What's the difference between A and C Weightings? How about personal vs. in-ear dosimetry? We've put together this basic glossary of terms used around the world to get you started in exploring noise measurements, administrative and engineering controls.
A filter applied to noise measurements, intended to replicate the frequency sensitivity of the human ear. The A-weighting is the most commonly-used weighting scale in hearing conservation programs, as it predicts quite well the damage risk of the ear. Sound level meters set to the A-weighting scale will filter out much of the low-frequency noise they measure, similar to the response of the human ear. Noise measurements made with the A-weighting scale are designated dBA. If A-weighted measurements are used to determine adequacy of hearing protection, OSHA instructions stipulate that a correction factor of 7 dB should be subtracted from the NRR of the hearing protector (as an error cushion for C-minus-A differences), then subtract the resulting lower NRR from the dBA noise measure to determine the protected noise level for the worker. (see also C-Weighting)
Most regulations state that when noise exposures exceed mandated levels, engineering and administrative controls are to be the first line of defense in reducing exposures to acceptable levels. Administrative controls include such actions as giving noise-exposed workers breaks in quiet areas, or rotating employees into noisy jobs for short durations so that no single employee is overexposed. If such controls are not feasible or practical, personal protective equipment (earplugs and earmuffs) should be implemented. (see also Engineering Controls)
One of the two methods recommended by OSHA to measure noise exposures (compare with Personal Monitoring). In area monitoring, a sound level meter is used to measure instantaneous noise levels in a given area. This method of noise monitoring is only valid when noise levels are fairly constant in a given area, and where workers remain fairly stationary throughout their work shift; in areas of high worker mobility, or where noise levels fluctuate, personal monitoring is the preferred method.
A filter applied to noise measurements. In contrast to the A-weighting, the C-weighting is a “flatter” filter, and allows more low frequencies to be measured. The C-weighting was originally conceived to be the best predictor of the ear’s sensitivity to tones at high noise levels. But the ear’s risk to damage from noise has since been found to be predicted much better by the A-weighting scale. Noise measurements made with the C-weighting scale are designated dBC. If C-weighted measurements are used to determine adequacy of hearing protection, U.S. OSHA instructions state that the NRR of the hearing protector should be subtracted directly from the dBC noise measure to determine the protected noise level for the worker. (see also A-Weighting)
The noise level that defines a 100% dose (maximum allowable exposure) if workers are exposed to it continuously for 8-hours (or equivalent). Also called the Daily Exposure Limit. In current AUS/NZ standards, the Criterion Level is 85 dBA of noise exposure over an 8-hour Leq, or a peak level of 140 dBC. Exposures over the Criterion Level warrant protective actions.
The maximum noise level allowed, beyond which protective measures are mandated. Also called the Criterion Level. In current AUS/NZ standards, the Daily Exposure Limit is 85 dBA of noise exposure over an 8-hour Leq. Exposures over this level warrant protective actions – administrative or engineering controls, or mandatory use of hearing protection.
Unit of measurement used for sound levels. The decibel scale is a logarithmic, not a linear scale. Since the physical measurement of acoustic energy typically spans an enormous range (from zero into the trillions), a logarithmic scale based on exponents is handier for day-to-day measurement of noise. This means that small changes in decibel levels represent enormous changes in sound level. It also means that decibels cannot simply be added mathematically: 80 dB plus 80 dB equals 83 dB (not 160 dB).
A measured percent of allowable noise exposure. For example, a noise dose of 75% indicates a worker was exposed to noise equivalent to 75% of the criterion level for an 8-hour work shift. A 200% noise dose indicates the worker was exposed to the equivalent of twice the allowable criterion noise level for an 8-hour work shift. The dose calculation takes two factors into consideration: the criterion level, and the exchange rate.
A noise monitoring device that integrates exposures over time. When worn by a noise-exposed worker, a dosimeter measures all continuous as well as intermittent noise exposures, and provides a read out of the worker’s average exposure at the end of the monitoring period (usually a few hours, or an entire work shift). Since noise monitoring with a dosimeter is specific to the wearer, dosimetry is also referred to as personal monitoring.
According to many regulations, when noise exposures exceed mandated levels, engineering and administrative controls are to be the first line of defense in reducing exposures to acceptable levels. If such controls are not feasible or practical, personal protective equipment (earplugs and earmuffs) should be implemented. Engineering controls include such actions as installing acoustic enclosures, barriers, mufflers, dampers, vibration isolators, or acoustic treatment of walls and ceilings. Varying the feed pressure or drive speed of production machinery can also reduce noise levels. (see also Administrative Controls)
For most workers, noise exposures fluctuate throughout the workday. The Leq is a single number used to represent the equivalent amount of sound energy as the fluctuating noise levels. It typically represents the 8-hour equivalent dBA noise level, as though the fluctuating noise had been a steady noise for 8 hours. We intentionally avoid using the term average noise level, because decibels do not add arithmetically to create averages: a 4-hour exposure at 80 dB plus a 4-hour exposure at 90 dB is equivalent to an 8-hour exposure at 87 dB (not the average of 85 dB).
The increase (or decrease) in noise level in decibels which warrants a doubling (or halving) of the noise dose.
The physical measurement of the oscillations in a sound wave (measured in units called Hertz). Subjectively, we hear frequency as “pitch” of a sound. The frequency range that can be perceived by human hearing generally extends from 20 – 20,000 Hertz, but the sounds that are most useful to us (in the speech and conversation range) are in the narrower range from 300 – 3,000 Hertz. Audiometric tests administered in industry generally test hearing at six or seven different standardized frequencies: 500, 1000, 2000, 3000, 4000, 6000 and sometimes 8000 Hertz. In noise monitoring and audiometric testing, frequency is often measured in thousands of Hertz, or kilohertz (kHz).
