Exposure to hazardous noise is a ubiquitous occupational hazard. In the United States alone, some 30 million workers are exposed to hazardous noise at work, according to NIOSH estimates. Repeated overexposure at or above the NIOSH recommended exposure limit (REL) can cause health effects ranging from hearing loss to more severe illnesses.

Although common, noise exposure can be one of the most difficult hazards to assess. This difficulty can be alleviated by a firm understanding of the basics of noise and the use of well-designed instruments for measuring it.

Even the most experienced industrial hygienists can find noise assessment overwhelming due to the following factors:

• the theory behind noise and the logarithmic mathematics
• noise measurement equipment that may be difficult to understand or operate

To address these challenges, let’s clarify some of the basic concepts.

Defining Noise Noise can be defined as unwanted sound. OSHA identifies measurement of noise as a key component of a workplace hearing conservation program, which strives to prevent initial hearing loss, preserve and protect remaining hearing, and equip workers with knowledge and protective devices to safeguard themselves. Noise measurement can determine exposure to elevated noise levels and identify appropriate controls to help eliminate hearing loss, physical and psychological stress, and impaired communication that may contribute to incidents and injuries. For a thorough discussion of the effects of noise on health, see the article "Auditory and Non-Auditory Effects of Noise on Health" in The Lancet.

Measuring Noise The following is a useful guide for organizing and performing a basic noise assessment:

1. Determine if a noise exposure issue is present. Some indications may include people raising their voices in the work area, complaints of hearing impairment after work hours (known as a temporary threshold shift), and the use of noise-generating equipment such as saws, compressed air, and heavy machinery.

2. Investigate, collect, and understand the initial information required. This information includes applicable noise regulations and recommendations, the sources and locations of noise (for example, machinery and activities), workday flow (such as work activities, job descriptions, and shift length), and previous noise measurements.

3. Measure noise levels in the areas of concern for screening purposes. The sound level meter (SLM) provides instantaneous noise measurements. A good SLM should have a clear, well-lit screen; navigation and controls that allow quick, easy, and accurate setup of measurement parameters; and easy, accurate calibration. Examples of SLMs include the SKC SoundCHEK Series, Casella CEL Series, TSI SoundPro, Larson Davis SoundTrack Series, and Svantek SV Series.

4. Measure personal noise exposure levels using a personal noise dosimeter during work shifts. A good noise dosimeter should have features that allow for secure shoulder mounting; easy calibration, navigation, and operation; a large display; and intuitive software for storage and retrieval of measurements. Examples of dosimeters include SKC NoiseCHEK, Casella dBadge, TSI Edge 5, Larson Davis Spartan, and Svantek SV 104 Series.

Measuring Noise Levels Using an SLM In the U.S., criterion levels for 8-hour time-weighted average (TWA) noise exposure include the OSHA permissible exposure limit (90 dBA with a 5 dBA exchange rate), OSHA’s benchmark for inclusion in a hearing conservation program (80 dBA with a 5 dBA exchange rate), NIOSH REL (85 dBA with a 3 dBA exchange rate), and ACGIH threshold limit value (80 dBA with a 3 dBA exchange rate). Industrial hygienists should use the regulations in their country or region. Some SLMs can simultaneously measure multiple thresholds; it is best practice to do so.

After setting up the desired parameters, the SLM should be calibrated using an acoustic calibrator according to the manufacturer’s instructions. Before starting a measurement, install a windscreen on the SLM microphone to avoid noise turbulence. Proper measurement can be made by holding the instrument microphone within an approximate 2-foot (0.30-meter) sphere surrounding the worker’s head. The hygienist should move the SLM left and right within the sphere during measurement to capture potential differences in noise sources on either side. OSHA defines this measurement sphere as the “hearing zone.”

A noise survey map is a great tool for identifying areas that require hearing protection and to plan for noise abatements. Note that a noise survey map works well for hearing protection devices and noise abatement, but because a map doesn’t account for how workers interact with noise sources it is not as useful for exposure assessment. When preparing a noise map, include information about equipment and machinery present in the assessment area, its operating status, and distances between the equipment. Consult the OSHA Technical Manual for guidance on how to create a noise map.

Noise levels may vary at different distances from the source. If the source is located outdoors, you may estimate the sound level by obtaining the sound level measured in dB at a specific distance from the source and subtracting 6 dB per doubling of that distance. For indoor sources, subtract 4 dB per doubling to account for the reverberant field. This type of estimate is based on the Inverse Square Law, or (I2/I1) = (d1/d2)2. The Inverse Square Law is a predictive idealization that does not consider possible reflective surfaces, barriers in the sound field, or other sources of noise that can affect sound level readings. The noise levels in decibels are logarithmic; therefore, a calculation of the average of the SLM readings cannot provide an accurate result and should not be used.

