1.10.2 - Noise reduction

Version 5


    Driver education


    How people drive, and particularly eco-friendly driving, can significantly reduce car noise pollution, increase fuel efficiency, and reduce greenhouse gas emissions. The main principles of eco-friendly driving that apply to noise reduction are the following:


    • Start slowly
    • Use lower engine speeds (for example, use 4th rather than 3rd gear at 50 km/h)
    • Respect speed limits
    • Drive smoothly (don’t accelerate or brake abruptly)
    • Maintain a steady speed
    • Turn the engine off for stops longer than 30 seconds
    • Eliminate extra weight and items that create wind-resistance (luggage carriers, roof racks, etc.).
    • Regularly check tire pressure (monthly for example).


    Awareness-raising workshops and eco-friendly driver training programs which teach these different techniques are becoming more widespread within companies, communities, and now the general public. The increasing popularity of eco-friendly driving is due to three main factors:


    • The integration of eco-friendly driving techniques into the driving test.
    • Companies providing specific training and developing Company Travel Plans in order to increase employee safety, reduce fuel consumption and fleet maintenance costs, and optimize travel time.
    • Governments, local and regional authorities, non-profit organizations, and road safety organizations developing activities and information or communication campaigns to promote safer and more eco-friendly driving.


    Example: In France, May 2013, the Paris Police Department conducted an extensive noise control campaign for two-wheelers in the Paris metropolitan area. Several hundred motorcycle and scooter drivers were given a preventative and educational test in order to measure the decibel level of their vehicle. If they exceeded the authorized level, the policemen described the impact of the noise they generated on the population, and the fines they would normally incur. One fourth of the two-wheelers that were tested had excessive noise levels, and 20 % had been derestricted (an illegal modification to the carburetor and exhaust pipe which is harmful to the ears).


    Soundproofing engines


    Soundproofing the engine is one of the main ways to reduce car noise level. This can be done to thermal engines and hybrid engines to a lesser extent. In fact, electric vehicles are already almost completely silent. A thermal engine emits an average of 60-75 decibels, depending on speed and power, whereas an electric motor emits 10-15 decibels - about the noise level of wind blowing through the trees.


    Reducing the noise


    To reduce the noise generated by thermal engines, and more generally the powertrain, manufacturers can improve different parts of the "chain of noise." The most effective soundproofing systems and technologies include:


    • Improvement of the 3 main types of materials used to reduce powertrain noise:
      • Damping materials to reduce the engine vibrations. These materials, chosen for their viscoelastic properties, are glued to the parts that are most exposed to the vibrations.
      • Soundproofing materials, placed inside the vehicle (floorboards, bulkhead, tunnel, etc.) to improve the acoustic comfort of the driver and passengers. A variety of materials can be used: injected polyurethane, polypropylene, glass fibers, cellulose fibers, etc. These are all extensively researched in order to obtain the best compromise between acoustics/weight/cost/safety/recyclability.
      • Absorbers placed all over the car, whose effectiveness is based on the thermo-acoustic effect (conversion of acoustic energy into heat).


    • Digital modeling of the car body in order to simulate vibrations and identify zones that need reinforcement, absorbers, or vibration damping.


    • Improvement of exhaust systems in order to reduce the pressure of exhaust gas and reduce or eliminate sound waves (sound traps, mufflers, etc.).


    • Improvement of fuel injector efficiency for better engine performance and reduced noise emissions (common rail


    • More widespread use of cruise control systems, which reduce the abrupt accelerations and decelerations that are a big source of noise.


    Soundproofing is now an automotive field in its own right, and car and car parts manufacturers have a number of acoustic engineering teams. Significant investments in soundproofing test and simulation labs have also been made. For example, the French car part manufacturer Faurecia has a 13,000 m2, ultramodern acoustic development and testing center in Germany, equipped with roller test beds and software to test vibrations and noise up to 500 hp at speeds of up to 240 km/hr. Similarly, Renault invested 25 million euros in 2005 to build an acoustic unit in its technical center.


