This section describes recommended procedures for the measurement of highway-related occupational noise exposure. Highway toll plaza and tunnel employees, highway maintenance and repair crews, and highway inspectors may be exposed to sound levels hazardous to hearing. Occupational noise exposure was developed to rate a person's susceptibility to hearing loss and to study noise environments that may be hazardous to hearing.(8) The following procedures are in accordance with ANSI S12.19-1996.(47)
For occupational noise exposures greater than 90 dB(A) in an 8-hour workday, the Occupational Safety and Health Administration (OSHA) requires mandatory hearing-conservation measures, such as audiometric testing or hearing protectors. OSHA defines a 90-dB(A) noise exposure as the criterion sound level, denoted herein by the symbol, LC; OSHA defines an 8-hour workday As the criterion duration, denoted herein by the symbol, TC.(48) A continuous criterion sound level over an entire criterion duration would result in 100 percent of an employee's allowable noise exposure. In addition, for exposures greater than 90 dB(A), some type of noise abatement action, such as machinery noise reduction via redesign or replacement, source/receiver isolation/enclosure, or employee exposure time limits, must be initiated.
For varying exposure durations, OSHA limits may be adjusted accordingly by the use of an exchange rate. For occupational noise exposure studies, OSHA requires the use of a 5-dB(A) exchange rate. In other words, for each additional 5 dB(A) of noise exposure up to 115 dB(A), the permitted duration is halved; for each reduction of 5 dB(A), the permitted duration is doubled. For example, if the noise exposure is 95 dB(A), a duration of 4 hours is permissible according to OSHA.
In addition, OSHA states that "exposure to impulsive or impact noise level should not exceed 140 dB." However, the regulations do not define what constitutes an impulsive or impact sound, nor do they address frequency weighting (See Section 126.96.36.199.2) of the measuring instrument, or whether the measurement uses one or none of the standard exponential time-averagings (See Section 188.8.131.52.4).(8) For the purposes of this document, it is recommended that the maximum A-weighted sound level, LAFmx, be used to ensure the 140 dB criterion is met.
For the purposes of noise exposure measurements, a noise dosimeter or a sound level meter can be used. To a certain degree, the particular instrument chosen will dictate the site-selection process.
The noise dosimeter should be worn by the employee during his/ her daily work routine. Its accompanying microphone should, preferably, be located on the employee's shoulder. If the employee is consistently exposed to noise from one particular side, the microphone should be placed on the associated Side. The microphone cable, which connects to the dosimeter, should be routed and fastened such that it does not interfere with the employee's safety or performance. The main body of the dosimeter may be located/attached to the employee's clothing at any convenient location. If the employee works at only one particular station, or if the employee will not be present during measurements, the dosimeter may be placed on a tripod at a representative position within the area.
Because of their larger size as compared with noise dosimeters, and due to the fact that they often do not have readily detachable microphones, sound level meters are often not logistically feasible to be worn directly by an employee. Consequently, they are typically positioned on a tripod within the work area. Specifically, the microphone should be positioned at a height approximately equal to that of the employee's head and as close as possible to the his/her ear. ANSI 12.19-1996 recommends a distance of 0.1 m (4 in) from the employee's ear, if feasible. In addition, the microphone should be placed Such that shielding by the employee or other objects is avoided. If the employee works at only one particular station, or if the employee will not be present during measurements, the microphone and sound level meter may be placed on a tripod at a representative position within the area.
The equivalent sound level, LAeq, and the duration of each measurement period should be recorded. The LAeq and the duration are then used to compute noise dose, which is, in turn, used to compute the time-weighted average sound level (LTWA(TC)), i.e., the employee's "noise exposure." As stated earlier, TC is the OSHA criterion duration of 8 hours. In addition, the maximum A-weighted sound level, LAFmx, should be recorded to ensure that the employee is not subjected to impulsive or impact noise levels greater than 140 dB(A).
The measurement duration should be sufficiently long, such that the resulting noise exposure is representative of the noise exposure associated with each task/location. For continually varying sound environments (sound level fluctuations greater ± 2.5 dB(A)), a longer sampling period is recommended. In most cases, noise exposure measurements are performed over a typical 8-hour work day.
Record the LAeq and the associated duration in addition to the LAFmx for each measurement period. Note: For a measurement to be considered valid:
(Note: Appendix B provides example field-data log sheets.)
The total noise dose for a typical workday is a summation of the individual task/location noise doses and is computed as follows:
D = 100 [nΣi=1 (Ci / Ti)] = 100[(C1 / T1) + (C2 / T2) + ... (Cn / Tn)] %
where: Ti = TC/2(LAeq, i - LC)/Q
The variables in the above equations are defined as follows:
|D||=Noise dose, expressed as a percentage;|
|Ci||=Measurement duration at task/location i;|
|Ti||=Permissible duration at task/location i;|
|LAeq,i||=Equivalent sound level measured during task/ location, i (Note: If the LAeq,i for a specific measurement period is below the OSHA-defined threshold level of 80 dB(A), it is not considered in the noise dose computation);|
|LC||=OSHA criterion level of 90 dB(A);|
|Q||=OSHA exchange rate of 5 dB(A); and|
|TC||=OSHA criterion duration of 8 hours;|
D = 100 [ (0.33/10.6) + (0.33/∞) + (2.6/8.0) + (3.5/1.0) + (1.24/0.66) + (2.0/4.0)] = 623.5%
The total noise exposure for a typical workday is computed as follows:
LTWA (TC) = [Q/log10(2)][log10(D/100)] + LC (dB)
The variables in the above equation are defined as follows:
|LTWA(TC) =||Noise exposure (time-weighted average sound level);|
|Q =||OSHA exchange rate of 5 dB(A);|
|D =||Noise dose, expressed as a percentage; and|
|LC =||OSHA criterion level of 90 dB(A).|
LTWA (8) = [5/log10(2)][log10(623.5/100)] + 90 =103.2 (dB)