Equipment noise impacts wildlife and people, and is a hazard to workers even more than the general public. Asking contractors to buy quieter equipment when they buy new equipment tops workers lists of requests; [N] unions, equipment manufacturers, and contractors are beginning to tackle the issue.[N]

Source controls, which limit noise emissions, are the most effective methods of eliminating noise problems. Source mitigation reduces the noise problem everywhere not just along a single path or for one receiver. Consequently, a project's noise mitigation strategy should emphasize noise control at the source. Source controls that limit noise emissions or restrict allowable types or operating times of heavy equipment are the easiest to oversee on a construction project.

The Noise Control Act of 1972 gives the Federal Environmental Protection Agency (EPA) the authority to establish noise regulations to control major sources of noise, including transportation vehicles and construction equipment. In addition, this legislation requires EPA to issue noise emission standards for motor vehicles used in Interstate commerce (vehicles used to transport commodities across State boundaries) and requires the Federal Motor Carrier Safety Administration (FMCSA) to enforce these noise emission standards. The EPA has established regulations that set emission level standards for newly manufactured medium and heavy trucks that have a gross vehicle weight rating (GVWR) of more than 4,525 kilograms and are capable of operating on a highway or street. For existing (in-use) medium and heavy trucks with a GVWR of more than 4,525 kilograms, the Federal government has authority to regulate the noise emission levels only for those that are engaged in interstate commerce. Regulation of all other in-use vehicles must be done by State or local governments.

NOAA Fisheries provided the following information on practices and conservation measures to reduce the effects of pile driving.[N] Systems successfully designed to reduce the adverse effects of underwater SPLs on fish have included the use of air bubbles. Both confined (i.e., metal or fabric sleeve or plastic sleeve) and unconfined air bubble systems have been shown to attenuate underwater sound pressures up to 28 dB.[N][N][N][N] When using an unconfined air bubble system in areas of strong currents, it is critical that the pile is fully contained within the bubble curtain. To accomplish this, adequate air flow and ring spacing both vertically and distance from the pile are factors that should be considered when designing the system. Recommended conservation measures to be included in project descriptions and specifications include:

• Install hollow steel piles with an impact hammer at a time of year when larval and juvenile stages of ESA-listed fish species or those with designated Essential Fish Habitat (EFH) are not present.
• If within the in-water work window described above, drive piles during low tide periods when located in intertidal and shallow subtidal areas.
• Use a vibratory hammer when driving hollow steel piles; however, impact hammers may be required for reasons of seismic stability or substrate type or to proof weight-bearing piles. Proofing is usually not required for non-weight bearing piles
• Monitor peak SPLs during pile driving to ensure that they do not exceed the 180 dB peak threshold for harm to fish (150 dBrms is protective with regard to harassment as defined under the ESA).[N]
• If sound pressure levels exceed acceptable limits, methods to reduce the sound pressure levels include, but are not limited to, the following:
• Surround the pile with an air bubble curtain system or air-filled cofferdam.
• Since the sound produced has a direct relationship to the force used to drive the pile, use of a smaller hammer should be used to reduce the sound pressures.
• Use a hydraulic hammer if impact driving cannot be avoided. The force of the hammer blow can be controlled with hydraulic hammers; reducing the impact force will reduce the intensity of the resulting sound.
• Drive piles when the current is reduced (i.e., centered around slack current) in areas of strong current to minimize the number of fish exposed to adverse levels of underwater sound.

Installing bubble curtains takes time and costs more, and can result in increasing the total amount of time the whole project takes to complete. Using a vibratory hammer can increase the amount of time it takes to drive a pile. Currently there is a trend to use fewer and larger piles on projects, requiring larger pile drivers, which in turn require that larger temporary piles be used to support the weight of the larger hammers.

Further research on the effects of noise mitigation measures and effectiveness in protecting fish life is underway and those who established the current thresholds believe thresholds will be raised when better information on pile driving effects is published.[N]

Instream and offshore blasting are used during bridge or culvert construction to fracture bedrock or free materials that are difficult to excavate. Fish near blast sites may be killed or severely injured as a result of swim bladder rupture, tissue and organ damage or internal bleeding. Fish habitat may be affected by changes in downstream water quality, sedimentation, or the physical destruction of habitat at the blast site. Blasting mitigation minimizes or eliminates the potential for negative effects on fish or fish habitat that might occur as a result of the instream or onshore use of explosives during bridge or culvert construction. The Alberta Transportation in Canada has developed the following best management practices for blasting: [N]

