Prasanth wrote:[justify]Noise control is becoming increasingly important for a wide variety of OEM designers. Examples of products that take noise control considerations into account during their design cycles include equipment such as computer hard drives, house appliances, material handling and transportation equipment etc,. In the transportation market, which includes aircraft, ground and marine segments, the demand is for low noise level goals. Achieving these goals is of primary importance for OEM to be continue to be competitive or to keep a given supremacy in the market. The automotive industry has been a leader in the adsorption of noise control technologies. Methods in use for several years for the prediction of interior noise levels include : finite element method(FEM), statistical energy analysis (SEA) boundary element analysis (BEA) etc. The internal combustion engine has mechanized the world. Since the early 1900s it has been our prime source of mechanical power. The vast number of internal combustion engines in the world today has resulted in air pollution, noise pollution etc.
CLASSIFICATION BY NOISE CHARACTERISTICS
One typical engine noise classification technique separates the aerodynamic noise, combustion noise and mechanical noise.
1. AERODYNAMIC NOISE
2. COMBUSTION NOISE
3. MECHANICAL NOISE
AERODYNAMIC NOISE-aerodynamic noise includes exhaust gas and intake air noise as well as noise generated by cooling fans, auxillary fans or any other air flow.
COMBUSTION NOISE- combustion noise refers to noise generated by the vibrating surfaces of the engine structure, engine components and engine accessories after excitation by combustion forces.
MECHANICAL NOISE-mechanical noise refers to noise generated by the vibrating surfaces of the engine components and engine accessories after excitation by reciprocating or rotating engine components.
EXHAUST SYSTEM NOISE:
Exhaust system noise includes the noise from exhaust gas pulses leaves the muffler or tail pipe and noise emitted from the vibrating surfaces of the exhaust system components. Noise emitted from the surfaces of exhaust system components results from two different types of excitation forces: those generated by the pulsating exhaust gas flow and those transmitted from the vibrating engine to exhaust system components. Additional considerations in the reduction of exhaust system noise include proper selection of piping lengths and diameters, proper mounting of exhaust system components and proper positioning of the exhaust outlet.
INTAKE SYSTEM NOISE:
Intake system noise includes noise generated by the flow of air through the systems air inlet and noise emitted from the vibrating surface components. As with exhaust systems surface radiated noise results from two different types of excitation process: those generated by the pulsating intake air flow and those transmitted from the vibrating engine to intake system components. In many instances, an engines air cleaner will provide significant attenuation of intake air noise. If additional attenuation is required, an intake air silencer can be added to the system. To minimize intake system surface radiated noise, proper design, selection and mounting of intake system components are essential.
COOLING SYSTEM NOISE:
Water cooled engines are typically cooled by using a radiator as a heat exchanger – with an axial flow fan is used to draw cooling air through the radiator. Air-cooled engines generally use a centrifugal fan in conjunction with shrouding to direct cooling air across the engine. Fan noise consists of both discrete frequency tones and broadband noise. The broadband components of fan noise are caused by the shedding of vortices from the rotating fan blades and by turbulence in the fans air stream.
The discrete frequency components are the result of pressure impulses that occur each time a fan blade passes an obstacle in the fans pressure field. When fan blades are spaced at equal angular intervals, the fundamental discrete tone will occur at the fans blade passing frequency.[/justify]