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What are the Sources of Traffic Noise?

Noise pollution is the focus of various studies and research today due to its proven significant impact on human health and work efficiency. Research shows that traffic noise in urban areas has increased tremendously since the turn of the century, mainly due to increased transport of people and goods. It can be concluded that the biggest source of noise in urban areas is traffic noise, which accounts for 80% of all common noise sources. Traffic noise from road traffic is the most common type of noise in urban areas and therefore poses a serious problem.

According to information provided by the International Union of Railways (UIC), all types of trains produce less noise than trucks, cars, airplanes and other means of transport. The railway is the most suitable mode of transport in terms of environmental degradation and noise as an influential factor for human health. Therefore, it can be determined that the railway has the lowest noise share among other means of transportation in urban areas.

Traffic Noise Sources

Road Traffic Noise

Road traffic noise depends on the following three factors:

• Type of road vehicles

• Friction of vehicle wheels on the surface of the road

• Driving style and driver behavior.

When considering vehicles with an internal combustion engine (ICE) as a noise source, most of the noise comes from the following sources or systems. These resources and systems are explained in detail in the following paragraph. Engine noise is generated during the compression and expansion process in the engine, which creates engine vibrations which then emit noise. Engine noise depends on engine size, speed and capacity. Intake system noise is caused by the opening and closing of the intake valves, and also the intensity of this noise depends on the engine operating mode, engine speed and the type of air filter. Noise from the exhaust system is caused by the sudden release of gas into the exhaust system to open the exhaust valve. Fan noise is caused by the operation of the fans in the vehicle, and the fans often make a wideband noise. Tire noise occurs when tires and road surfaces come into contact. This type of noise depends on the type of road surface and tire structure.

In terms of noise pollution, electric vehicles represent the future, especially when compared to vehicles with internal combustion engines. However, at low speeds, electric vehicles generate very low levels of noise, meaning that the existing acoustics are practically inaudible in urban environments. For example, the noise level difference between an electric vehicle and an internal combustion engine (ICE) vehicle can be more than 6 dB (A) at 10 km/h. Unfortunately, much later at high speeds both types of cars become equally noisy, mainly due to tire noise. Considering how traffic flow affects the subjective perception of noise levels, it can be concluded that it depends on the number of vehicles, their speed and structure as explained in the paragraph below.

A traffic flow of 2000 vehicles per hour produces twice the perceived noise level than 200 vehicles per hour. If the traffic speed is 105 km/h, it produces twice the perceived noise level than 50 km/h traffic flow. A heavy vehicle (HV > 3.5 tons) at a speed of 70 km/h generates a perceived noise level from 28 light vehicles (AG < 3.5 tons).

Railway Traffic Noise

The main sources of rail traffic noise are noise from:

• Vehicles traveling on the railway

• Maneuvers

• Wagons

• Electromotor trains

• Motor trains

• Warning signals

Apart from the main sources mentioned above, there are several other important sources of noise:

• Drive systems for railway and rolling stock

• Interaction of wagon wheels, locomotives and trains with rails

• Braking operation

• Additional equipment such as ventilation, sirens, air conditioning and heating

• Aerodynamic noise, especially in high-speed trains

Noise in the drive system is mainly caused by the operation of the traction engine (intake and exhaust operation in the diesel engine, which is also the noisiest engine type), engine cooling system, transmission system and ventilation. Wheel-rail interaction produces the dominant noise in rolling stock and is directly dependent on the travel speed and the geometric configuration of the railway track. When moving on straight railway sections, the noise is mainly caused by the roughness of the wheel and rail surfaces, that is, their friction. When driving on railway bends, the wheels make more noise not only due to rolling but also due to the sliding of metal wheels, which can be seen as squeaking along the railway track. The reason for this phenomenon is the constructive nature of the wagons themselves, in which the wheels are fixed by parallel axles, so the outer wheels have to slip when passing a longer path than the inner wheels, therefore producing noise.

The noise produced by the braking process, in addition to the roughness of the wheels and track contact surfaces, is highly dependent on the type and shape of the brakes used. Noise from additional equipment is mostly produced by fans and their motors. It is also important to mention the noise produced by warning and notification signals. Aerodynamic noise is caused by the train passing through the airspace. The noise level produced by air turbulence on or near the moving train surface is logarithmically proportional to the train speed; therefore aerodynamic noise only matters at higher speeds.

Aircraft Traffic Noise

Aircraft traffic also causes various environmental problems, or in other words, noise increase. Nowadays, when the rapid development of all kinds of traffic, especially aircraft traffic, is observed, it can be concluded that there has been a significant increase in noise levels. In particular, the population living near airports is affected by the negative effects of exposure to noise. Aircraft noise can be divided into the following groups:

• Noise from different engine types

• Noise from aircraft structure

The sources of noise produced by engine groups are:

• Turbojet engine

• Turbofan engine

• Propeller drive (conventional or turbine engine)

The noise produced by a turbojet engine can be divided into the following groups:

• Compressor noise

• Exit jet noise

Turbojet engine noise was a major problem in the 1960s, especially the intake noise of this type of engine. The noise source of this type of input noise is the compressor blades. As technology has improved, airplanes have become quieter, and noise reduction in this sense continues even today. The turbofan engine is designed to reduce aviation noise levels. The biggest sources of noise in early turbofan engines are compressor, turbine and jet exhaust. Newer turbofan engine models have managed to reduce the noise levels mentioned above. The turbofan engine consists of blades and a turbojet engine. This type of engine is often used in the commercial aircraft industry.

Other Types of Noise Sources

Other sources of noise include industrial noise, noise from various construction works and noise produced by different music and sporting events. Industrial noise is the amount of acoustic energy received by the human hearing system while working in an industrial hall. Occupational noise or industrial noise is a commonly used term when it comes to occupational safety because prolonged exposure to such noise can cause various health problems (eg boredom, loss of concentration, sleep disturbances, headaches, etc.). The worst consequence of prolonged exposure to this type of noise is permanent hearing impairment. Considering all of the above, it can be concluded that this type of noise certainly affects work efficiency.

Given the noise from different construction sites, such noise can have extremely high noise levels. Also, given that there are many different stages of the construction process, such noise levels are very variable. Therefore, depending on the type and stage of construction, this category of noise can have internal and external sources of noise, and sometimes both at the same time. Activities at construction sites include noise produced by hand tools such as hammers, off-road trucks, cement mixers, cement cutters, chainsaws, welding machines and drills. Therefore, such noise poses a challenge for workers as well as the population located near the construction site. This type of noise can have the same health consequences as those described in the previous section for industrial noise.

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