🔇Noise Control Engineering Unit 7 – Transportation Noise

What's Transportation Noise?

• Refers to the unwanted sound generated by various modes of transportation including road traffic, rail, and aviation
• Consists of a combination of engine noise, tire-road interaction, aerodynamic noise, and other mechanical sounds
• Characterized by its frequency content, duration, and intensity which can vary depending on the type of vehicle and operating conditions
• Considered a major environmental pollutant in urban areas contributing to reduced quality of life and potential health effects
• Can be quantified using various metrics such as sound pressure level (SPL), equivalent continuous sound level (Leq), and day-night average sound level (Ldn)
  • SPL represents the instantaneous sound pressure at a given location measured in decibels (dB)
  • Leq is the average sound level over a specified time period taking into account the total sound energy
  • Ldn accounts for the increased annoyance of noise during nighttime hours by applying a 10 dB penalty to sounds occurring between 10 pm and 7 am
• Influenced by factors such as traffic volume, vehicle speed, road surface, and distance from the source
• Propagation of transportation noise can be affected by atmospheric conditions, ground absorption, and the presence of barriers or reflective surfaces

Sources of Transportation Noise

• Road traffic noise generated by cars, trucks, buses, and motorcycles is the most prevalent form of transportation noise in urban environments
• Tire-road interaction is a significant contributor to road traffic noise especially at higher speeds (above 50 km/h) due to the rolling and friction of tires on the road surface
• Engine noise from internal combustion engines is dominant at lower speeds and during acceleration events such as at intersections or on steep grades
• Rail noise originates from various components of the train including the engine, wheels, and brakes as well as from the interaction between the wheels and the rails
  • Wheel-rail noise is caused by roughness on the wheel and rail surfaces leading to vibrations that radiate sound
  • Squeal noise can occur when trains navigate tight curves due to the lateral slip between the wheel and rail
• Aircraft noise is generated by the engines (jet or propeller) and the aerodynamic flow around the aircraft structure during takeoff, landing, and flyover events
  • Jet noise is caused by the turbulent mixing of high-speed exhaust gases with the surrounding air
  • Propeller noise is a combination of thickness noise (due to the displacement of air by the rotating blades) and loading noise (resulting from the forces acting on the blades)
• Other sources of transportation noise include horns, sirens, and public address systems used for communication or warning purposes

Measuring Transportation Noise

• Sound level meters are the primary tools used to measure transportation noise consisting of a microphone, preamplifier, and signal processing unit
• Measurements are typically conducted outdoors at a specified distance from the source (e.g., 15 m from the centerline of a road or 1.2 m above the ground)
• Time-averaging sound level meters can compute metrics such as Leq and Ldn over a desired period (e.g., 1 hour, 24 hours) to characterize the noise exposure
• Frequency analysis can be performed using octave or 1/3-octave band filters to determine the spectral content of the noise and identify dominant frequencies
• Traffic noise models (e.g., FHWA TNM, RLS-90) can predict noise levels based on input parameters such as traffic volume, speed, and road geometry
  • These models use a combination of empirical data and mathematical algorithms to estimate the noise emission and propagation from transportation sources
  • Model validation is performed by comparing predicted levels with actual measurements to assess the accuracy and make necessary adjustments
• Noise mapping techniques using geographic information systems (GIS) can visualize the spatial distribution of transportation noise levels over a large area
  • Noise contours representing lines of equal noise level are generated based on the model predictions or interpolation of measurement data
  • Noise maps can identify hotspots and evaluate the effectiveness of mitigation measures such as noise barriers or land-use planning strategies

