The measurement of sound pressure levels throughout a workplace is a fundamental component of noise-control and hearing-conservation initiatives. It is the basis for exposure assessment and regulatory guidance. Sound measurement and audiometry are opposite sides of the same coin.
This installment of the “Occupational Soundscapes” series introduces basic concepts of sound level measurement and exposure assessment. Equipment used, frequencies analyzed, calculation of a “dose,” and more are presented. Like the presentation of audiometry (Part 5), its aim is to provide a level of understanding, within the constraints of this format, that engenders trust in an organization’s noise-related practices.
Types of Sound
There are three types of sound to which a person may be exposed: continuous, intermittent, and transient. These are not unique to occupational settings, but understanding each is critical to defining an occupational soundscape. Accurate definition of the soundscape is required to identify and implement appropriate noise-control and hearing-protection measures.
Continuous or “steady-state” sound is nearly constant over long periods of time. Generally, sound that varies less than ± 3 dB is considered continuous. OSHA also treats sounds with level maxima at intervals up to one second as continuous. Electric motor-driven equipment, fans, and turbines are common examples of continuous sound generators. Continuous sounds are the most predictable and easily measured. Thus, it is relatively straightforward to identify and implement appropriate countermeasures.
Sounds that vary by more than ± 3 dB are termed intermittent. This can include the “extreme” case of a sound being absent (“turned off”) at intervals during the measurement. Less-extreme sound level variations are often called time-varying or fluctuating. For example, automated equipment that performs a series of operations in a repeating cycle – workpiece transfers, pressurization and exhaust, enclosure closing and opening, clamping and unclamping, etc. – can generate highly-variable sound pressure levels (SPLs).
Fluctuating sounds may or may not be predictable. A single automated workcell may generate predictable sound levels as it executes highly-repeatable process cycles. In contrast, the interaction of a variety of equipment, processes, and schedules can make predicting sound levels highly impractical. Quantifying such a variable soundscape may require the use of averages or other sound level definitions for a “typical” workday. For example, L10, L50, and L90 levels may be cited to describe the variation in the environment. These correspond to the sound level exceeded during 10%, 50%, and 90%, respectively, of the measurement period. For a “normal” workday, the notation L90,8hr, for example, may be used to specify the measurement period. Varying sound levels may also necessitate the use of dose calculations (see “Exposure Indices,” below).
Transient sounds exist only for a very brief period of time (often less than one second); these may also be called instantaneous sounds. There are two types of transient sounds – impact and impulse (or impulsive) sounds. Impact sounds are generated by the collision of two solid objects, such as parts falling into a bin. Impulse sounds are generated by rapid releases of energy, such as explosions, gunfire, or pressurized fluid release (i.e. burst or relief).
Transient sounds pose the greatest risk to hearing for three interrelated reasons. They often occur at sufficiently-high levels (e.g > 140 dB) to cause immediate permanent hearing loss. They are often unpredictable; therefore, proper protections may not be in place (e.g. use of HPDs) when they occur. They are also more difficult to measure, defying the accurate definition needed to implement appropriate countermeasures.
Results of some studies suggest that transient sounds exhibit a synergistic effect with continuous sound. That is, adding transient sounds to a “background” of continuous sound has a greater effect on hearing than SPL addition might indicate. This combined-sound scenario is common in industrial settings, reinforcing the need to pay special attention to transient sounds in the occupational soundscape.
Sound levels are measured in octave bands identified by their nominal center frequencies. An octave band is a range of frequencies where the upper end is twice the lower end (fupper = 2 x flower Hz). All sound transmitted at frequencies within an octave band are aggregated and associated with the band’s center, or nominal, frequency. A “full” octave is called a 1/1 octave band; the “1/1” designation is often dropped, as it is assumed when absent.
For a more-refined analysis of a soundscape, sound can be measured in 1/3 octave bands. As the name implies, each octave is subdivided into three bands, each with its own center frequency to which measured sound levels correspond. The 1/1 and 1/3 octave bands in common use for workplace noise assessments are tabulated in Exhibit 1. Measurements can be conducted in narrower frequency ranges, with the right instrument, but the 1/1 and 1/3 octave bands should suffice until noise control practitioners gain experience and sophistication.
Several frequency-weighted sound level scales have been developed for various purposes. Those relevant to hearing conservation programs are A-weighting and C-weighting. A-weighting (dBA) is used to model human perception of loudness of sound. C-weighting (dBC) is typically used in the assessment of hearing protection device (HPD) effectiveness. Z-weighting (dBZ) is often used when measuring exposures to transient sounds. It is an unweighted measurement scale (making it something of a misnomer); therefore, dBZ ≡ dB and the Z-weighting designation is often dropped.
The A- and C-weighting factors are tabulated in Exhibit 2 and shown graphically in Exhibit 3. Although 1/3 octave bands are shown, it is common practice to use 1/1 octave band (highlighted rows) measurements in workplace assessments.
An example data set converted to A- and C-weighted sound levels is shown in Exhibit 4. The sound level measurement in each frequency band is adjusted by the corresponding weighting factor. To determine the overall weighted sound level, the sound levels at all frequencies are added (see “SPL Addition” in Part 4). Comparing sound levels attained for the different scales demonstrates how significant weighting can be to assessment results.
