Noise control or noise mitigation is a set of strategies to reduce noise pollution or to reduce the impact of that noise, whether outdoors or indoors. The main areas of noise mitigation or abatement are: transportation noise control, architectural design, urban planning through zoning codes, [1] and occupational noise control. Roadway noise and aircraft noise are the most pervasive sources of environmental noise worldwide, and little change has been affected in source control in these areas since the start of the problem, a possible exception being the development of hybrid and electric vehicles. Social activities may generate noise levels that consistently affect the health of populations residing in or occupying areas, both indoor and outdoor, near entertainment venues that feature amplified sounds and music that present significant challenges for effective noise mitigation strategies.

Multiple techniques have been developed to address interior sound levels, many of which are encouraged by local building codes; in the best case of project designs, planners are encouraged to work with design engineers to examine trade-offs of roadway design and architectural design. These techniques include design of exterior walls, party walls, and floor and ceiling assemblies; moreover, there are a host of specialized means for damping reverberation from special-purpose rooms such as auditoria, concert halls, entertainment and social venues, dining areas, audio recording rooms, and meeting rooms.

Many of these techniques rely upon materials science applications of constructing sound baffles or using sound-absorbing liners for interior spaces. Industrial noise control is really a subset of interior architectural control of noise, with emphasis upon specific methods of sound isolation from industrial machinery and for protection of workers at their task stations.

Noise control techniques include the following:

・ Sound insulation: prevent the transmission of noise by the introduction of a mass barrier. Common materials have high-density properties such as brick, thick glass, concrete, metal etc.

・ Sound absorption: a porous material which acts as a “noise sponge” by converting the sound energy into heat within the material. Common sound absorption materials include decoupled lead-based tiles, open cell foams and fiberglass

・ Vibration damping: applicable for large vibrating surfaces. The damping mechanism works by extracting the vibration energy from the thin sheet and dissipating it as heat. A common material is sound deadened steel.

・ Vibration isolation: prevents transmission of vibration energy from a source to a receiver by introducing a flexible element or a physical break. Common vibration isolators are springs, rubber mounts, cork etc.

The noise levels can be reduced through redistributing the traffic flows. It has advantageous to concentrate the traffic to a few streets and to locate these as for from receiver as possible [5] . The absorptive material can be used to reduce the noise levels as well.

The strategic assessment of environmental noise, US has introduced on indicator L_den, calculated according to their environmental noise directive [6] , L_Aeq. With a 5dB penalty for evening noise and 10 dB penalty for night- time noise levels.

The reverberation time of room is focused by [7] . The measurement of the reverberation time has been carried out, the RT measured is quite similar, and decreases fairly uniformly as the frequency increases. Moreover, in each frequency band the greater the volume of the room, the greater the RT observed.

The design of the barrier shape 1) gives a better sound absorption at the total frequencies of interest 2) protects the inside absorption layer from deterioration due to harsh weather [8] [9] . For a lightly damped thin- walled barrier, the absorption treatment not only absorbs acoustical energy but also damps the barrier structure resulting in a negligible energy transmission. The results demonstrated that for total noises the proposed barrier is more effective than reflecting and absorbent barriers [10] .

The Planning and Development Regulations [11] [12] provide planning exemptions for renewable technologies for commercial, public, industrial and agricultural buildings where noise levels do not exceed 43 dB (A), during normal operations as measured from the nearest party boundary.

Within the last few years, concern about the protection of the environment has grown rapidly as it has become generally recognized that steady rise in pollution of all kinds cannot be allowed to continue indefinitely. The acoustic environment has likewise suffered from the increase in use and power of the machines in the workplace, increasing road traffic, larger aircrafts etc. To combat this, many countries have introduced legislation making it a legal requirement to measure noise levels to reduce noise from vehicles at the source and maintain acceptable noise levels in factories to prevent hearing loss. India has emerged as fast developing country resulting in an increase in activity of the workforce. In 1989, Central Pollution Control Board (CPCB) [13] , promulgated the Ambient Air Quality Standards for Noise, hereby establishing the noise limits for residential, commercial and silence zone areas. For assessing the urban noise problem and suggesting the mitigation measure, it is imperative that accurate data be obtained/measured of the noise levels at different locations at different times of the day.

The sources of vehicle noise have been identified as the power unit (engine, air inlet and exhaust), cooling fan, transmission (gear box and rear axle), rolling noise (aerodynamic and tyre/road surface), bakes, body rattles and load. In general sources related to the power unit and transmission upto the lay shaft are referred to as power train noise and all other sources are termed rolling noise [9] .