• Limit the charge size and detonation velocity. Shock wave intensity and blast radius may be minimized by keeping the weight of individually detonated charges small and by selecting explosives that minimize detonation velocity
• No explosive that produces, or is likely to produce, an instantaneous pressure change greater than 100 kPa (14.5 psi) in the swim bladder of a fish should be detonated in or near fish habitat. Setback distances from the land-water interface or burial depths from fish habitat are included in the Alberta's Construction BMP Fact Sheet on Blasting Practices.
• No explosive that produces, or is likely to produce, a peak particle velocity greater than 13 mm/s in a spawning bed should be detonated during incubation. Setback distance or burial depths are given in Alberta's Construction BMP Fact Sheet on Blasting Practices.
• Increase the delay between charges. For multiple charges, time delay detonators (blasting caps) should be used to reduce the overall detonation to a series of single explosions separated by a minimum of 25 millisecond delay.
• Perform blasting work during non-critical or less sensitive time periods for the fish. Avoid blasting during periods of fish migration, spawning and overwintering, when fish are often concentrated in smaller, critical habitats.
• Select blasting sites to minimize the blast area and any impacts to fish habitat. Blast in shallow water as substantial blast energy dissipation occurs as the shockwave reaches the water surface. Important fish habitat such as riffles or deep pools should be avoided
• Keep fish out of the blast area. Methods include scare blasting (detonation of a length of primer cord or a blasting cap, 30 to 60 seconds before the main blast); electrofishing to remove or scare away fish shortly before the blast; and setting block nets upstream and downstream of the blast area. The applicability of each method depends on site conditions (e.g., blocknets are only effective in small, slow moving streams). Care should be taken to avoid unplanned, dangerous detonations during pre-blast detonations and electrofishing
• Blastholes should be filled, or stemmed, with sand or gravel to grade or flush with streambed to confine the blast. Blasting mats should be placed on top of the holes to minimize the scattering of blast debris around the area.
• Ammonium nitrate-fuel oil mixtures (ANFO) should not be used in or near water due to the production of toxic by-products (ammonia).

The following best practices have been used in the U.S. ( Montgomery County MD, Boston's Central Artery), suggested by FHWA for night operations in particular, and used abroad in some cases. Great Britain, Australia, and Hong Kong have been active in implementation of construction noise control measures.

• Communicate with the surrounding community, early and often. Put a human face on the project and the company. Let people know what is happening and, most important, when it should be over. People are more tolerant when they know what to expect. As one noise control manager notes, excessive construction noise can motivate distressed communities to threaten a project's progress if not adequately managed; a project's "physical noise" can lead to generating more "political noise" than project managers may be able to handle.[N] Community outreach practices, and particularly those employed on Boston's "Big Dig," are discussed in greater detail under "Receiver Controls."
• Require construction operations planning. A construction noise control plan pro-actively evaluates anticipated construction noise consequences at all identified noise sensitive receptors within each contract area by: 1) identifying where and what type of construction equipment will be used during respective time periods, 2) predicting noise levels at receptor locations using accepted point-source-strength propagation algorithms, 3) comparing those predicted results against noise criteria limits, 4) if warranted, identifying proposed noise mitigation measures required to ensure compliance with the agency or project noise plan, and 5) demonstrating the expected beneficial noise reduction affects in both a qualitative and quantitative manner.[N] Emphasize noise control and "Work Quiet." Equal to all of the above is the awareness that noise control is an important part of the job. Everyone likes a good neighbor. Managers and supervisors should communicate that noise control is part of the job.
• Incorporate noise control considerations in all phases of project design and planning.
  For example, the project may specify:
  • Where practical and feasible, construction sites should be configured to minimize back-up alarm noise. For example, construction site access should be designed such that delivery trucks move through the site in a circular manner without the need to back up.
  • Limit Equipment On-Site — Have only necessary equipment on-site.
  • Restrict the movement of equipment into and through the construction site. Long-term impacts are generated along haul routes when there are large quantities of materials to be moved.
  • Re-route truck traffic away from residential streets.
  • Impose seasonal limitations on construction noise as spring and fall are critical times in residential areas due to windows left open at night.
• "Buy Quiet — Rent Quiet" ― Require modern equipment and quiet alternatives. Newer equipment is noticeably quieter than older models due primarily to better engine mufflers, refinements in fan design and improved hydraulic systems. Low noise equipment is also often of better quality and durability. The emission levels specified should reflect levels that can reasonably be achieved with well-maintained equipment. DOT specification of equipment noise emission limits forces the use of modern equipment having better engine insulation and mufflers. Most manufacturers can provide noise emission specs. Electric or hydraulic powered equipment is usually quieter than a diesel-powered machine. For example, electric tower cranes can be used instead of diesel power mobile cranes.
• Employ timing restrictions, such as those required by Montgomery County, Maryland: Between the hours of 7:00 a.m. and 5:00 p.m., Monday through Friday, noise from construction activities must not exceed 75 dBA, measured at the nearest receiving property line, but no less than 50 feet from the source. Several construction activities, such as demolition or pile driving, may inherently exceed 75 dBA, depending upon the circumstances. In those cases, the Ordinance allows up to 85 dBA, provided a Noise Suppression Plan is implemented. Essentially, these plans require the best reasonably available control technology or strategy. They may involve equipment selection, scheduling and temporary noise control devices to block or absorb the sound. At all times other than 7:00 a.m. to 5:00 p.m. weekdays, construction activities must meet the time of day receiving property line limits specified above, almost always 65 dBA daytime and 55 dBA nighttime. Construction activities are also subject to the Noise Disturbance provisions. Noise Disturbance incidents normally arise during the late night or early morning hours and involve delivering, loading and unloading equipment or materials and the associated back-up beepers. [N]