    Tire noise



    The noise provoked by the tires when they come into contact with the road, or rolling noise, increases significantly at certain speeds: 30-40 km/h for light vehicles, 50-60 km/h for trucks. And the quieter engines become, the louder tire noise becomes in comparison, even at low speeds.


    The development of low-noise tires and road surfaces that absorb sound waves are two important contributions to reducing noise pollution.


    Minimizing tire noise always requires finding the best possible compromise with other tire performance criteria: grip (wet and dry surfaces), handling (wet and dry surfaces), emergency avoidance, hydroplaning, grip and braking on snow and ice (winter tires), rolling resistance and fuel consumption, longevity, cost, etc. In fact, in an ideal situation the quietest tire would be smooth, with no treads at all. But such a tire would of course be disastrous in terms of grip and safety.


    To design and produce these high-tech tires, manufacturers optimize:


    • The composition of the multi-layer tread structure and the external layer. New technologies to produce materials (polymers, silica), the mix of rubbers, and the layering process all contribute to the optimization of rolling resistance and thus a reduction in tire noise and improved fuel efficiency. These new materials, as well as the tread design, also ensure even tire wear, which reduces vibration and noise problems.


    • The tread pattern. Manufacturers use advanced modeling to design a tread that reduces or eliminates different airflows that produce sound waves: vibrations, air pumping, the horn effect, whistling, etc. Producing low-noise tires thus requires a complex combination of parameters: pitch sequence, asymmetric tread design, integration of transverse grooves, arrangement and density of sipping and tread blocks, creation of cellular depressions in the walls of the oblique grooves, etc.


    • Development of pressure and temperature sensors that are integrated into the tire and inform the driver of optimal tire pressure according to vehicle load. These intelligent tires, which are still being researched, could reduce the risk of accidents caused by insufficient tire pressure, and at the same time the rolling noise. 


    • Introduction of absorbent materials or insulating foam into the tire, in order to reduce rolling vibrations and the associated sound waves. Some initial studies indicate that this new technology could reduce the rolling noise by 2 or 3 decibels, which represents a 30 to 50% reduction in sound energy, but would increase the rolling resistance thus the fuel consumption.


    Lowering tire noise is also the result of increasingly strict regulations. For example, since November 1, 2012, tires sold in the European Union must bear a label with 3 criteria: rolling resistance (and thus fuel efficiency), rated from A to G; wet grip, rated from A to G; and external rolling noise classified according to 3 sound waves. The third sound wave corresponds to the maximum rolling noise currently permitted in the European Union. Starting in 2016, this limit will be lowered to the 2nd sound wave listed on the label. Tires with three sound waves will no longer be allowed on the market.


    Absorbent road surfaces


    Rolling noise (the noise tires make when they come into contact with the road) greatly contributes to a vehicle noise pollution at speeds of 30-40 km/h for individual cars and 50-60 km/h for trucks (1). The quality of both the road surface and the tires thus play a large role in reducing traffic noise, even more so outside of urban centers where vehicles drive at very high speeds.


    Public work companies and infrastructure constructors are therefore developing better and better road surfaces that absorb part of the rolling noise. In most cases, these new products all adhere to the same principle: instead of smooth, dense asphalt which reflects and propagates sound waves created by friction between the tires and the road, sound-absorbing surfaces are porous and made up of very small interstices which trap sound waves. The irregularities in the asphalt also limit air pumping caused by compression and expansion of air in the contact zone of the tires. And they can also trap some of the sound waves emitted by the powertrain, which makes them useful even at lower speeds in urban areas.


    The most effective among this latest generation of sound-absorbing surfaces reduce noise by 8-9 decibels, the equivalent of an 8-fold reduction in traffic. They last as long as traditional coatings, around 15 years on average. Towards the end of their life span they still reduce noise by around 3 decibels, which cuts traffic noise in half. They cost 10-15% more than traditional road surfaces.


    Example: In Switzerland, Geneva County is investing heavily in cutting-edge asphalt made of very fine particles with a high sound-absorbing capacity. By 2016, three-fourths of the noisiest roads (a total of 150 kilometers) will be covered by so-called quiet road surfaces. For the roads that have already been re-done, the noise reduction represents an 85% reduction in traffic noise.


