Appendix III:A-1. Noise Instrument Settings
The exchange rate is the increase or decrease in decibels (db) corresponding to twice (or half) the noise dose. For example, when using a 5 dB exchange rate, a dose of 90 dB is twice the dose of 85 dB, assuming that the duration of exposure is the same.
- Only instruments using a 5 db exchange rate may be used for OSHA compliance measurements.
- Noise dosimeters used by the Department of the Navy use a 4 dB exchange rate.
- Instruments used by the Department of the Army, the Department of Air Force, the National Institute for Safety and Health (NIOSH), the Environmental Protection Agency (EPA), and most foreign governments use a 3 db exchange rate. Additionally, the American Conference of Governmental Industrial Hygienists (ACGIH) Physical Agents Threshold Values (TLV) Committee revised its noise TLV to also use the 3 dB exchange rate.
The threshold level is the A-weighted sound level at which a personal noise dosimeter begins to combine noise into a measured exposure.
For example, if the threshold level on a sound level meter is set at 80 dB, it will capture and integrate all noise in the employee’s hearing zone that equals or exceeds 80 dB into the dose computation. Sound levels below this threshold would not be included in the computation of noise dose.
Differences between sound-measuring instrument settings must be taken into account when measuring employee noise exposure. A dosimeter with an 80 dBA threshold integrates all noise above 80 dB into the dose, and so the 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. (Note: In high noise levels (all noise above 90 dBA), the two dosimeters will read exactly the same.)
The criterion level is the permissible exposure limit (PEL).
- For OSHA purposes, the value for the criterion level is 90 dB, 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.
Additional factors to consider include frequency weighting and instrument response.
- For compliance measurements, use the A-weighted network and slow response setting exclusively. The computation of noise dose for compliance with OSHA noise standards includes the following types of noise:
- Continuous
- Intermittent (aircraft flyovers, automobile traffic, trains, mobile equipment, sporadically operated tools and machines)
- Impulse (drop forge hammer, dog barking, pistol shot, door slamming)
- Repeated impulse (riveting, pneumatic hammers, machine guns)
- The standard requires that all continuous, intermittent, and impulsive sound levels from 80 dB to 130 dB be included in the measurement of dose.
Appendix III:A-2. Sound Level Meter Calibration
Calibration refers to checking or adjusting the accuracy of the measuring instruments.
- All instruments must be calibrated (according to the manufacturer’s instructions) to ensure measurement accuracy. [29 CFR 1910.95(d)(2)(ii)]
- Calibration should be performed before and after each day of use and whenever the temperature or relative humidity changes significantly.
- Prior to and immediately after taking measurements, it is a good practice to check, using a calibrator, the ability of the sound level instrument to correctly measure sound levels.
As long as the sound level readout is within 0.2 decibels (dB) of the known source, it is suggested that no calibration adjustments be made. If large fluctuations in the level occur (more than 1 dB), then either the calibrator or the instrument may have a problem.
Appendix III:A-3. Effects of the Environment on Instrumentation
There are various environmental factors that can affect the performance of noise-measuring instruments and their readings, including:
Temperature
Sound-measuring equipment should perform within design specifications over a temperature range of -20 °F to 140 °F (-29 °C to 60 °C). If the temperature at the measurement site is outside of this range, refer to the manufacturer’s specifications to determine if the sound level meter or dosimeter is capable of functioning 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.
Humidity
Most 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 an exposure situation, refer to the manufacturer’s instructions or other noise professionals for technical support.
Atmospheric Pressure
Atmospheric pressure affects the output of sound level calibrators. 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 a pressure greater that its surroundings (for example, in underwater tunnel construction), use the following equation to correct the instrument reading:
Air Pressure Correction Equation
|
|||||||||||
| 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 | |||||||||
| NOTE: | For high altitude locations, C will be positive; in hyperbaric conditions (above atmospheric pressure), 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 (for example, 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 instruments must be used near such devices or operations, the extent of the field’s interference should be determined by consulting the manufacturer’s instructions.
Sound
Forsound level meters and noise dosimeters equipped with omni-directional microphones, the effects of the microphone placement and orientation are negligible in an environment that is typically reverberant.If the measurement site is non-reverberant and the noise source is highly directional, consult the manufacturer’s literature 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), use the unweighted peak mode setting of the sound level meter or equivalent impulse precision sound level meter.
Appendix III:A-4. Special Consideration for the Use and Care of Sound Level Meters and Dosimeters
Special considerations for use and care may include:
- Always check the batteries prior to use. Be very careful with the microphone cable. Never kink, stretch, pinch, or otherwise damage the cable.
- Use the microphone windscreen to protect the microphone when the wearer will be outdoors or in dusty or dirty areas. (The windscreen will not protect the microphone from rain or extreme humidity. Refer to the manufacturers instructions when using equipment in extreme conditions.)
- Never use any type of covering over the microphone (e.g., plastic bag or plastic wrap) to protect it from moisture. Such materials will distort the noise pickup, and the readings will be invalid.
- Never try to clean a microphone, particularly with compressed air, since damage is likely to result. Although dirt and exposure to industrial environments will damage the microphones, regular use of an acoustical calibrator will detect such damage so that microphones can be replaced.