City councils in Arizona, Colorado, Florida, Illinois, Maine, New Jersey, Ohio, Tennessee, and Virginia have also imposed construction timing limits, limiting night and weekend work.[N]

• Schedule the more noise intense activities for less intrusive times, such as mid-morning to mid-afternoon, whenever possible.

  Shift work to weekend days rather than weeknights. Because of the operating limitations placed on the contractor due to high traffic volumes during the day, some work operations on the Boston Central Artery project were necessarily scheduled for nighttime periods. At the community's urging, project schedulers evaluated the implications of performing some of this necessary work on weekend days rather than at night during the week, where the cost for double-time work on Sundays added approximately $1 million to the project. Noise mitigation measures such as noise barriers were still used during these weekend shifts, but full compliance with Noise Spec limits could not be guaranteed during these accelerated weekend day work shifts.[N]
• Employ special nighttime equipment restrictions. Night construction is becoming increasingly common for urban widening/rehabilitation work where daylight construction closures of the routes cause unacceptable congestion problems. Heightened attention to noise control is particularly important at night. The human ear can judge sound beyond absolute terms, sensing the intensity of how many times greater one sound is to another. Because nighttime sound levels are generally lower than daytime levels and because nighttime noise can interrupt sleep, it is important to mitigate nighttime construction, including the following noise generators:[N]

Table 12 : Critical Nighttime Construction Noise Generators

Table 13 : Construction Equipment Noise Control Options

• Use a noise control specification. The Boston Central Artery and Tunnel Project Construction Noise Control Spec 721.56 is one of the most comprehensive noise control specifications in the country. It specifies a Noise Monitoring Plan be submitted prior to construction and every six months thereafter, calibration and certification of noise monitoring equipment, and Noise Measurement Reports on a weekly basis during construction including all noise level measurements taken during the previous week, construction compliance monitoring and any required complaint response investigations. Equipment Noise Compliance Certification measurements are required every six months or less if subsequent field inspection noise compliance measurements indicate that a given piece of equipment no longer meets its respective 50-foot noise emission limit. Construction noise limits are set and the spec requires the contractor to use equipment with efficient noise-suppression devices and employ other noise abatement measures such as enclosures and barriers necessary for the protection of the public. Work is required to be performed in a manner that prevents nuisance conditions such as noise that exhibits a specific audible frequency or tone (e.g., backup alarms, unmaintained equipment, brake squeal) or impact noise (e.g., jackhammers, hoe rams). The Engineer has the authority to make final interpretations on nuisance noise conditions and to stop work until nuisance noise conditions are resolved, without additional time or compensation for the Contractor. The requirements of the specification must be overseen by an approved Acoustical Engineer employed by the Contractor and the Noise Control Plan must be signed by him/her and include: contract-specific noise control commitments made previously by the Project, description of the anticipated construction activities, and an inventory of construction equipment and associated noise levels. The following example specifications have been used on the state and municipal levels:
  • Noise reduction materials may be new or used. Used materials should be of a quality and condition to perform their designed function.
  • Use concrete crushers or pavement saws for concrete deck removal, demolitions, or similar construction activity.
  • Pre-augur pile holes to reduce the duration of impact or vibratory pile driving.
  • Attach noise-deadening material to inside of hoppers, conveyor transfer points, or chutes.
  • Internal combustion equipment should be equipped with proper well-maintained intake and exhaust mufflers, shields, or shrouds. In particularly noise sensitive areas, use "critical" mufflers.
  • All equipment used on the construction site, including jackhammers and pavement breakers, shall have exhaust systems and mufflers that have been recommended by the manufacturer as having the lowest associated noise.
• Maintain equipment mufflers and lubrication.
• Maintain precast decking or plates to avoid rattling.
• Limit 1) the number and duration of equipment idling on the site; 2) the use of annunciators or public address systems; 3) the use of air or gasoline-driven hand tools.
• The use of impact pile drivers should be prohibited during evening and nighttime hours.
• The use of pneumatic impact equipment (i.e., pavement breakers, jackhammers) should be prohibited within 200 feet of a noise-sensitive location during nighttime hours.
• The local power grid should be used wherever feasible to limit generator noise. No generators larger than 25 KVA should be used and, where a generator is necessary, it shall have a maximum noise muffling capacity. All variable message/sign boards should be solar powered or connected to the local power grid.
• Engine idling for trucks should be limited to 5 minutes maximum.
  

