Acoustic criteria: how to protect your workforce from permanent hearing damages
One of the first stages for designing a soundproofing strategy? Finding the right acoustic requirements.
Different “failure criteria” exist for various scenarios. And just like mechanical design, acoustic design encompasses diverse criteria for different applications.
Some designs aim to reduce noise that hinders workers’ communication or their ability to perform assigned tasks. In other cases, the goal is to avoid negative reactions from the surrounding communities near a noisy plant.
In this article, we focus on how acoustic designers should seek to reduce noise to a level that doesn’t prevent permanent hearing loss among industrial workers.
The human ear and hearing loss risks
In order to comprehend the detrimental impact of sound on the human ear, it is necessary to briefly grasp the anatomical structure of the ear.
The human ear is an extraordinary auditory system. It can perceive sounds within a frequency spectrum ranging from approximately 16 to 20 Hz up to frequencies in the 16 to 20 kHz range.
Furthermore, the ear has the capacity to detect acoustic pressures as low as 20 mPa at a frequency of 1000 Hz and endure acoustic pressures as high as 2000 mPa for brief durations.
Due to the acoustic characteristics of the outer ear and the mechanical features of the middle ear, the human ear does not act as a linear transducer for sound pressure levels.
At the same time, due to the poor acoustic impedance matching between the air outside the ear and the outer ear at frequencies below about 500 Hz, the ear can detect only sounds that have a pressure level greater than about 12 dB for frequencies of 250 Hz and lower.
For a sound pressure level of approximately 120 dB with a frequency between 500 Hz and 10 kHz, an individual will experience a tickling sensation in the ears. This level represents the threshold of ‘‘feeling’’ or the beginning of discomfort due to noise.
When the sound pressure level is increased above approximately 140 dB, the threshold of pain is reached. Continuous exposure to noise above 140 dB for a few minutes can result in permanent damage to the ears.
Industrial noise criteria and exposure standards: a model from the United States
Therefore, one of the primary reasons for implementing soundproofing solutions today is to safeguard workers from hearing loss caused by occupational noise exposure.
In the United States, the government has implemented several frameworks to assist in establishing acoustic parameters that define acceptable noise levels.
In 1965, the National Academy of Sciences and the National Research Council’s Committee on Hearing, Bioacoustics, and Biomechanics (CHABA) developed noise exposure criteria, according to which acceptable noise level should not result in a permanent threshold shift (NIPTS) exceeding 10 dB at 1 kHz and below, 15 dB at 2 kHz, and 20 dB at 3 kHz or higher after 10 or more years of exposure.
In 1970, the Occupational Safety and Health Administration (OSHA) established a noise exposure limit of 90 dBA for an 8-hour workday, allowing higher noise exposures for shorter durations. For every 5 dBA increase above 90 dBA, the permissible exposure time was reduced.
Moreover, according to OSHA’s criteria, exposure to noise levels exceeding 115 dBA is not allowed for any duration. The action level, which triggers the initiation of hearing conservation measures, was set at 85 dBA. The upper limit for impulsive noise exposure was established at 140 dBA.
If the noise level surpasses the permissible limits set by governmental criteria, the employer must take necessary steps.
Firstly, a noise survey should be conducted to identify areas where limits are exceeded and determine the specific source of noise.
Secondly, engineering measures or controls should be implemented to reduce worker exposure to noise.
Some examples of engineering control measures:
- Substituting machinery with quieter options: this can involve using larger, slower machines, employing belt drives instead of gear drives, or redesigning the equipment to emit lower levels of noise.
- Substituting manufacturing processes: switching to quieter alternatives, such as using welding instead of riveting, can help reduce noise emissions.
- Replacing worn or loose parts: worn-out or loosely fitting components should be replaced to minimize noise generation.
- Installing vibration dampers and isolators: these measures help reduce the transmission of vibrations and subsequently lower noise levels.
- Installing flexible mountings and connectors: using flexible materials for mounting and connecting equipment can help to absorb vibrations and reduce noise propagation.
- Enclosing the noise source: placing the noise source within an enclosure or employing acoustic barriers between the worker and the noise source can help contain and diminish noise levels.
- Isolating the worker from the noise source: creating an acoustically treated room where the worker and machine controls are situated can effectively isolate the worker from the noise source.
By implementing these engineering control measures, employers can mitigate excessive noise levels and minimize the risk of hearing damage to their workers.
As well as implementing engineering control measures, employers can adopt Industrial Silencers to perform acoustic attenuation.
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