The relative importance of these sources depends on the type of the vehicle and the operating condition. With light vehicles the engines is dominant at low road speed order or higher than power train noise. However with heavy diesel engine Lorries the engines, exhaust and cooling fan noise are the dominant sources under most operating conditions although the noise of the tyres rolling on the road surface can be noticeable at high speeds, particularly with tyres having pronounced transverse ribs.

The noise for typical light vehicles (less than 1.5 times) as well as heavy vehicles increases at a rate of about 9 dBA per doubling of speed. At higher speeds power train noise, and hence total noise, increases at a similar rate, with the rolling noise remaining approximately the same. At lower speeds in urban areas where lower gears are used, power train noise tends to be independent of road speed and hence rolling noise becomes insignificant as speed falls. A comparison of these parameters has been tabulated in Table 1.

The relative magnitude of noise from separate sources for a vehicle operating in a top gear at a steady speed of approximately 50 km/h is given Table 2.

Mechanical noise can originate from many different sources of an engine and can be much more difficult to locate and quantify than combustion noise. The most common sources of mechanical noise are piston slap, timing drive noise, bearing forces and rotations, and other including value train impacts, fuel injection pump, etc. The main sources of tyre noise are aerodynamic noise, air pumping and tyre vibration. The most efficient methods of vehicle noise reduction should be achieved in early stages of vehicle design and during proto type development of the vehicles and engines. Further improvements such as in tyre surface noise can be achieved by proper design of the road surface.

The legislative constrains on today’s automotive industry are considerable. In USA the Society of Automotive Engines (SAE) makes recommendations in the form of standards for the noise testing of a variety of vehicles, for example SAE J986 for passenger car and light trucks. SAE J968 similar to the ISO R362 procedure, distance of 15 m truck length and microphone measuring distance of 15 m [12] . There are detailed differences in the use of gears and the interpretation of results but basically, the test is still a low gear, wide open throttle test, measuring the maximum sound pressure level on a sound level meter mid way along the truck. Noise regulation in USA exist for two categories of vehicles, namely,

1) Medium and heavy trucks (gross weight > 4500 kg).

2) Motor cycle and motor-cycle exhaust systems. The road vehicle noise limits in USA are shown Table 3. And the nominal limits for light vehicle in USA are given Table 4.

The levels of noise generated y road traffic depend mainly upon the type of vehicle flow, the volume of traffic, the speed and composition of the traffic, the road gradient and the type of road surface. The type of vehicle flow is 1) Free flowing and 2) Interrupted traffic. As the flow rate of traffic increases, the traffic, Leq also increases upto saturated condition, beyond which, the effect of reducing vehicle speed on noise predominates. Whether

considering hourly values of Leq, L₁₀ etc., as a function of mean hourly flow or longer term averages of these indices, it is generally accepted that over a wide range of traffic flows the variation of these indices with vehicle flow, Q, can be adequately represented by a logarithmic relation of the form L = Clog₁₀Q.

The propagation of traffic noise is influenced by a number of factors. These include the attenuation due to distance, the interaction of the propagating wave with the ground surface, the barriers, the effect of vegetation, and, for long distance propagation, the effect of varying weather conditions.

Noise radiated from an Omni-directional point source into a free space attenuates according to the inverse square law, i.e. the acoustic field decreases as 20logd decibels. Although road vehicles are neither Omni-direc- tional nor point sources a similar attenuation function can be obtained in the far field for isolated vehicle noise propagating over a hard reflecting ground surface. Consequently, provided these conditions are observed, in practice the peak noise levels from individual vehicles will attenuate by approximately 6 dBA per doubling of distance from the vehicles on a road segment approaches the distance separating the receiver from the traffic lane. As the flow increases further, the geometric spreading approaches that of a line source such that, theoretically, the acoustic field attenuates as 10log₁₀d decibels.

When an obstacle intercepts the line of sight from the source to a receiver, the noise at the receiver is reduced. The amount of screening provided caries according to the amount of sound energy that is diffracted over the top of the barrier. For traffic noise problems, the sound energy transmitted through a roadside screen can generally be neglected since the transmission loss through most barrier material is much higher than the potential screening performance determined by the barrier geometry. For example, a barrier material with a surface mass of 20 kg/m² will generally provide a transmission loss of more than 20 dB. This of course, assumes that there are no haps in the structure.

Trees, brushes and plants are of great value in improving the aesthetics of road environment However; the noise attenuation provided by vegetation is generally overestimated. Vegetation may affect the propagation of low-fre- quency sound by ground absorption which can be enhanced in wooden areas because of the high porosity of the ground resulting from tree roots and fallen leaves, etc. High-frequency propagation is affected be scattering by tree trucks, braches and partly by leaf absorption.