B-weighting (dBB) is also included in the example for comparison, though it is no longer in common use. It was originally conceived of as a scale for “medium-level” sounds, but has failed to offer sufficient utility to justify its retention in addition to A- and C-weighting. D-weighting (dBD) was developed for the unique characteristics of aircraft noise. Its use is currently limited to non-bypass jet engines on military aircraft. It is included in the graph of Exhibit 3, but precise weighting values are not readily available.
If only A- and C-weighted sound levels (LA and LC, respectively) are known, some insight into the frequency composition of the sound can still be gained. When higher frequencies are prevalent, LA > LC and when lower frequencies are prevalent, LA < LC. When LA and LC are nearly equal, the sound is not dominated by frequencies at either end of the range. Review the weighting factors in Exhibit 2 or the graph in Exhibit 3 to see why this is true.
The magnitude of a person’s exposure to sound energy can be expressed in various ways. The choice of exposure index may be influenced by several factors, such as:
Exposure to continuous sound is well-defined by a single measurement, utilizing A-weighting and expressed in dBA. OSHA defines the maximum duration of exposure allowable at A-weighted SPLs from 80 – 130 dBA; this information is tabulated in Exhibit 5. This “reference duration” can also be calculated:
The valid range for this calculation is also 80 – 130 dBA. Below 80 dBA, indefinite exposure is acceptable; there is no appreciable risk to hearing. Continuous exposures above 130 dBA are impermissible for any duration (TOSHA, >130dBA = 0).
Other organizations determine the reference duration with different parameters. The generic form of the reference duration calculation is:
The exchange rate is also called the “doubling rate.” It is the increase in sound level exposure permissible when exposure duration is halved. Alternatively, it is the decrease in sound level exposure required to double the permissible exposure duration. OSHA’s exchange rate is 5 dB, while NIOSH prescribes a 3-dB ER.
NIOSH, in its “Criteria for a Recommended Standard,” provides some history of ER development and the current disparity between its specified exchange rate and that of OSHA. NIOSH demonstrates how a 3-dB ER accurately reflects the amount of sound energy a person is exposed to when conditions change. That is, the exchanged time and sound level are shown to be equivalent in energy terms. This equivalency is known as the equal-energy rule.
Intermittent sounds are not fully defined by a single measurement. The L10, L50, and L90 levels introduced previously can provide significant information about a soundscape, but its definition remains incomplete. To fully account for the entirety of exposure, a person’s “noise dose” can be determined. Doing so can require a large number of measurements if a person’s exposure changes frequently during a workday.
A person’s noise dose, D, is calculated as follows:
where Ci is the duration of exposure at a specific sound level and Ti is the reference duration at that sound level. The OSHA reference duration calculation is shown above; for NIOSH, use the generalized formula with Lc = 85 dB and ER = 3 dB.
A 100% noise dose indicates that a person’s average exposure to sound levels equals the permissible exposure limit (PEL). It is important to remember that a 100% dose may be reached long before the workday has ended!
If the variations in a person’s exposure are cyclical and, therefore, predictable, the anticipated daily noise dose can be determined with fewer measurements. For example, a worker performs a series of tasks that require two hours to complete and repeats the pattern four times per workday. The person’s exposure data for one 2-hour cycle can be extrapolated to an anticipated 8-hour dose using the dose rate, DR:
For our example, the worker receives a 25% dose in 120 minutes; DR = 0.21%/min. Multiplying by the full duration of the workday (480 min) yields a 100% dose (rounding error may be introduced). Use of dose rates can significantly reduce the monitoring workload in predictable soundscapes.
A time-weighted average (TWA) may be a more intuitive index. To convert a daily dose to an 8-hour TWA, the following calculation is performed:
The exchange rate (ER) and criterion level (Lc) must be consistent with the standard used to determine the dose (e.g. NIOSH, OSHA, etc.). At 100% dose, TWA = Lc. OSHA provides a table of converted values (see Exhibit 6) that may expedite TWA determinations.
An alternative representation of a person’s sound exposure is provided by the equivalent sound exposure level, LAeq,T, known in recent standards as the equivalent continuous sound level, LAT. Conceptually, LAT is the logarithm of the ratio of sound exposure to exposure duration. Exposure can be integrated over any time period; when the measurement period is eight hours, LAT ≡ TWA.
The pressure changes associated with transient sounds can occur so rapidly that the sound exposure can be integrated over a one-second time period. This “equivalent” measure is called the sound exposure level (SEL). Integrating the same exposure over an 8-hour time period yields an Leq that is 44.6 dB lower than the SEL (SEL = Leq,8hr + 44.6 dB), an indication of the ferocity of transient sounds.
Development of sound-measurement equipment over several decades has provided sophisticated measurement and analysis tools to industrial hygienists and researchers alike. A thorough treatment of these instruments and their capabilities is beyond the scope of this series. However, an introduction to the types of equipment commonly used to support hearing conservation in occupational settings could prompt participants to conduct necessary research and enlist the assistance of knowledgeable and experienced specialists.