    (1) Source: France – Paris area - Observatoire du bruit (Bruitparif)


    Sound barriers


    Sound barriers are generally placed along major transportation routes (highways, heavy traffic roads, beltways, etc.) to protect nearby residents from excessive noise pollution. In an increasing number of countries, this protection is guaranteed by laws that define the maximum level of decibels permitted close to residential buildings and/or require noise reduction systems such as noise barriers. This is particularly applied to the new significant land transportation infrastructure construction or modification.


    Acoustic parameters


    The design and position of sound barriers require complex calculations and simulations in order to optimize the effect of a number of acoustic phenomena. In particular:


    • Dispersion of the sound wave energy depending on the distance of propagation.
    • Absorption of sound waves by materials used in the noise barriers.
    • Reflection, diffraction, and transmission of sound waves by the materials that are used to build the noise barriers.
    • Ground effect: interference between the sound waves emitted by vehicles and those that are reflected by the ground and the noise screens.
    • Effect of weather: variation of the speed and direction of sound wave propagation depending on wind, temperature, and air humidity.


    Effectiveness criteria


    Optimizing the different acoustic phenomena and effectively attenuating noise requires the screens to:

    • be installed as close as possible to the road and as high as possible, in order to stop the sound waves before they spread and direct them upwards (vertically in relation to the road).
    • be installed as close as possible to the road and as high as possible, in order to stop the sound waves before they spread and direct them upwards (vertically in relation to the road).
    • be very dense (weight/m3) with no openings, in order to block the maximum amount of sound waves either by reflecting or absorbing them.
    • include an absorbent material to trap some of the sound waves and limit the amount of reflection between the cars and the screen. This reflection and interference is likely to significantly increase the noise level on the road and to propagate sound waves above the screen, creating a great deal of noise pollution for residents living on the upper floors of apartment buildings.
    • include barrier caps that prevent the sound waves from going above the screens and send them back down to the road.
    • be long enough to prevent sound waves from getting around the noise screens on the sides.


    Types of screens and effectiveness


    The different types of noise barriers on the market all include one or some of the following effectiveness characteristics:

    • Metal screens (aluminum or steel, corrugated or sheet metal, etc.), which either absorb or just reflect sound waves. The absorbent screens are made of a material which traps the sound waves: fiberglass, stone wool, etc.
    • Concrete screens which either absorb or reflect sound waves. Concrete can be made absorbent in various ways: integration of "sound traps" into the manufacturing process (wood fibers, expanded clay, ceramic, rubber, etc.), production of porous cement, grooved patterns on the surface for better sound wave diffraction or reflection.
    • Wooden screens, which are more aesthetically appealing but less absorbent than specially-treated cement.
    • Transparent screens, made of plastic or glass, which permit visibility and reflect the sound waves (without absorbing them).
    • Perforated PVC screens made with absorbent materials.
    • Gabion screens, with a central core made of sand or acoustic cement and a pile of stones within a metal framework.
    • Noise screens covered with plants, which lessen the aesthetic problems associated with plain noise screens and can also help to purify the air. However, the vegetation has a low impact on noise reduction.
    • Talus, earthen embankments, and barricades which have a more natural look but are much less effective in reducing noise. The highest part is necessarily farthest from the road, with a rounded angle that sounds waves cross more easily.
    • Structures that cover the road completely, which cost more but are the most effective in reducing noise, as long as a specific acoustic treatment is used at tunnel entry and exit points. These structures can also serve as eco-ducts and thus help to reduce habitat fragmentation.


    Though the principle of noise screens is fairly simple, they are the focus of extensive acoustic research and make use of advanced technologies. For example, manufacturers are developing screens made of sonic crystals, or using fractal geometry - among other recent innovations - in order to better trap sound waves.


    The effectiveness of noise screens is measured according to how well they insulate, transmit, and absorb sound. This performance is evaluated and codified for instance in a European standard (EN 1793). Furthermore, since May 1st, 2007, every roadside noise reduction system marketed in the European Union should obtain a "CE" mark which guarantees its compliance with EC acoustic efficiency requirements.