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.
DEFINITIONS OF SOUND
Sound can be defined as the perception of vibrations stimulating the ear. If scientifically taken into account, sound is a periodic disturbance in fluids density or in the elastic strain of a solid, generated by vibrating objects. These waves or vibrations propagate in two basic ways.
1. longitudinal waves.
2. Transverse waves
DECIBEL â€œ Sound level is measured in decibel. Sound level in decibels is a logarithmic rather than a linear measure of change in pressure with respect to a reference pressure level. A small increase in decibels can represents a large increase in sound energy.
Difference in decibel = 10 log10 (W1/W2)
Technically an increase of 3db represents a doubling of sound energy, and an increase of 10 db represents a ten-fold increase. The measurement for measuring noise is the basic sound level meter or no: of its derivatives, including noise dosimeters.
SOUND PRESSURE LEVEL:
We have difference in decibel, 20 log10 n1/2 = 20 log10 (P1/P2). If P2 were given an absolute value and 20log10 n1/2 were known then P1 would also have an absolute value. In audio â€œ acoustics, P1 is frequently given the absolute r.m.s value of 2X10-5 N/M2which is the minimum sound pressure fluctuation discernible to human ear. Consequently P1 must also become an r m s pressure expressed in N/M2. 20 log10 n1/2 is still in decibels but is now defined precisely as the sound pressure level (SPL).
SPL = 20 log10 P1 2X10-5 db
In terms of sound pressure level the sensitivity of ear ranges from O db (2X10-5 N/M2). Everyday levels very b/w 35 db and 90db.
WEIGHTING CURVES The apparent loudness of sound varies with frequency as well as with sound pressure. To adjust the frequency response of necessary systems to be similar to that of human ear, several weighting curves proposed.
ËœAâ„¢ weighted curve was designed to appropriate to human response let sound pressure levels (<55db) similarly ËœBâ„¢ and ËœCâ„¢ weighting were intended for use at sound pressure levels of 55 db to 85 db and above 85 db respectively. For almost all practical sound measurement purposed in industrial as well as automotive applications we follow the dBA weighting curve. Because dBA curve is very similar to the perception of human ear.
Available range of sound in human beings are 20-20,000 Hz.
There has been a direct relationship between the improvement in manâ„¢s physical standard of living and the degree of his development of machines. The industrial revolution was really a series of social and industrial transformations, beginning in England with the use of coal in place of charcoal for the smelting of iron, progressing through the stages of steam engines and electric motors and all the producing and processing made possible by these devices. of the age of gasoline, sea and air for various types of transportation. For that matter, sweeping mechanical progress witness automation and the utilization of nuclear energy; but with every new machine, a little noise is created, with every mechanism employed to do manâ„¢s work, some mechanical or electrical power is converted into acoustical power, so that with
the rise of peopleâ„¢s standard of living there occurs also a rise in the noise level of peopleâ„¢s confines.
TYPES OF SOURCES OF NOISE
Sources of noise are numerous but may be classified broadly into two classes as
2. NON INDUSTRIAL
The industrial may included noises from various industries operating in cities like transportation, air crafts, rockets, defense equipments, explosions etc. The disturbing qualities of noise emitted by industrial premises are generally its loudness, its distinguishing features such as tonal or impulsive components and its intermittency and duration.
Various sound levels and its effects on Human Beings
Sound sources Sound level dBA Subjective feeling or human being Effects of human being
Rockets and missiles heavy explosives 150-160 Unbearable Above 150 dBA may cause severe damage to the whole body such as loss of hearing of both ears, dizziness, nausea, disturbance of speech, confusion or psychosis
Above 90 dBA mat cause headache, dizziness, tinnitus, insomnia, deafness, heart disease, blood hyper-tension, gastric ulcers, neurosis, temporary hearing threshold shift.
Jet planes and cannons, explosives 140 Unbearable
Aircraft propeller and machine guns 130 Unbearable
Diesel, steam engine and ball mills, crackers 120 Unbearable
Electric saws and looms, heavy trucks 110 Unbearable
Lorries, highway vehicles and very busy streets 90-100 Very noisy
50-90 dBA may cause various degrees of effects in sleeping, studying, working and talking
Commercial place, air conditioners, loud voice and busy streets 70-80 Noisy
Office complex, average loudness of voice 60 Noisy
Sense of noisy feeling
Ordinary room 50 Quiet
Silent night, library 30-40 Very quiet
Hospital, bedroom at night, church 20-30 Very quiet
Sound proof room, broadcasting studio 10-20 Very quiet
Lower limit of hearing 0 Very quiet Threshold of hearing
NON INDUSTRIAL (TRAFFIC)
In non industrial source , major one is traffic mopeds and scooters with have a maximum of 75 dBA and 50 dBA respectively, according to new norms from 1st January 2003. the present norms for petrol and diesel driven two wheelers are 50dBA and 82 dBA respectively. Present norms for three wheelers are 82 dBA for petrol and 85 dBA for diesel will be modified as three wheelers upto 175 cc, 75 dBA and above 175 cc, 80dBA. Passenger cars can produce maximum noise of 75dBA present being 82dBA.
