Threshold and criterion levels
The threshold level is the A-weighted sound level at which a personal noise dosimeter begins to integrate noise into a measured exposure. For example, if the threshold level on a sound level meter is set at 80 decibels (dB), it will capture and integrate into the computation of dose all noise in the employee's hearing zone that equals or exceeds 80 dB. Sound levels below this threshold would not be included in the computation of noise dose.
The criterion level is the continuous equivalent 8-hour A-weighted sound level that constitutes 100% of an allowable noise exposure. In other words, the criterion level is the permissible exposure limit (PEL). For OSHA purposes, this is 90 decibels, averaged over an 8-hour period on the A scale of a standard sound level meter set on slow response. Noise measurements taken with an instrument set on the A weighting scale are expressed as dBA.
Paragraphs 29 CFR 1910.95(a) and (b) of the OSHA Occupational Noise Exposure Standard date back to the 1969 Walsh-Healey Act. Because this early standard predated noise dosimetry and OSHA had no instructions for taking noise measurements, the first dosimeters that were developed used 90 dBA both as the threshold and criterion levels.
Paragraph 1910.95(c) of the 1983 Hearing Conservation Amendment to the Occupational Noise Exposure Standard requires employers to administer a continuing, effective hearing conservation program for all employees whose noise exposures equal or exceed an 8-hour time-weighted average (TWA8) of 85 dBA or, equivalently, a noise dose that is equal to 50 percent of the PEL. The standard requires that all continuous, intermittent, and impulsive sound levels from 80 dB to 130 dB be included in the measurement of dose. In other words, the threshold level for noise measurement purposes is 80 dBA.
Differences between sound-measuring instrument settings must be taken into account when measuring employee noise exposure. For example, because a dosimeter with an 80 dBA threshold integrates all noise above 80 dB into the dose, such a dosimeter will report a higher noise dose than the dose reported by a dosimeter with a 90 dBA threshold if both instruments are used side-by-side to evaluate the same noise exposure (see Table 1).
The noise dose provided by dosimeters can be used to calculate both the continuous equivalent A-weighted sound level (LA) and the 8-hour TWA for the time period sampled, using the following formulas:
Equation 1a LA Sound Calculation
Equation 1b TWA Sound Calculation
LA = the continuous equivalent A-weighted sound level in decibels for the time period sampled
D = dosimeter readout in percent noise dose
t = the sampling time in hours
TWA = the 8-hour time-weighted average in decibels, dBA
Equation 1b is used for enforcement purposes, and Equation 1a can be used to assist in evaluating hearing protectors and engineering controls.
The exchange rate is the increase or decrease in decibels corresponding to twice (or half) the noise dose. This means that the sound level of 90 dB produces twice the noise dose that 85 dB produces (assuming that duration is held constant). The OSHA exchange rate is 5 dB (see Table D-2 of the construction noise standard, 29 CFR 1926.52, and Tables G-16 and G-16a of the general industry noise standard, 29 CFR 1910.95).
Only instruments using a 5-dB exchange rate may be used for OSHA compliance measurements. CSHO's should be aware that noise dosimeters used by the Department of the Navy use a 4-dB exchange rate, while instruments used by the Department of the Army and the Department of the Air Force use a 3-dB exchange rate. Instrument used by the National Institute for Occupational Safety and Health (NIOSH) and the Environmental Protection Agency (EPA), as well as most foreign governments also use a 3-dB exchange rate. Additionally, the ACGIH Physical Agents Threshold Limit Values (TLV) Committee recently revised its noise TLV to also use the 3-dB exchange rate.
The hypothetical exposure situations shown in Table 1 illustrate the relationship between criterion level, threshold, and exchange rate and show the importance of using a dosimeter with an 80-dBA threshold to characterize an employee's noise exposure. For example, an instrument with a 90-dBA threshold will not capture any noise below that level, and will thus give a readout of 0% even if the employee being measured is actually being exposed to 89 dBA for 8 hours (i.e., to 87% of the allowable noise dose over any 8-hour period).
Chronic noise-induced hearing loss is a permanent sensorineural condition that cannot be treated medically. It is initially characterized by a declining sensitivity to high-frequency sounds, usually at frequencies above 2,000 Hz.
Exposure of a person with normal hearing to workplace noise at levels equal to or exceeding the PEL may cause a shift in the worker's hearing threshold. Such a shift is called a standard threshold shift and is defined as a change in hearing thresholds of an average 10 dB or more at 2,000, 3,000, and 4,000 Hz in either ear. Workers experiencing standard threshold shifts are required by 29 CFR 1910.95(g)(8) to be fitted or refitted with hearing protectors, trained in their use, and required to use them.
