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Sound Meters

Recommended Standard: Occupational Noise Exposure

(NIOSH Publication No. 98-12, June 1998)

No single method or process exists for measuring occupational noise. Hearing safety and health professionals can use a variety of instruments to measure noise and can choose from a variety of instruments and software to analyze their measurements. The choice of a particular instrument and approach for measuring and analyzing occupational noise depends on many factors, not the least of which will be the purpose for the measurement and the environment in which the measurement will be made. In general, measurement methods should conform to the American National Standard Measurement of Occupational Noise Exposure, ANSI S12.19-1997 [ANSI 1996a]. More detailed discussions about instrumentation and measurement protocols appear in reference sources such as NIOSH [1973], Earshen [1986], Johnson et al. [1991], and Harris [1991].

Sound Level Meters

The sound level meter is the basic measuring instrument for noise exposures. It consists of a microphone, a frequency selective amplifier, and an indicator. At a minimum, it measures sound level in dB SPL. An integrating function may be included to automate the calculation of the TWA or the noise dose.

4.1.1 Frequency Weighting Networks
The human ear is not equally responsive to all frequencies; it is most sensitive around 4000 Hz and least sensitive in the low frequencies. The responses of the sound level meter are modified with frequency-weighting networks that represent some responses of the human ear. These empirically derived networks approximate the equal loudness-weighting networks or scales; some also have a B-scale. The A-scale, which approximates the ears response to moderate-level sounds, is commonly used in measuring noise to evaluate its effect on humans and has been incorporated in many occupational noise standards.

4.1.2 Exponential Time Weighting
A sound level meters response is generally based on either a FAST or SLOW exponential averaging. FAST corresponds to a 125-millisecond (ms) time constant; SLOW corresponds to a 1-s time constant. The meter dynamics are such that the meter will reach 63% of the final steady-state reading within one time constant. The meter indicator reflects the average SPL measured by the meter during the period selected. In most industrial settings, the meter fluctuates less when measurements are made with the SLOW response compared with the FAST response. A rapidly fluctuating sound generally yields higher maximum SPLs when measured with a FAST response. The choice of meter response depends on the type of noise being measured, the intended use of the measurements, and the specifications of any applicable standard. For typical occupational noise measurements, NIOSH recommends that the meter response on a sound level meter be set at SLOW.*

4.2 Noise Dosimeter

Measuring noise with a sound level meter is relatively simple when the noise levels are continuous and when the worker remains essentially stationary during the work shift. A noise dosimeter is preferred for measuring a worker's noise exposure when the noise levels are varying or intermittent, when they contain impulsive components, or when the worker moves around frequently during the work shift.

The noise dosimeter may be thought of as a sound level meter with an additional storage and computational function. It measures and stores the sound levels during an exposure period and computes the readout as the percent dose or TWA. Many dosimeters available today can provide an output in dose or TWA using various exchange rates (e.g., 3, 4, and 5 dB), 8-hr criterion levels (e.g., 80, 84, 85, and 90 dBA), and sound measurement ranges (e.g., 80 to 130dBA). The choice of FAST or SLOW meter response on the dosimeter does not affect the computed noise dose or TWA when the 3-dB exchange rate is used, but it will when other exchange rates are used [Earshen 1986].

In noise dosimetry, the microphone is attached on the worker whose exposure is being measured. The placement of the microphone is important in estimating the worker's exposure, as Kuhn and Guernsey [1983] have found large differences in the sound distribution about the body. ANSI [1996a] specifies that the microphone be located on the midtop of the worker's more exposed shoulder and that it be oriented approximately parallel to the plane of this shoulder.

4.3 Range of Sound Levels

OSHA requires that, for the purposes of the Hearing Conservation Amendment, all sound levels from 80 to 130 dBA be included in the noise measurements [29 CFR 1910.95(d)(2)(I)]. This range was specified on the basis of instrument capabilities available at that time [ANSI 1978], and OSHA had intended to increase the upper limit of the range to 140 or 150 dB as improved dosimeters became readily available [46 Fed. Reg. 4135 (1981b)].

To measure all sound levels from 80 to 140 dBA, a noise dosimeter should have an operating range of at least 63 dB and a pulse range of the same magnitude. In contrast, the ANSI S1.25-1991 standard specifies that dosimeters should have an operating range of at least 50 dB and a pulse range of at least 53 dB [ANSI 1991a]. Today, noise dosimeters with operating and pulse ranges in excess of 65 dB are quite common. Therefore, NIOSH considers that measuring all sound levels from 80 to 140 dBA with a noise dosimeter is technically feasible.

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