks previously raising concerns.
Use current references and investigate reported concerns
Spain’s LPRL requires risks to be assessed and controlled. Older technical notes can explain physical principles, but their historical tables must not be presented as current statutory limits.
Reported discomfort or changes are investigated through prevention and health surveillance without automatically attributing them to one source or dismissing them because it is inaudible. Assessment must also consider other agents present.
Hypothetical example: introducing ultrasonic cleaning
A workshop installs ultrasonic cleaning equipment. Before use, it reviews instructions, guards, loading arrangements and the presence of nearby workers.
A specialist defines the appropriate acoustic assessment. After installing a control solution, the workshop verifies effectiveness and records how to operate and maintain the equipment without accessing active areas.
