Most noise is not a pure tone, but rather consists of many frequencies simultaneously emitted from the source. To effectively evaluate the total noise of a most sources, it is usually necessary to measure it across its frequency spectrum. One reason for this is that people react differently to low, mid, and high-frequency sounds. Additionally, for the same sound pressure level, high-frequency noise is much more disturbing and more capable of producing hearing loss than low-frequency noise. Furthermore, engineering solutions to reduce or control noise are different depending on the predominant frequency of the noise. As a general guideline, low-frequency noise is more difficult to control.
One of the most common noise sources within manufacturing equipment is pneumatic- or compressed-air-driven devices such as air valves, cylinders, and solenoid valves. High-velocity air is also a major contributor to worker noise exposure where hand-held air wands or guns are used to remove debris from work areas. Finally, compressed air nozzles are often used to eject parts from a machine or conveyor line. All these forms of pneumatic systems generate undesirable noise as the high-velocity air mixes with the atmospheric air, creating excessive turbulence and particle separation. It is important to note that the intensity of sound is proportional to the air flow velocity raised to the 8th power. Therefore, as a source modification, it is recommended that the air-pressure setting for all pneumatic devices be reduced or optimized to as low a value as practical. As a general guideline, the sound pressure level can be reduced by approximately 6 dBA for each 30% reduction in air velocity. Additional noise controls for high-velocity air are presented in the retrofit and relocation sections below.
Just because a surface area vibrates, it is not correct to assume it is radiating significant noise. In fact, probably less than 5% of all vibrating panels produce sufficient airborne noise to be of concern in an occupational setting. However, vibration damping materials can be an effective retrofit for controlling resonant tones radiated by vibrating metal panels or surface areas. In addition, this application can minimize the transfer of high-frequency sound energy through a panel. The two basic damping applications are free-layer and constrained-layer damping. Free-layer damping, also known as extensional damping, consists of attaching an energy-dissipating material on one or both sides of a relatively thin metal panel. As a guide, free-layer damping works best on panels less than ¼-inch thick. For thicker machine casings or structures, the best application is constrained-layer damping, which consists of damping material bonded to the metal surface covered by an outer metal constraining layer, forming a laminated construction. Each application can provide up to 30 dB of noise reduction.
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.
The room shown in Figure 35 has been treated with absorption panels in the ceiling space. Note that adding this material to reduce the reverberant sound does not reduce the direct sound coming from the equipment: that sound will always exist, even if the equipment is placed outside, where little to no reflection exists. When treating a ceiling with absorptive material, a useful guideline is that the noise level will not be significantly reduced for workers at ground level when acoustical panels are installed at ceiling heights greater than 15 feet. In this situation, workers are most likely affected primarily by the direct sound wave. Vertically hung panels can create new problems, such as interference with ventilation, lighting, and sprinkler patterns. Also, for this form of treatment to provide a measurable noise reduction, the original room must be acoustically "hard." In other words, the room surfaces must be made of highly reflective materials, such as concrete or painted cinder block.
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.
General guideline 2: Workers' compensation costs for hearing loss average about 0.2% of payroll. (Workers' compensation averages about 2% of payroll; 10% percent of that is associated with hearing loss compensation.)
General guideline 3: Reducing compressed air pressure and volume used can reduce noise levels substantially and can also save on energy costs. It is almost always cost-effective. Other good opportunities for noise reduction are associated with routine maintenance and machine guarding (why not build in noise reduction at the same time?).
General guideline 1: Whenever possible, include noise control at the design phase (equipment or facilities). Considering noise exposure only at a later stage and then retrofitting existing equipment can cost more than 10 times as much as designing the noise control before construction begins. The cost of purchasing new production equipment comes into play somewhere between the two.
NASA developed a comprehensive program to guide quieter equipment purchases. This program, termed the "Buy-Quiet Process Roadmap," is part of the NASA EARLAB Auditory Demonstration Laboratory website.
General guideline 1: The cost of a dual-ear, full-disability claim across the United States reported in The Noise Manual (Berger et al., 2003) averages approximately $66,000 in 2011 dollars (assuming a long-term average of 4.2% inflation)
SHAs have flexibility in developing their noise policies and documenting the results of the noise studies. This guide provides technical guidance and is a tool for SHA practitioners to support and promote comprehensive and efficient reviews of highway traffic noise studies. This guide provides guidance for reviewing noise models developed with FHWA's Traffic Noise Model (TNM) version 3.0 (TNM 3.0). Some SHAs already provide guidance for TNM models review. This guide serves as a supplemental resource but is not intended to replace SHA guidance.
Section 2 of this guide provided a Tier 1 approach that provides basic details and checklists for reviewers performing accelerated reviews and includes information on how to review noise models developed using TNM 3.0. Section 3 of this guide provides a Tier 1 approach that provided in-depth advanced scenario details and checklists useful for detailed reviews of complex projects and reviews specific project scenarios using the National Cooperative Highway Research Program (NCHRP) Report 791, Supplemental Guidance on the Application of FHWA's Traffic Noise Model (TNM). Section 4 identifies challenges in TNM 3.0 analysis (Section 4). Appendix A includes a checklist tool for review of noise models.
TNM 3.0 differs from its predecessor, TNM 2.5, in the way it names models. TNM 2.5 used the term "run" to define a specific model while TNM 3.0 uses the term "project". Therefore, all references in this guide to "project" are for the TNM model and not the Type I project for which the noise study is being conducted.
Sections 2 and 3 are in a statement and response format. The reviewer can move through each section of a guide to evaluate the completeness and accuracy of a submitted noise model. The reviewer can use the associated TNM 3.0 noise modeling review checklist in Appendix A to determine if it provides all required information. A reviewer can complete a checklist for each report noting items that are complete and add notes on items that are missing, incorrect, or need attention.
The reviewer should also use the FHWA's review guide, Techniques for Reviewing Noise Analyses and Associated Noise Reports, to review the associated noise study report to ensure that the TNM modeling results are reported accurately.
FHWA TNM has no built-in function for modeling deceleration. NCHRP Report 311 defined two "zones of influence" (ZOIs) to represent the last two segments of a roadway being used to model deceleration, as illustrated in Figure 3-1. The reviewer should check NCHRP Report 791 for guidelines on the lengths of these segments, as a function of approach speed, and "equivalent speeds" to use for each vehicle type on each segment. Generally, the noise analyst reduces the modeled speed as the vehicles approach the roundabout.
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