The Boston Central Artery and Tunnel Project Construction Noise Control Spec 721.56 contains both "relative" noise criteria limits at identified noise-sensitive receptor locations as well as "absolute" noise emission limits for any/all specific equipment used on site. The Noise Spec's lot-line criterion states that construction induced L10 noise levels cannot exceed baseline (pre-construction) L10 noise levels by more than 5 dBA at identified noise-sensitive receptor locations. L10 noise limits are intended to address, and have in practice been shown to correlate well with, more steady construction noise averaged over some time interval (20 minutes). To be allowed to work on a job site, each piece of construction equipment must comply with Equipment Noise Emissions Limits (Lmax, dBA, slow, at 50 ft) that are also contained in the Noise Spec for various generic types of construction equipment. Construction equipment groups were assumed for the various phases of the work. Equipment assumed to work day and night included cranes, backhoes, loaders, dump trucks, concrete pumps, mixer trucks, delivery trucks, pneumatic tools, graders, pavers, compactors, and generators. Particularly loud equipment which was only assumed to operate during the day included pile drivers, jackhammers, how rams, and saws. Noise emission source strength levels were taken from CA/T Noise Spec databases that provided equipment Lmax emission levels expressed in A-weighted decibels (dBA, slow) at a reference distance of 50 feet. Equipment acoustic usage factors, or the percent of time the equipment is assumed to operate at full power, were taken from CA/T databases. Then, the noise contribution from each piece of equipment was projected over the distance from the equipment to each respective receptor location.[N]

The City of Seattle averted a major showdown with the company building the new Seattle Seahawks stadium. The city placed noise-specific rather than time-specific restrictions on the construction project, which would initially have disallowed construction from 7 a.m. to 6 p.m.[N]

• Employ measures to address and minimize back-up alarm issues, the most common public complaint, using OSHA approved alternatives. Consider 1) use of self-adjusting ambient-sensitive backup alarms, 2) manually-adjustable alarms on low setting, 3) use of observers, 4) scheduling of activities so that alarm noise is minimized, 4) construction site access should be designed such that delivery and dump trucks move through the site in a forward manner without the need to back up. "Smart Alarms" or video systems have also been used.
  
  Ambient-sensitive self-adjusting backup alarms increase or decrease their volume based on background noise levels. These alarms work best on smaller equipment such as backhoes and trucks. The alarm self-adjusts to produce a tone that is readily noticeable over ambient noise levels (a minimum increment of 5 decibels is typically considered readily noticeable), but not so loud as to be a constant annoyance to neighbors. The typical alarm adjustment is 82 or 107 dBA. Close attention must be give to the alarm's mounting location on the machine in order to minimize engine noise interference, which can be sensed by the alarm as the ambient noise level. These alarms should be mounted as far to the rear of the machine as possible. An alarm mounted directly behind a machine's radiator will sense the cooling fan's noise and adjust accordingly. Such a mounting will negate the purpose of the device.
  
  Manually-adjustable alarms are effective in reducing backup alarm noise nuisance but their use requires that each alarm be set at the beginning of each day and night shift. The manual setting feature eliminates the machine mounting location problem of the ambient-sensitive self-adjusting backup alarms. The manually adjustable alarms typically have an 87 and 107 dBA setting range, with the 87 dBA setting used for nighttime operations.
  
