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Jonathan Morris
Jonathan Morris

The Ultimate Guide to Double Exposure, the Reality-Altering Card Effect by Asi Wind



Also in 1969, the Bureau of Labor Standards promulgated an occupational construction noise standard under the Construction Safety Act, which was later adopted by OSHA in 1971. Soon after, in 1972, NIOSH published recommendations for an OSHA occupational noise standard, which included a recommended 8-hour TWA exposure limit of 85 dBA, an 80 dBA sampling threshold, and a 5-dBA exchange rate. However, in 1973, OSHA's Standards Advisory Committee maintained the 90-dBA 8-hour TWA, 90 dBA sampling threshold, and 5-dBA exchange rate. Even though noise energy exposure doubles every 3 dB, OSHA thought it important to account for the time during the workday that a worker was not exposed to noise hazards. At the time, using a 5-dB exchange rate was viewed as a sufficient way to account for this.




Asi Wind Double Exposure Download



Enclosures, or personnel shelters, can provide a cost-effective means for lowering worker noise exposure instead of lowering equipment noise levels. Control booths or rooms are commercially available from a number of manufacturers, many of which are listed in the Noise and Vibration Control Product Manufacturer Guide (see Section VII-Resources). The cost for these units typically ranges from $5,000 to $35,000 depending on the size and sophistication of their design and their need for electronic controls, video monitoring, number of observation windows, and other features. Any of the vendors listed in the manufacturer's guide can provide a cost estimate upon request. As a minimum requirement, all control rooms should maintain an interior sound level lower than 80 dBA, which will minimize worker noise exposure. Should there be a need to communicate with workers inside a control room, however, then a better design criterion would be to limit sound levels to 60 dBA or less.


Special consideration will need to be taken to determine microphone placement when monitoring workers who are wearing protective head gear such as abrasive blasting helmets or supplied air respirators. In many of these cases, the helmet/hood of this type of equipment would not be considered as a hearing protective device, and the microphone should be placed under the helmet/hood when measuring employee exposures. Care should be taken to ensure that the microphone does not contact surfaces inside the helmet/hood which may incur inaccurate noise measurements. Similarly, the microphone should also be positioned so that it is not located within any direct air streams such as from a supplied air respirator, which may also cause erroneous readings. Additional care may be necessary in running the dosimeter cable under any respirator or hood seals so that it does not interfere with such seals and as approved by the respirator manufacturer, as applicable. In some special cases, protective headwear such as abrasive blasting helmets may be considered as a secondary hearing protective device (earplugs worn under the helmet would be considered the primary hearing protective device). When considering the possibility for inclusion of the helmet as providing hearing protection, consultation with the manufacturer is necessary to determine the design, intent, and attenuation performance data associated with this scenario. If the helmet is determined to act as a hearing protector, the microphone should be placed outside the helmet when determining noise exposures and evaluating hearing protection worn by the employee. However, as previously mentioned, particular care is likely needed in order to protect the microphone in harsh environments; a wind screen would be necessary but for extremely harsh conditions it may not be feasible to position the microphone outside the hood/helmet. For questions related to assessing exposures and microphone placement associated with protective headwear and respirators, CSHOs should contact their regional OSHA office enforcement personnel or the OSHA Health Response Team for guidance, as necessary.


Workers are understandably curious about the noise dosimeter, and particularly the microphone. Take time to explain that it only collects information on how loud the sounds are--it does not record speech. Activate the dosimeter and replace its screen cover, or lock out the controls before the worker begins working. Take sound level measurements frequently during the course of the noise dosimetry. The sound level measurements document the noise in the area at specific points in time and from specific sources. These values both validate the dosimeter reading and provide insight into how and when exposure is occurring. Some noise dosimeters log data that can be downloaded to a computer and later graphed against time to show how the worker's noise exposure varies over the course of a shift. This is a useful feature, but is not a substitute for good notes on the workplace and the sources of noise in specific times and places.


Table V-3 and V-4 show various transmission loss values for common building materials at specific frequencies and material thicknesses. Note that the values in these tables are measured under ideal laboratory conditions as a resource for comparing different materials. In the workplace, the noise exposure experienced by the receiver would not actually be reduced by the reported transmission loss value, because imperfections in enclosures, barriers, or other noise controls made of these materials permit sound to go around the material, leak through cracks or utility paths, or pass through other materials with lower transmission loss values (e.g., a door jamb, window glass) that were also used in construction.


Dose (%): Related to the criterion level, a dose reading of 100% is the maximum allowable exposure to accumulated noise. For OSHA, 100% dose occurs for an average sound level of 90 dBA over an 8-hour period (or an equivalent exposure). If a TWA reading is used rather than the average sound level, the time period is no longer explicitly needed. A TWA of 90 dBA is the equivalent of 100% dose. The dose doubles every time the TWA increases by the exchange rate. Table A-1 shows the relationship between dose and the corresponding 8-hour TWA exposure.


Dual hearing protection involves wearing two forms of hearing protection simultaneously (e.g. earplugs and ear muffs). The noise exposure for workers wearing dual protection may be estimated by the following method: Determine the hearing protector with the higher rated NRR (NRRh) and subtract 7 dB if using A-weighted sound level data. Add 5 dB to this field-adjusted NRR to account for the use of the second hearing protector. Subtract the remainder from the TWA. It is important to note that using such double protection will add only 5 dB of attenuation. For a more extensive discussion of how to use the NRR, see the NIOSH website. NIOSH has developed guidelines for calculating and using the NRR in various circumstances (Hearing Protector Devices ). Also see 29 CFR 1910.95 Appendix B.


The RPBZ retrofit post base is designed to reinforce existing posts and columns. The single, versatile model will fit on any size post consisting of a double 2x4 or larger. RPBZ can also be used to reinforce new post-base connections, such as braced carports, patio covers, decks and other structures. The RPBZ can be installed with the CPS composite plastic standoff to meet a 1" post standoff code requirement. A single RPBZ can be installed on a post that is flush to a corner, and two RPBZs can be installed at away-from-edge conditions to fortify the post-base connection to resist both wind and seismic forces.


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