Unit of measurement for frequency, equal to the number of oscillations (or cycles) per second of a sound wave. For example, the lowest note on a piano has a frequency of about 27 Hz (the thick wire-wound piano strings oscillate 27 times per second), while the highest note on a piano has a frequency of about 4,186 Hz (these thinnest strings oscillate over 4,000 times per second). In noise monitoring and audiometric testing, frequency is often measured in thousands of Hertz, or kilohertz (kHz).
Defined as noise bursts with peaks more than one second apart (as opposed to continuous noise with peaks less than one second apart). If the noise bursts occur very rapidly (noise from a running jackhammer, for example), the noise would not be considered impulse noise. Most industrial noise is steady and continuous, but much of it is impulse noise – a hammer or punch press, for example. When monitoring noise for OSHA compliance, all noise sources (both continuous and impulsive) are figured into the measurements. This is most easily accomplished by using a noise dosimeter.
Integrates a personal dosimeter into a hearing protection device. In-ear dosimeters, such as QuietDose™ by Howard Leight, measure and record a worker's actual noise dose over their entire work shift. When it is worn as hearing protection, it measures sound pressure levels interior to the protector. During periods when it is not worn, in-ear dosimeters measure the ambient noise level, which is equal to the exposure impinging on the ear. Overall, it accounts for the ear’s actual noise exposure, and provides real-time monitoring throughout the workday.
The daily noise exposure level at which precautionary measures must be in place. If noise levels exceed 80 dBA, then hearing protectors must be available to exposed workers (usage is voluntary) and training must be provided to exposed workers.
An analysis of noise levels broken down by component bands one octave apart in center frequency (e.g. 125, 250, 500, 1000, 2000, 4000, 8000 Hz). Such noise measurements offer more precise information about the spectrum of the noise, but are more time-consuming. Octave band analysis is also used to match appropriate hearing protection to specific noise environments, by comparing the published attenuation values (found on boxes or bulk packages of hearing protectors) with noise levels at each octave band of noise. In Appendix B of OSHA’s Hearing Conservation Amendment, this octave band analysis is referenced as NIOSH Method #1 for estimating the adequacy of hearing protector attenuation. Other methods (such as using the NRR) are simpler but less accurate.
In addition to the Daily Exposure Limit for noise exposures throughout the 8-hour workday, the AUS/NZ standards also define a maximum allowable single-event exposure: 140 dBC. No worker may be exposed without protection to peak noise levels above this limit. Note that the Peak Level is measured in dBC, not dBA, since the C-weighted scale is more accurate at high noise levels.
The maximum instantaneous value of a C-weighted noise measurement.
The maximum noise level allowed, beyond which protective measures are mandated. Also called the Criterion Level in Australia/New Zealand. In current OSHA regulations, the Permissible Exposure Limit is 90 dBA of noise exposure over an 8-hour time-weighted average. Workers exposed continuously at this level throughout a work shift will have a dose of 100%. Exposures over this PEL warrant protective actions – administrative or engineering controls, or mandatory use of hearing protection.
One of the two methods recommended by OSHA to measure noise exposures (the other being Area Monitoring). In personal monitoring, a noise dosimeter is used to average exposures over time. When worn by a noise-exposed worker, a dosimeter measures all continuous as well as intermittent noise exposures, and provides a read out of the worker’s average exposure at the end of the monitoring period. In contrast to area monitoring, personal monitoring is the required measurement method when there is significant fluctuation in noise levels, or high worker mobility, during the course of an employee’s work shift.
A sensitivity dampener that is built into the circuitry of most sound level meters. The Slow Response restricts the rapid fluctuations of a sound level meter, making it easier to read and less prone to transient noise. OSHA regulations state that noise measurements for hearing conservation purposes should be taken using the Slow Response mode.
A noise monitoring device that measures instant noise levels. Since noise monitoring with a sound level meter is specific to the immediate area where the measurement is being taken, these measurements are also referred to as area sampling. The input to a sound level meter can be filtered through different weightings (see A-weighting and C-weighting) to mimic the reception of the human ear. Optional attachments, such as Octave Band filters, can further restrict the noise measurement only to specific frequency bands. Sound level meters used for regulatory compliance must meet specifications in ANSI Standard S1.4-1971, “Specifications for Sound Level Meters.”
A computed average of all incoming sound levels, that represents what the average noise level would be if that level remained constant over an 8-hour work shift. For example, a worker wears a noise dosimeter for a 2-hour sampling period, during which time he works in fluctuating noise ranging from 83 dB to 98 dB. The resulting noise dose can be converted into an equivalent 8-hour time-weighted average. Even though the sampling period may be less than 8 hours, a time-weighted average allows us to apply the sample measurement to an 8-hour work shift. According to current OSHA standards, the Permissible Exposure Limit is an 8-hour time-weighted average of 90 dBA.
The daily noise exposure level at which protective measures must be in place. If noise levels exceed 85 dBA, the employer must ensure the use of hearing protectors among exposed workers (usage is mandatory).
Weighting refers to different sensitivity scales for noise measurement. Filters in the circuitry of noise meters (either sound level meters or noise dosimeters) affect the sound level reading, depending on which weighting (if any) is used. A noise measurement using no filters is called unweighted. Weightings allow the noise meter to respond more like the sensitivity of the human ear. The most common weighting scales used in hearing conservation are the A- and C-weightings. When used in noise measurements, these are designated dBA and dBC.