  Example specifications include the following:
  • All equipment with backup alarms operated by the Contractor, vendors, suppliers, and subcontractors on the construction site should be equipped with either audible self-adjusting ambient-sensitive backup alarms or manually adjustable alarms. The ambient-sensitive alarms shall automatically adjust to a maximum of 5 dBA over the surrounding background noise levels. The manually adjustable alarms should be set at the lowest setting required to be audible above the surrounding noise. Installation and use of the alarms should be consistent with the performance requirements of the current revisions of Society of Automotive Engineering (SAE) J994, J1446, and OSHA regulations, and as described in an Exhibit at the end of Division II Special Provisions.
• Purchase an approved sound level meter for self-monitoring and documentation.
• Operate at minimum power. Noise emission levels tend to increase with equipment operating power. This is a critical issue with older street sweepers, demolition work using a hoe-ram, and equipment such as vac-trucks. Require that such equipment operate at the lowest possible power levels necessary to get the job done. This saves fuel too.
• Use noise monitoring methods identified in Boston Central Artery and Tunnel Project Construction Noise Control Spec 721.56. Results were submitted on a standard form and plotted in 24-hour noise measurements showing L10 and Lmax noise levels vs. time along with appropriate lot-line criteria limits for daytime, evening, and nighttime periods. Diagrams of the location of noise measurement equipment in relation to noise monitoring locations were required, including the location of all construction equipment operating during the monitoring period and the distance between the noise measurement location and the construction equipment. Activities occurring while performing noise measurements are noted, such as "auger banging on ground to clean soil from threads" or "heavy traffic passing near the sound level meter." Any noise level of 85 dBA or greater requires an explanation. Elements of the Boston Central Artery's noise control spec related to monitoring include the following.[N]

Example 9 : Monitoring Related Elements of the Boston Central Artery Noise Control Specification

Noise Monitoring - General

1. The sound level meter and the acoustic calibrator should be calibrated and certified annually by the manufacturer or other independent certified acoustical laboratory. The sound level meter should be field calibrated using an acoustic calibrator, according to the manufacturer's specifications, prior to and after each measurement.

2. All measurements should be performed using the A-weighting network and the "slow" response of the sound level meter.

3. The measurement microphone should be fitted with an appropriate windscreen, should be located 5 feet above the ground, and should be at least 5 feet away from the nearest acoustically-reflective surface.

4. Noise monitoring shall not be performed during precipitation or when wind speeds are greater than 15 mph, unless the microphone is protected in such a manner as to negate the acoustic effects of rain and high winds.

Background Noise Monitoring

Background noise measurements (in dBA, slow ) should be collected for at least 24 hours over two non-consecutive days Monday through Saturday and one Sunday at noise monitoring receptor locations as specified in paragraph 1.06.B.1 prior to the start of construction. Background noise measurements should be performed in the absence of any contributing construction noise for each of the noise monitoring receptor locations identified in Table 3 and Figure 1 of this Section. Background noise L10 levels should be arithmetically averaged into single L10 levels defining the background noise for daytime (7 AM - 6 PM ), evening (6 PM - 10 PM ), and nighttime (10 PM - 7 AM ) time frames, respectively.

Construction Noise Monitoring

1. Noise level measurements should be taken at each noise-sensitive location during ongoing construction activities at least once each week during the applicable daytime, evening, and nighttime period. All other noise monitoring locations as specified in paragraph 1.06.B.1 should be measured at least once each week during the daytime period.

2. The time period for each noise measurement should be 20 minutes.

3. Construction noise measurements shall coincide with daytime, evening, and nighttime periods of maximum noise-generating construction activity, and should be performed during the construction phase or activity that has the greatest potential to exceed noise level limitations as specified in Article 1.04 of this Section. Compliance noise measurements for the noise limits in Table 1 should be performed at a point on a given lot-line which is the closest to the construction activity.

4. If, in the estimation of the person performing the measurements, outside sources contribute significantly to the measured noise level, the measurements should be repeated with the same outside source contributions when construction is inactive to determine the ambient noise level contribution.

5. All measurements should be taken at the affected lot-line. In situations where the Work site is within 50 feet of a lot-line, the measurement should be taken from a point along the lot-line such that a 50 foot distance is maintained between the sound level meter and the construction activity being monitored.

6. Up to four 24-hour noise monitors should be maintained at the lot-line of noise receptor locations and shifted among locations corresponding to construction activity as directed by the Engineer. These monitors should be capable of recording the Lmax and L10 values in 20-minute intervals over 24-hour periods. These monitors should be durable and enclosed in weather resistant cases, and located in a manner that will prevent vandalism.