Impacts on People and Environment

• Exposure to transportation noise can lead to annoyance, sleep disturbance, and stress-related health effects such as hypertension and cardiovascular disease
• Annoyance is a subjective response to noise that can interfere with daily activities, communication, and overall well-being
  • The degree of annoyance depends on factors such as the noise level, duration, frequency of events, and individual sensitivity
  • Dose-response relationships have been developed to quantify the percentage of highly annoyed individuals as a function of the noise exposure (e.g., Ldn)
• Sleep disturbance can occur when transportation noise events exceed a certain threshold (e.g., 45 dBA indoors) and cause awakenings or shifts in sleep stages
  • Chronic sleep disturbance can lead to daytime sleepiness, reduced cognitive performance, and long-term health consequences
• Cognitive impairment in children exposed to high levels of transportation noise has been observed including difficulties with reading comprehension, attention, and memory
• Wildlife and ecosystems can also be affected by transportation noise through changes in behavior, communication, and habitat use
  • Birds may alter their singing patterns or abandon noisy areas leading to reduced reproductive success
  • Marine mammals (e.g., whales, dolphins) rely on acoustic communication for navigation and social interactions which can be masked or disrupted by underwater noise from ships
• Economic impacts of transportation noise include reduced property values in noise-exposed areas and potential productivity losses due to health effects and decreased quality of life

Noise Control Strategies

• Source control measures aim to reduce noise at the point of generation through technological improvements and operational changes
  • Examples include quieter engines, improved tire designs, smooth road surfaces, and optimized train wheel profiles
  • Electric and hybrid vehicles have the potential to significantly reduce engine noise compared to traditional internal combustion engines
• Path control measures focus on interrupting the propagation of noise between the source and receiver
  • Noise barriers are the most common form of path control consisting of solid walls or earth berms placed along transportation corridors
    • Effective noise barriers typically reduce levels by 5-15 dB depending on their height, length, and proximity to the source and receiver
    • Absorptive materials (e.g., porous concrete, vegetation) can be used on the barrier surface to minimize reflections and improve performance
  • Building insulation and soundproofing techniques can reduce indoor noise levels by blocking or absorbing sound transmission through walls, windows, and doors
• Receiver control measures involve land-use planning and zoning strategies to minimize the number of people exposed to high levels of transportation noise
  • Locating sensitive land uses (e.g., residential areas, schools, hospitals) away from major transportation routes can reduce the impact of noise on these populations
  • Encouraging compatible land uses (e.g., commercial or industrial) near transportation facilities can provide a buffer zone and limit noise exposure
• Traffic management strategies can help reduce transportation noise by smoothing traffic flow and reducing congestion
  • Examples include speed limits, restrictions on heavy vehicles during nighttime hours, and the use of roundabouts instead of intersections with stop signs or traffic lights
• Operational procedures for aircraft such as noise abatement flight paths and preferential runway use can minimize noise exposure over populated areas
  • Continuous descent approaches and steeper climb-outs can reduce the time aircraft spend at low altitudes where noise is more noticeable
  • Shifting flight paths over less populated areas or bodies of water can help mitigate the impact of aircraft noise on communities

Regulations and Standards

• Noise regulations and standards aim to protect public health and welfare by setting limits on transportation noise levels and guiding noise control efforts
• The U.S. Federal Highway Administration (FHWA) has established noise abatement criteria (NAC) for different land use categories exposed to road traffic noise
  • The NAC range from 57 dBA Leq for lands on which serenity and quiet are of extraordinary significance to 72 dBA Leq for developed lands
  • If predicted or measured noise levels approach or exceed the NAC, noise abatement measures must be considered as part of highway projects
• The U.S. Federal Railroad Administration (FRA) has similar noise impact criteria for rail projects based on the increase in noise levels relative to existing conditions
• The International Civil Aviation Organization (ICAO) sets noise certification standards for aircraft based on their maximum takeoff weight and number of engines
  • Aircraft must meet increasingly stringent noise limits (Stage 3, Stage 4, Stage 5) to be certified for operation in member countries
  • Airports may also have their own noise abatement procedures and restrictions on aircraft operations to minimize community noise exposure
• The World Health Organization (WHO) has published guidelines for environmental noise exposure to protect against adverse health effects
  • The guidelines recommend noise levels below 45 dB Lnight (outdoor annual average) to prevent sleep disturbance and below 53 dB Lden (day-evening-night annual average) to avoid annoyance
• Local and state governments may have their own noise ordinances and zoning regulations to control transportation noise levels in specific areas
  • These can include restrictions on vehicle types, hours of operation, and maximum allowable noise levels at property boundaries
  • Enforcement of noise regulations often involves measuring noise levels using standardized procedures and imposing fines or penalties for violations