Measuring Personal Noise Levels Using a Dosimeter A noise dosimeter is worn by a worker to determine exposure to personal noise during a specific sampling period. Noise dosimeter firmware runs algorithms that calculate the average noise exposure over time and reports results as a TWA and as a dose.

The dosimeter should be set up using very specific criteria based on regulations. For an example of these criteria, visit SKC's website (PDF). Improper setup will cause incorrect TWA and dose results. Some manufacturers make dosimeter PC software or mobile apps available to facilitate setup.

There are several practical considerations when measuring personal noise with a dosimeter:

Placement of the dosimeter is important. The dosimeter should be clipped outside of any clothing or personal protective equipment. If an article of clothing, such as a flame-retardant jacket, is removed during monitoring, the dosimeter should be reattached in the hearing zone to the worker’s outermost layer of clothing.

Workers who wear noise dosimeters may be very active during the work shift. When selecting a dosimeter, consider features such as durability and a well-protected microphone with secure windscreen.

Workers are often curious about the instrument. Selecting a dosimeter that has security features may be advised, and clear communication with workers is essential. Advise workers not to press buttons on the instrument and to perform work tasks in a normal fashion.

Workers often view noise dosimeters as recorders. Reassure workers that the unit records noise only, not speech, and that it will not affect their privacy. This helps increase workers’ acceptance of the instrument.

Following personal noise monitoring, obtain sound level measurements with an SLM to confirm the noise levels in an area at specific times and from particular sources and then compare these results with personal noise measurements from the dosimeter. Some dosimeter software and mobile apps allow users to download data and generate reports that make comparison easier.

Good field notes also are essential to noise assessments. Carefully document activities during the noise assessment and identify noise sources. Some noise dosimeters offer the ability to record a voice note while in the field; the note will be attached to the data upon downloading.

Using an Octave Band Analyzer Use of an octave band analyzer (OBA) can be very helpful, but confusing. OBAs divide noise into its frequency components and are typically used for more complex noise studies, especially noise abatement. OBAs are also useful for determining the effectiveness of hearing protective devices. The standard octave band filter sets provide filters within the following center frequencies: 16, 31.5, 63, 125, 250, 500, 1,000, 2,000, 4,000, 8,000, and 16,000 hertz (Hz).

Many SLMs and some noise dosimeters now contain integrated octave band filters, which allow users to assess the different sound frequencies simultaneously.

What to Do with the Data After collecting the appropriate sound level and personal dosimeter measurements, compare the TWA results with the applicable noise regulations per country or region. Share the results with management, and then post results for employees to view. Develop a written report of the findings that explains the results and possible controls to minimize exposures. The hierarchy used to determine controls for airborne exposures applies to noise as follows:

• Elimination or substitution: Determine if the task can be performed differently or if alternative equipment or machinery can be used.
• Engineering controls: Build acoustic barriers, fix equipment that may be generating the noise, and redesign the machinery.
• Administrative controls: Rotate personnel to minimize length of exposure.
• PPE: Implement hearing protection such as earplugs, earmuffs, or both with the adequate noise reduction rating (NRR).

It is best practice to define and document decisions regarding controls in the event that regulators visit the workplace for an inspection.

Noise assessments can appear intimidating, but reviewing the basics can simplify matters even for experienced industrial hygienists. Given the challenges of noise assessment, having well-designed, accurate, and reliable instruments can ease the process and provide the essential workplace assessment needed to help minimize hearing losses and illnesses.

Resources The Journal of the Acoustical Society of America: “Occupational Noise Exposure: A Review of its Effects, Epidemiology, and Impact with Recommendations for Reducing its Burden” (November 2019).

The Lancet: “Auditory and Non-Auditory Effects of Noise on Health” (April 2014).

OSHA: OSHA Technical Manual, Section III: Chapter 5.

OSHA: Safety and Health Topics, Occupational Noise Exposure.

SKC: NoiseCHEK Personal Noise Dosimeter Operating Instructions (PDF).

LUCINETTE ALVARADO, CIH, has worked as an industrial hygienist for fourteen years and currently serves on the AIHA Board of Directors. She is the corporate CIH and technical services manager for SKC Inc.

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