• Employ on-site technician to ensure compliance with noise control requirements. The cost to retain several staff as noise patrol and community liaisons on the Central Artery Project was estimated to amount to $1 million over the four years the contract; such positions undertook the following:[N]
  • Performing special noise studies and project-change impact analyses, such as evaluating noise consequences through measurements and predictive modeling and preparing noise sections for Notice of Project Change (NPC) regulatory filings.
• Overseeing contractor compliance with contract-specific noise limits by:
• Performing short-term and long-term noise compliance monitoring
• Providing a presence in the field during nighttime periods (Noise Patrol). Many noise complaints were proactively avoided and better management and control of conditions in the field were accomplished through the use of a dedicated noise technician to patrol the project at night. Should the project receive a noise complaint, the noise technician is able to immediately respond to the scene and investigate the circumstances that led to the complaint. The noise technician is empowered to intervene directly and shut down otherwise unmitigatably noisy operations that are exceeding Noise Spec limits and/or causing noise nuisances.
• Ensuring that contractors are fulfilling their noise control plans.
• Being prepared to shut down otherwise unmitigatably noisy work at night.
• Providing technical and field support to construction managers by:
• Responding to and supporting resident engineers to keep work progressing.
• Documenting contractor noise compliance for QA purposes.
• Presenting noise issues before the city and affected communities.
• Training field staff on noise issues, measurement, evaluation, and control through presentation of the CA/T Noise Control Workshop to all field staff and providing on-site mentoring and mitigation recommendations.
• Providing defendable technical advice in noise-related legal challenges, such as preparing expert witnesses for supporting courtroom testimony, defending the project's position when challenged by abutters, and documenting reasons to avoid contractor claims for noise-related work stoppages.
• Developing noise mitigation programs and strategies for policy adoption, such as developing area-specific noise mitigation measures (noise sheets), designing large-scale noise barrier/curtain systems, developing and implementing an acoustical window treatment program, developing noise-related policies (e.g. Off-Site Mitigation Policy).
• Incorporate noise control costs in appropriate parts of the project. CA/T primary elements as follows: 1) restricting certain noisy equipment from night work, 2) the provision of extensive noise barriers and noise curtain systems, 3) an expansion of the successful bedroom window acoustical treatment program, 4) a prohibition of backup alarms at night, and 5) an option to perform some work on Sundays (at a cost premium) that would otherwise need to be done at night due to traffic restrictions. In all, these noise mitigation measures cost the Project an estimated $2-3 million.[N] Noise control costs were absorbed into different parts of the project, per specification. Payment for the Noise Monitoring Plan and first Noise Control Plan were considered part of the payment for Mobilization. Payment for the 6 month Noise Control Plans, equipment certifications, and complaint response and weekly construction noise monitoring reports were considered part of the payment for related Construction. Payment for temporary noise and acoustical barriers and noise control curtains were at the Contract unit price per square foot, which included constructing, providing, placing, maintaining, moving, relocating, and disposing of temporary noise barrier walls.

Source noise controls are frequently inadequate at adequately minimizing noise impacts on abutting sensitive receptors because of the close proximity to residences and businesses in urban areas and because of the very nature of the construction work. When source controls are inadequate, controlling noise radiation along its transmission path should be considered a second line of defense.

Once established, only reflection, diffraction insulation, or dissipation can modify an airborne sound field. In other words, it is necessary to increase the distance from the source or to use some form of solid object to either destroy part of the sound energy by absorption, or to redirect part of the energy by wave deflection.

Noise path barriers should provide a substantial reduction in noise levels, should be cost effective, and should be implementable in a practical manner without limiting accessibility. Barriers can increase a project's visual impact and thus aesthetic effects must be considered as well, when designing barrier systems.

Path control measures include:

• Move equipment farther away from the receiver. Some noisy activities may be able to be moved farther away from receptors.
• Use landscaping as a shield and dissipator; however, research conducted by FHWA has shown that very dense trees or shrubs would be needed (100 feet deep and much taller than can normally be achieved) to get noise reductions in the 1-3 dBA range. [N]
• Enclose especially noisy activities or stationary equipment with noise barriers or curtains. Noise barriers are semi-permanent or portable wooden or concrete barriers. Noise curtains are flexible intervening curtain systems hung from supports. Enclosures encase localized and stationary noise sources. Enclosures can provide a 10 to 20 dBA sound reduction. Additionally the visual impact of roadwork activities has an affect on how construction sounds are perceived. An important noise mitigation issue, therefore, is the audio-visual sensing factor. It is common to require all jackhammers and pavement breakers used at the construction site to be enclosed with shields, acoustical barrier enclosures, or noise barriers. Example specifications include:
  • Noise reduction materials may be new or used. Used materials should be of a quality and condition to perform their designed function.
  • Noise reduction equipment and materials may include, but not be limited to:
  • Shields, shrouds, or intake and exhaust mufflers.
    Noise-deadening material to line hoppers, conveyor transfer points, storage bins, or chutes.
    