In addition to effects on hearing, noise:
Interferes with understanding speech;
Causes a stress reaction;
Interferes with sleep;
Interferes with concentration; and
Effects of the environment
Temperature, humidity, atmospheric pressure, wind, and dust can all affect the performance of noise-measuring instruments and their readings. Magnetic fields can also affect the performance of instruments. Each of these factors is discussed below.
Temperature. Sound-measuring equipment should perform within design specifications over an ambient temperature range of -20°F to 140°F (-29°C to 60°C). If the temperature at the measurement site is outside this range, refer to the manufacturer's specifications to determine if the sound level meter or dosimeter is capable of performing properly.
Sound-measuring instruments should not be stored in automobiles during hot or cold weather because this may cause warm-up drift, moisture condensation, and weakened batteries, all of which can affect instrument performance.
Humidity. OSHA noise instruments will perform accurately as long as moisture does not condense or deposit on the microphone diaphragm. If excessive moisture or rain is a problem in a given exposure situation, the Assistant Regional Administrator (ARA) for Technical Support should be consulted.
Atmospheric Pressure. Both atmospheric pressure and temperature affect the output of sound level calibrators; atmospheric pressure is the more important of these two factors. When checking an acoustical calibrator, always apply the corrections for atmospheric pressure that are specified in the manufacturer's instruction manual.
In general, if the altitude of the measurement site is less than 10,000 feet above sea level, no pressure correction is needed. If the measurement site is at an altitude higher than 10,000 feet, or if the site is being maintained at greater-than-ambient pressure (e.g., in underwater tunnel construction), use the following equation to correct the instrument reading:
Equation 2 Air Pressure Correction
C = correction, in decibels, to be added to or subtracted from the measured sound level
t = temperature in degrees Fahrenheit
B = barometric pressure in inches of mercury
For high altitude locations, C will be positive; in hyperbaric conditions, C will be negative.
Wind or Dust. Wind or dust blowing across the microphone of the dosimeter or sound level meter produces turbulence, which may cause a positive error in the measurement. A wind screen should be used for all outdoor measurements and whenever there is significant air movement or dust inside a building (e.g. when cooling fans are in use or wind is gusting through open windows).
Magnetic Fields. Certain equipment and operations, such as heat sealers, induction furnaces, generators, transformers, electromagnets, arc welding, and radio transmitters generate electromagnetic fields that can induce current in the electronic circuitry of sound level meters and noise dosimeters and cause erratic readings. If sound level meters or dosimeters must be used near such devices or operations, the extent of the field's interference should be determined by consulting the manufacturer's instructions.
Effects of sound
For sound level meters and noise dosimeters equipped with omnidirectional microphones, the effects of microphone placement and orientation are negligible in a typically reverberant environment. If the measurement site is nonreverberant and/or the noise source is highly directional, the manufacturer's literature should be consulted to determine proper microphone placement and orientation.
For determining compliance with the impulse noise provision of 29 CFR 1910.95(b)(1) or 29 CFR 1926.52(e), the unweighted peak mode setting of the sound level meter, or equivalent impulse precision sound level meter should be used.
Several sound measuring instruments are available to CSHO's. These include noise dosimeters, sound level meters, and octave-band analyzers. The uses and limitations of each kind of instrument are discussed below.
Noise Dosimeter. The noise dosimeters used by OSHA meet the American National Standards Institute (ANSI) Standard S1.25-1978, 'Specifications for Personal Noise Dosimeters,' which set performance and accuracy tolerances. For OSHA use, the dosimeter must have a 5-dB exchange rate, use a 90-dBA criterion level, be set at slow response, and use either an 80-dBA or 90-dBA threshold gate, or a dosimeter that has both capabilities, whichever is appropriate for the evaluation.
Sound Level Meter.
a. All sound level meters used by OSHA meet ANSI Standard S1.4-1971 (R1976) or S1.4-1983, 'Specifications for Sound Level Meters,' which set performance and accuracy tolerances. Sound level meters are used for the following purposes:
To spot-check noise dosimeter performance;
To determine an employee's noise dose whenever a noise dosimeter is unavailable or inappropriate;
To identify and evaluate individual noise sources for abatement purposes;
To aid in the determination of the feasibility of engineering controls for individual noise sources for abatement purposes; and
To evaluate hearing protectors.
b. For practical purposes, this procedure should be followed for all sound level measurements:
The microphone should be in the monitored employee's hearing zone. OSHA defines the hearing zone as a sphere with a two-foot diameter surrounding the head. Considerations of practicality and safety will dictate the actual microphone placement at each survey location.
When noise levels at an employee's two ears are different, the higher level must be sampled for compliance determinations.
Sound level readings in a nonreverberant environment should be taken in accordance with the manufacturer's instructions.