Due to their use in musical and theatrical performances, public address systems, and other applications, microphones seem familiar to most people. However, few understand the principles of operation or why a particular microphone is used in each application.
Three types of microphones may be used in sound measurement, the choice depending on the nature of the sound field. Proper orientation of a microphone in a sound field is defined by the type in use; a pictorial summary is provided in Exhibit 7.
A direct-incidence microphone is placed parallel to the sound wave’s direction of travel, or “pointing at” the sound source. This type is also known as a free-field or normal-incidence microphone. This type of microphone is well-suited for use with a single sound source and in the absence of reflections.
A pressure microphone is positioned perpendicular to the sound wave’s travel. For this reason, this type is sometimes called a grazing microphone. Pressure microphones are often used in calibration of audiometric equipment or flush-mounting in a wall, baffle, or similar surface.
A random-incidence microphone should be oriented at approximately 70° from the primary sound source’s wave propagation direction. This type is also called an omni-directional microphone to describe its ability to process multiple sound sources and reflections simultaneously. It is this capability that makes them well-suited for many industrial environments.
In addition to its type, a microphone’s physical characteristics (e.g. diameter) can determine its suitability to a particular application. Exhibit 8 demonstrates this by comparing the frequency responses of several microphones. To get the best results, properties of the sound field and measurement equipment must be compatible.
Sound Level Meters
Though presented separately, above, microphones are typically integral to sound level meters (SLMs) used in occupational noise assessments. These are usually handheld devices that also carry a display and setup capabilities onboard. This discussion of SLMs is somewhat superficial by necessity. The technology behind their operation is far beyond the scope of this series; here, the focus is on basic functionality and operator interface.
Two classes of SLM are defined in current standards. Class 2 devices are “general purpose” units and may suffice for preliminary assessments and other applications with less-stringent requirements. Class 1 devices are precision instruments with higher performance characteristics. A comparison of specifications for Class 1 and Class 2 SLMs is provided in Exhibit 9.
Preparing an SLM for an assessment consists of selecting several parameters according to the sound field to be studied and the purpose of the study. Modern SLMs typically offer A-, C-, and Z-weighting options (see “Frequency Weighting,” above) to accommodate a variety of needs. The user can also choose to make 1/1 or 1/3 octave band measurements. Some models offer additional octave band options to refine measurements further.
The time constant (τ) to be used for measurements must also be selected. An SLM’s time constant is a measure of its responsiveness to changes in sound levels; the shorter the time constant, the more rapidly the instrument responds to changes in the input. The time constant is defined as the time required for a response curve to reach 63% of the maximum value of a step change in the input.
In the occupational noise context, a slow (S) time constant (τ = 1 s) is typically used to smooth the response curve when the input is highly variable. A fast (F) time constant (τ = 0.125 s) is used when the range of sound levels (i.e. min, max) is of greatest interest. If fast-response (F) readings fluctuate more than ±4 dB, the relative stability of slow-response (S) readings may be preferable.
Two other time constants are considered “legacy” settings and may not be available on all instruments. The impulse (I) time constant (τ = 0.005 s) is well-suited for measurements of impact noises. The peak hold (τ = 0.00005 s) setting is used to capture the maximum level of extremely-short-duration transient sounds, such as gunshots.
Compliance with a standard will dictate some or all of the settings and equipment types needed. Understanding the context of the noise assessment is critical to obtaining useful results.
A dosimeter is, in essence, a sound level meter with additional computing capability and a modified physical form. Either miniaturization of the instrument or separation of the microphone from the body of the unit makes it feasible for a person to wear a dosimeter for an entire workday without interfering with normal activities. Positioning a microphone in a worker’s “hearing zone” – within 12 in (30.5 cm) of the ear canal – is untenable with a standard SLM.
The requirements of various standards (OSHA, NIOSH, etc.) may be preprogrammed in the dosimeter; assessment per such standards are as simple as a setting selection. Some units allow custom profiles, called virtual dosimeters, to be programmed; various exchange rates, criterion levels, and threshold levels can be entered manually. The threshold level is the sound level below which the instrument does not integrate the exposure. Stated another way, sounds below the threshold level are not included in the noise dose calculation.
The large amount data that is often required for dose calculations is held in onboard memory until it can be transferred to a computer for further analysis or long-term retention. Lengthy measurement periods, in addition to onboard data processing, make battery life another important factor for consideration in the selection of devices.
Frequency analyzers, spectrum analyzers, real-time analyzers (RTAs), and similarly-named instruments are also variations on the SLM theme. Narrower octave bands, simultaneous measurements, graphical displays, and other features support advanced analysis. Further exploration of these devices here is unwarranted; readers are encouraged to investigate the capabilities of these instruments after mastering the foundational elements of sound measurement and hearing conservation covered in this series.
For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.
For a directory of “Occupational Soundscapes” volumes on “The Third Degree,” see Part 1: An Introduction to Noise-Induced Hearing Loss (26Jul2023).
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Jody W. Phelps, MSc, PMP®, MBA
JayWink Solutions, LLC
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