- Remove the batteries when the dosimeter will be stored for more than 5 days. Protect dosimeters from extreme heat and humidity.
- No field maintenance is required other than replacement of batteries.
Appendix III:A-5. Dosimeter Settings
Settings
According to OSHA’s noise standard 29 CFR 1910.95, the noise dosimeter is the primary instrument for making compliance measurements. The following dosimeter settings must be utilized:
- Exchange rate: 5 decibels (dB)
- Frequency weighting: A
- Response: slow
- Criterion level: 90 dBA
- Threshold: 80 dBA or 90 dBA
A dosimeter with a threshold of 80 dBA as well as one with a threshold of 90 dBA should be used to measure noise exposures (most modern dosimeters utilize simultaneous 80 and 90 dBA threshold settings), as follows:
- The 80 dBA threshold dosimeter is used to measure the noise dose of those employees identified during the walkaround whose exposure may exceed the 85 dBA time-weighted average (TWA) limit.
- The 90 dBA threshold dosimeter is used to measure the noise dose of those employees identified during the walkaround whose exposure may exceed the 90 dBA permissible exposure level (PEL).
- Note: Paragraphs 29 CFR 1910.95(a) and 29 CFR 1910.95(b) of OSHA’s noise standard date back to the 1969 Walsh-Healey Act. This early standard predated noise dosimetry and OSHA had no instructions for taking noise measurements, and so the first dosimeters that were developed used 90 dBA both as the threshold and criterion levels. Paragraph 29 CFR 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.
Dosimeter Readout
The hypothetical exposure situations shown in the table below 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. An instrument with a 90 dBA threshold will not capture any noise below that level, and will thus give a readout of 0 percent even if the employee being measured is actually being exposed to 89 dBA for eight hours (equivalent to 87 percent of the allowable noise dose over any eight hour period).
| DOSIMETER READOUT, IN PERCENT OF MEASURED DOSE | ||
|---|---|---|
| Exposure conditions | Dosimeter with threshold set at 90 dBA | Dosimeter with threshold set at 80 dBA |
| 90 dBA for 8 hours | 100.0% | 100.0% |
| 89 dBA for 8 hours | 0.0% | 87.0% |
| 85 dBA for 8 hours | 0.0% | 50.0% |
| 80 dBA for 8 hours | 0.0% | 25.0% |
| 79 dBA for 8 hours | 0.0% | 0.0% |
| 90 dBA for 4 hours plus 80 dBA for 4 hours | 50.0% | 62.5% |
| 90 dBA for 7 hours plus 89 dBA for 1 hour | 87.5% | 98.4% |
| 100 dBA for 2 hours plus 89 dBA for 6 hours | 100.0% | 165.3% |
| * Assumes 5 dB exchange rate, 90-dBA PEL, ideal threshold activation, and continuous sound levels. | ||
Some dosimeters indicate when a 115 dBA (A-weighted decibel) sound level has been exceeded. Do not use this indication for compliance determination.
Using the Noise Dose Reading
The noise dose provided by dosimeters can be used to calculate both the continuous equivalent A-weighted sound level (LA) and the eight hour TWA for the time period sampled.
| Equation One: A-weighted Sound Level Calculation |
| LA = 16.61 log10 | D
12.5t |
+ 90 |
Equation Two:
Eight Hour TWA Sound Calculation
| TWA = 16.61 log10 | D
100 |
+ 90 |
| 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 eight hour time-weighted average in decibels (dBA) |
Equation Two is used for enforcement purposes and Equation One can be used to assist in evaluating hearing protectors and engineering controls. Note: Most dosimeters perform the above calculation and automatically provide data for LA and TWA.
Appendix III:B. General Sampling Protocol
Follow this sampling protocol:
- Inform the employee being monitored that the dosimeter should not interfere with his/her normal duties, and emphasize that the employee should continue to work as usual.
- Explain the purpose of the dosimeter to each employee being sampled and emphasize that the dosimeter is not a speech recording device.
- 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 it from its installed position. Inform the employee when and where the dosimeter will be removed.
- The microphone should be located in the employee’s hearing zone. OSHA defines the hearing zone as a sphere with a two-foot diameter surrounding the head. Clip the microphone to the employee’s clothing according to the manufacturer’s instructions. Most manufacturers recommend that the microphone be placed on the shoulder area midway between the head and the point of the shoulder. Practicality and safety will dictate the actual microphone placement at each survey location.
- Use the microphone windscreen to protect the microphone when the wearer will be outdoors or in dusty or dirty areas. (The windscreen will not protect the microphone from rain or extreme humidity).
- When noise levels at an employee’s two ears are different, the higher level must be sampled for compliance determinations.
- Position and secure any excess microphone cable to avoid snagging or inconvenience to the employee. If practical, the cord should be run under the employee’s shirt or coat.
- Check the dosimeter periodically to ensure that the microphone is properly oriented.
- Obtain and note sound level meter readings during different phases of work the employee performs during the shift. There is no minimum regarding the number of readings to obtain, but it is important to take enough readings to identify work cycles. For statistical reasons, more readings should be taken when noise levels fluctuate widely.
- Record the information required on the OSHA-92 Noise Survey Report.
Source: OSHA