1. Are there guidelines for noise exposure on shifts longer than 8 hours? 1) Equal energy rule Many regulatory agencies recommend a time- weighted average (TWA) sound level of 85 dB(A) to 90 dB(A) as a noise exposure limit for 8-hour work day. The ISO Standard 1999-1990 produced by the International Organization for Standardization recommends the use of the equal energy principle (3 dB exchange rate) in calculating the TWA for a work shift: Limit for a given shift = 90 - 10 log (T/8)

2. where T = duration of work shift in hours. Results of such calculation for various extended work shifts are listed in Table 1. Table 1 also shows the noise exposure limit for extended shifts when the 5 dB exchange rate is used. The formula used for calculating these exposure limits for extended shifts is: Limit for a given shift (5 dB rule) = 90 - 16.61 log (T/8)

3. Table 1 - TWA Method Work Shift Duration (Hours) Noise Exposure Limit, dB(A) [Criterion level = 90 dB(A)] Using 3dB exchange rate Using 5 dB exchange rate 890 989.589.2 1089.088.4 1188.687.7 1288.287.1 1387.986.5 1487.686.0 1587.385.5 1687.085.0

4. INSTRUMENT PERFORMANCE A. 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. 1. 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.

5. 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 don’t prefer to take result or concern the specification.

6. 1. 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:

7. Air Pressure Correction whe re: 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, C will be negative.

8. 1. 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). 2. 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.

9. When and how do you make corrections for background noise? Sometimes it is necessary to determine whether or not the background noise is influencing the total noise level measured when the noise source is "on". In such cases, two readings of noise level are taken - one with the noise source "on" and the other with the noise source "off". The following table can be used to determine noise level due to the noise source. For example if the total noise level is 97 dB and the background noise is 90 dB, the noise due to source is 96 dB (97-1). If the difference is more than 10 dB, no correction is needed.

10. Table 2 Background Noise Level Correction TOTAL NOISE LEVEL(dB) minus BACKGROUND NOISE LEVEL (dB) dB SUBTRACTED FROM TOTAL NOISE LEVEL TO GET NOISE DUE TO THE SOURCE 8 - 100.5 6 - 81 4.5 - 61.5 4 - 4.52 3.52.5 33