Octave-Band Noise Analyzers.
a. The Type 1 sound level meters (such as the GenRad 1982 and 1983 and the Quest 155) used by OSHA have built-in octave band analysis capability. These devices can be used to determine the feasibility of controls for individual noise sources for abatement purposes and to evaluate hearing protectors.
b. Octave-band analyzers segment noise into its component parts. The octave-band filter sets provide filters with the following center frequencies: 31.5; 63; 125; 250; 500; 1,000; 2,000; 4,000; 8,000; and 16,000 Hz.
c. The special signature of a given noise can be obtained by taking sound level meter readings at each of these settings (assuming that the noise is fairly constant over time). The results may indicate those octave-bands that contain the majority of the total radiated sound power.
d. Octave-band noise analyzers can assist CSHO's in determining the adequacy of various types of frequency-dependent noise controls. They also can be used to select hearing protectors because they can measure the amount of attenuation offered by the protectors in the octave-bands responsible for most of the sound energy in a given situation.
e. The Assistant Regional Administrator for Technical Support can provide assistance in the use of octave-band analyzers.
For compliance purposes, readings with an ANSI Type 2 sound level meter and readings with a noise dosimeter are considered to have an accuracy of ±2 dBA. Readings with ANSI Type 1 sound level meter are considered to have an accuracy of ±1 dBA. For unusual measurement situations, refer to the manufacturer's instructions and appropriate ANSI standards for guidance in interpreting instrument accuracy.
Calibrate all noise-measuring instruments according to the manufacturer's instructions before and after each day of use and whenever the temperature or relative humidity changes significantly.
For compliance measurements, the A-weighted network and slow response setting shall be used exclusively. All continuous, intermittent, and impulse noise is included in the computation of noise dose for compliance with all OSHA noise standards.
The results of the walkaround survey are used to assist the CSHO in planning the workshift noise sampling strategy.
Walkaround Survey. When screening for noise exposures, sound level meter measurements and estimates of exposure duration are sufficient. The resulting spot readings can be used to determine the need for a more complete evaluation. A sound level meter may be useful for this purpose.
a. If the results of the walkaround survey indicate that OSHA's noise standard may be exceeded, the noise exposures of representative employees from each job classification potentially overexposed should be sampled.
b. The CSHO should use a dosimeter with a threshold of 90 dBA as well as one with a threshold of 80 dBA to measure the noise exposure of employees identified as potentially overexposed during the walkaround.
c. The CSHO shall use the 80-dBA threshold dosimeter to measure the noise dose of those employees identified during the walkaround as having noise exposures that are in compliance with Table G-16 of 29 CFR 1910.95 but whose exposure may exceed the levels specified in Table G-16a. The CSHO shall use the dosimeter with a threshold of 90 dBA to measure the noise dose of those employees identified during the walkaround as having potential noise exposures that exceed the sound levels in Table G-16 of 1910.95 or Table D-2 of 1926.52.
Note: 29 CFR 1926 does not contain the requirements of the hearing conservation amendment (29 CFR 1910.95, paragraphs (c) through (p)). In other words, 29 CFR 1926.52 addresses only allowable noise doses.
d. As a minimum strategy, the CSHO shall conduct sampling for the time necessary to establish that an exposure above the limits permitted by Tables G-16, G-16a, or D-2 exists (in general industry or construction workplaces, respectively), taking instrument accuracy into account.
CSHO's evaluating employee exposures to workplace noise should:
a. Inform the monitored employee that the dosimeter should not interfere with his/her normal duties, and emphasize that the employee should continue to work in a routine manner.
b. Explain to each employee being sampled the purpose of the dosimeter, and emphasize that the dosimeter is not a speech recording device.
c. Instruct the employee being sampled not to remove the dosimeter unless absolutely necessary and not to cover the microphone with a coat or outer garment or move the microphone from its installed position. Inform the employee when and where the dosimeter will be removed. When the dosimeter is positioned (generally in the shirt pocket or at the waist), clip the microphone to the employee's shirt collar at the shoulder, close to the employee's ear. Clips should be placed in accordance with the manufacturer's instructions.
d. Position and secure any excess microphone cable to avoid snagging or inconveniencing the employee. If practical, the cord should be run under the employee's shirt or coat.
e. Check the dosimeter periodically to ensure that the microphone is oriented properly.
f. Obtain and note sound level meter readings during different phases of the work performed by the employee during the shift. Take enough readings to identify work cycles. For statistical reasons, more readings should be taken when noise levels fluctuate widely.
g. Record the information required on the OSHA-92.
Some dosimeters indicate when a 115-dBA sound level has been exceeded. This indication is not to be used for compliance determination.