Case Studies and Real-World Examples

• The Big Dig in Boston, Massachusetts involved the construction of an underground highway system to replace an elevated expressway that was a major source of traffic noise
  • Noise barriers and absorptive materials were used in the tunnel design to minimize noise levels for nearby residents
  • The project also included the creation of new green spaces and parks above the tunnels, providing a buffer zone between the highway and adjacent neighborhoods
• The Øresund Bridge connecting Denmark and Sweden is a combined road and rail link that incorporates several noise reduction measures
  • The bridge deck includes noise barriers and absorptive coatings to minimize tire-road noise and reflections
  • The rail tracks are mounted on resilient pads to reduce vibration and structure-borne noise transmission
  • Underwater noise from the construction process was monitored to minimize impacts on marine life, particularly harbor porpoises
• The Schiphol Airport in Amsterdam, Netherlands has implemented a comprehensive noise management system to balance the needs of the airport and surrounding communities
  • The system includes a network of noise monitoring stations, a preferential runway use program, and noise abatement flight procedures
  • A noise insulation program provides funding for soundproofing homes and buildings in the highest noise exposure zones
  • The airport also has a noise quota system that limits the total annual noise exposure and incentivizes the use of quieter aircraft through higher landing fees for noisy planes
• The city of Phoenix, Arizona has adopted a Complete Streets policy that prioritizes the design of streets for all users, including pedestrians, bicyclists, and transit riders
  • The policy includes guidelines for reducing traffic noise through the use of landscaping, street trees, and traffic calming measures such as speed humps and chicanes
  • The city has also invested in the expansion of its light rail system, which provides a quieter and more sustainable alternative to automobile travel in the urban core

Future Trends in Transportation Noise

• The increasing adoption of electric and hybrid vehicles is expected to significantly reduce transportation noise levels, particularly in urban areas
  • Electric motors are inherently quieter than internal combustion engines, producing minimal noise at low speeds
  • However, tire-road noise will still be a dominant source at higher speeds, requiring continued efforts in tire and road surface design
• The development of autonomous vehicles (AVs) may lead to changes in traffic patterns and driving behaviors that could affect transportation noise levels
  • AVs have the potential to reduce congestion and smooth traffic flow through optimized routing and platooning, which could lead to lower noise levels
  • However, the increased availability and convenience of AVs could also lead to higher vehicle miles traveled (VMT) and potentially offset some of the noise reduction benefits
• The growth of urban air mobility (UAM) and the use of drones for passenger and cargo transport presents new challenges for transportation noise management
  • Electric vertical takeoff and landing (eVTOL) vehicles are being developed for short-range, intra-city travel, but their noise impact on urban environments is still largely unknown
  • Noise certification standards and operational guidelines will need to be established to ensure the compatibility of UAM with existing urban soundscapes
• Advancements in materials science and engineering could lead to the development of new noise reduction technologies
  • Meta-materials with unique acoustic properties could be used to design more effective noise barriers and absorptive coatings
  • Active noise control (ANC) systems that generate counter-noise to cancel out unwanted sounds could be integrated into vehicles and infrastructure
• The increasing use of big data and machine learning techniques in transportation planning and management could enable more targeted and effective noise mitigation strategies
  • Predictive models could be developed to identify areas with high noise exposure risk and optimize the placement of noise barriers and other control measures
  • Real-time noise monitoring networks could be used to dynamically adjust traffic flow and aircraft operations to minimize noise impacts on nearby communities