Specifications and materials descriptions are from Boston Central Artery and Tunnel Project Construction Noise Control Spec 721.56, an excerpt follows: [N]

Example 10 : Specifications and Materials Descriptions for Barriers from the Boston Central Artery and Tunnel Construction Noise Control Spec

Temporary Noise Barriers
The Contractor shall erect temporary noise barriers to mitigate construction noise at locations specified in the Noise Control Plan or as directed by the Engineer. Temporary noise barriers should be readily moveable so that they may be re-positioned, as necessary, to provide noise abatement for non-stationary, as well as stationary, processes.

A. Temporary barriers should be constructed of 3/4-inch Medium Density Overlay (MDO) plywood sheeting, or other material of equivalent utility and appearance having a surface weight of 2 pounds per square foot or greater. The temporary noise barriers shall have a Sound Transmission Class of STC-30, or greater, based on certified sound transmission loss data taken according to ASTM Test Method E90.

B. The temporary barriers should be lined on one side with glass fiber, mineral wool, or other similar noise curtain type noise-absorbing material at least 2-inches thick and have a Noise Reduction Coefficient rating of NRC-0.85, or greater, based on certified sound absorption coefficient data taken according to ASTM Teat Method C423.

C. The materials used for temporary barriers should be sufficient to last through the duration of construction for this Contract, and should be maintained in good repair.

D. Construction Details

1. Barrier panels should be attached to support frames constructed in sections to provide a moveable barrier utilizing the standard "Temporary Precast Concrete Median Barrier" for the Project, as shown on Standard Drawing SD-H-401 and SD-H-403 for Construction Barricade, or other supports designed to withstand 80 mph wind loads plus a 30 percent gust factor.
2. When barrier units are joined together, the mating surfaces of the barrier sides should be flush with each other. Gaps between barrier units, and between the bottom edge of the barrier panels and the ground, should be closed with material that will completely fill the gaps, and be dense enough to attenuate noise.
3. The barrier height should be designed to break the line-of-sight and provide at least a 5 dBA insertion loss between the noise producing equipment and the upper-most story of the receptor(s) requiring noise mitigation. If for practicality or feasibility reasons, which are subject to the review and approval of the Engineer, a barrier can not be built to provide noise relief to all stories, then it must be built to the tallest achievable height.

E. Prefabricated acoustic barriers are available from various vendors. An equivalent barrier design can be submitted as specified in paragraph 1.03.G in lieu of the plywood barrier described above.

F. Installation, Maintenance, and Removal

1. The barriers should be installed such that the noise-absorptive surfaces face the construction noise source.
2. The Contractor shall maintain the temporary noise barriers and repair all damage that occurs, including, but not limited to, keeping barriers clean and free from graffiti and maintaining structural integrity. Gaps, holes, and weaknesses in the barriers, and openings between or under the units, should be repaired promptly or replaced by the Contractor with new material.
3. The Contractor shall remove and dispose of the temporary noise barriers at the end of the Contract or sooner at the direction of the Engineer.
   

Acoustical Barrier Enclosures

A. Materials

1. The acoustical barrier enclosure shall consist of durable, flexible composite material featuring a noise barrier layer bonded to sound-absorptive material on one side.
2. The noise barrier layer shall consist of rugged, impervious material with a surface weight of at least one pound per square foot. The sound absorptive material shall include a protective face and be securely attached to one side of the flexible barrier over the entire face.
3. The acoustical material used should be weather and abuse resistant, and exhibit superior hanging and tear strength during construction. The material shall have a minimum breaking strength of 120 lb/in. per FTMS 191 A-M5102 and minimum tear strength of 30 lb/in. per ASTM D117. Based on the same test procedures, the absorptive material facing shall have a minimum breaking strength of 100 lb/in. and a minimum tear strength of 7 lb/in.
4. The acoustical material should be corrosion resistant to most acids, mild alkalies, road salts, oils, and grease.
5. The acoustical material should be fire retardant and be approved by the City of Boston Fire Department prior to procurement. It shall also be mildew resistant, vermin proof, and non-hygroscopic.
6. The acoustical material shall have a Sound Transmission Class of STC-25 or greater, based on certified sound transmission loss data taken according to ASTM Test Method E90. It shall also have a Noise Reduction Coefficient rating of NRC-0.70 or greater, based on certified sound absorption coefficient data taken according to ASTM Test Method C423.
7. The Contractor shall submit the name of the manufacturer, properties of the material to be furnished, and two one-foot square samples to the Engineer for review prior to submittal of design and detailed engineering as specified in paragraph 1.03.G.

B. Construction Details

1. The acoustical barrier enclosure should be designed similar to the example shown in Exhibit II-C, "Construction Noise Control Specification and Guidelines".
2. The acoustical material should be installed in vertical and horizontal segments with the vertical segments extending the full enclosure height. All seams and joints shall have a minimum overlap of 2 inches and be sealed using double grommets. Construction details should be performed according to the manufacturer's recommendations.
3. The Contractor should be responsible for the design, detailing, and adequacy of the framework and supports, ties, attachment methods, and other appurtenances required for the proper construction of the acoustical barrier enclosure.
4. The design and details for the acoustical noise barrier enclosure framework and supports should be prepared and stamped by a Registered Professional Engineer licensed in the Commonwealth of Massachusetts. The Contractor shall submit the design and detailed engineering drawings to the Engineer as specified in paragraph 1.03.G.

Noise Control Curtains

A. Materials

1. The noise control curtain shall consist of durable, flexible composite material featuring a noise barrier layer bonded to sound-absorptive material on one side. The noise barrier layer shall consist of a rugged, impervious material with a surface weight of at least one pound per square foot. The sound absorptive material shall include a protective face and be securely attached to one side of the flexible barrier over the entire face.
2. The noise curtain material used should be weather and abuse resistant, and exhibit superior hanging and tear strength during construction. The curtain's noise barrier layer material shall have a minimum breaking strength of 120 lb/in. per FTMS 191 A-M5102 and minimum tear strength of 30 lb/in. per ASTM D117. Based on the same test procedures, the noise curtain absorptive material facing shall have a minimum breaking strength of 100 lb/in. and a minimum tear strength of 7 lb/in.
3. The noise curtain material should be corrosion resistant to most acids, mild alkalies, road salts, oils, and grease. It also should be mildew resistant, vermin proof, and non-hygroscopic.
4. The noise curtain material should be fire retardant and be approved by the City of Boston Fire Department prior to procurement.
5. The noise control curtain shall have a Sound Transmission Class of STC-30 or greater, based on certified sound transmission loss data taken according to ASTM Test Method E90. It shall also have a Noise Reduction Coefficient rating of NRC-0.85 or greater, based on certified sound absorption coefficient data taken according to ASTM Test Method C423.
6. The Contractor shall submit the name of the manufacturer, properties of the material to be furnished, and two one-foot square samples to the Engineer for review prior to submittal of the design and detailed engineering drawings as specified in paragraph 1.03.G.

B. Construction Details

1. The noise control curtains should be designed such as described in an Exhibit at the end of Division II Special Provisions, "Construction Noise Control Specification and Guidelines." The curtains should be secured above, at the ground, and at intermediate points by framework and supports designed to withstand 80 mph wind loads plus a 30 percent gust factor.
2. The curtains should be installed in vertical and horizontal segments with the vertical segments extending the full curtain height to the ground. All seams and joints shall have a minimum overlap of 2 inches and be sealed using Velcro or double grommets spaced 12 inches on center. Curtains should be fastened to framework and guardrails with wire cable 12 inches on center. Construction details should be performed according to the manufacturer's recommendations.
3. The curtain height should be designed to break the line-of-sight and provide at least a 5 dBA insertion loss between the noise producing equipment and the upper-most story of the receptor(s) requiring noise mitigation. If for practicality or feasibility reasons, which are subject to the review and approval of the Engineer, a curtain system can not be built to provide noise relief to all stories, then it must be built to the tallest achievable height.
4. The Contractor should be responsible for the design, detailing, and adequacy of the framework and supports, ties, attachment methods, and other appurtenances required for the proper installation of the noise control curtains.
5. The design and details for the noise control curtains framework and supports should be prepared and stamped by a Registered Professional Engineer licensed in the Commonwealth of Massachusetts. The Contractor shall submit the design and detailed engineering drawings to the Engineer as specified in paragraph 1.03.G.

 

The Central Artery Project noise specification outlines a complaint procedure to ensure that public and agency complaints are addressed and resolved consistently and expeditiously. If the Contractor receives a complaint regarding construction noise, the Contractor must immediately notify the Engineer and the Interim Operations Center (IOC) or successor to the IOC. Upon receipt or notification of a noise complaint from the Engineer, the Contractor must promptly perform noise measurements at the complainant's location during activities representative of the offending operation. The noise measurements must be performed using equipment and methods specified and reported as specified, and immediately submitted to the Engineer. In the event that the measured noise level exceeds allowable limits or result in nuisance conditions, the Contractor must immediately use noise reduction materials and methods such as, but not limited to, those described in the specification, to reduce noise levels or to alleviate the nuisance conditions.