DECIBEL RANGES FOR CERTAIN PARAMETERS
Gun shot, jet engine 140dB(immediate danger to hearing)
Fire cracker 125 dB (pain threshold)
Ambulance siren, rock concert 120dB(risk of hearing damage in 7 mnts)
Jet ski 115 dB (risk of hearing damage in15 mnts)
Chain saw, snow mobile 110dB (risk of hearing damage in 30 mnts)
Impact wrench 100 dB( risk of hearing damage in 2 hrs)
Movie trailor 93 dB
Lawn mover 90dB (OSHA permissible exposure level )
Ringing telephone 80 dB
Normal conversation 65dB
Faint sound, whisper 30dB
The elementary as well as complex analysis of noise problems can be simplified by following a set of guide lines or principles. The principles of noise control can be summarized as identifications of sources of noise and their relative importance, listening and evaluation of possible noise control procedures as they applied to source, path and receiver. Identifications of the relative contributions from both directed and reflected sound. Difference between absorption and attenuation of noise. Identification and evaluation of flanking paths and also identification and evaluation of certain significance of flanking path and structure
INTERNAL COMBUSTION ENGINE NOISE
Several alternative methods can be used to classify internal combustion engine noise. The two most typical classification techniques are discussed below.
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.
CLASSIFICATION BY ENGINE NOISE SOURCES
A second approach to the classification of piston engine noise involves the separation of engine noise into the following categories â€œ
1. EXHAUST SYSTEM NOISE
2. INTAKE SYSTEM NOISE
3. COOLING SYSTEM NOISE
4. ENGINE SURFACE RADIATED NOISE
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 â€œ 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.
F0 = fan rotational speed (rpm) x number of fan blades
a).WATER COOLED ENGINES
A variety of design parameter affect at the sound-emission levels of axial-flow fans, but fan blade tip speed is the dominant factor. To minimize fan tip speed, while still providing sufficient engine cooling, the cooling systemâ„¢s efficiency must be as high as possible. To maxmise cooling system efficiency in water-cooled engines, the following consideration should be made-
1. use water pump and radiator that have adequate capacities, furthermore, be sure that the radiator core has sufficient surface and air flow areas.
2. use a fan with proper aerodynamic blade design.
3. use a shroud to prevent recalculation of air from the high pressure side of the fan in the low pressure side. Clearance between the tips of the fan blades and the shrouding should be minimal.
4. reduce air flow resistance and turbulence in the system. This can be achieved through proper shroud design, proper spacing between the radiator, proper radiator core design.
b).AIR COOLED ENGINES
For most air cooled engines, a centrifugal fan is used to provide cooling air for the engine. As with axial flow fans, a centrifugal fanâ„¢s blade tip speed is the primary factor influencing fan sound levels. Consequently, cooling system efficiency must be as high as possible. So that fan blade tip speed can be minimized.
1. Use a fan with proper aerodynamic blade design.
2. Use a properly designed fan shroud.
3. On any engine application, prevent the cooling fan from drawing in air that has been elevated in temperature by exhaust system components.
ENGINE SURFACE NOISE
Engine surface noise refers to sound emitted from vibrating surfaces of engine components and accessories and other than items included in the engine exhaust, in take and cooling systems. Techniques used to reduce engine-surface radiated noise include a reduction in running clearances and/or machining tolerances of the engine components , acoustical treatment or re-design of engine components, use of acoustically treated shields and vibrating isolation and damping of engine covers and diesel engines than for gasoline engines. Turbo charging of a diesel engine can result in some reduction of engine surface-radiated noise at high engine loads.
ENGINE APPLICATIONS AND NOISE
Application of the internal combustion engine covers a broad spectrum. The internal combustion engine is now used to power anything from a small hand-held weed cutter to a large ocean liner. One typical application is as given below.
The automobile represents the largest single application of the internal combustion engine in our society; and the automotive industry is primarily responsible for rapid advances in internal combustion engine designs in the late 1800s and early1900s. The automotive industry also became more fervent in implementing measures to reduce both interior and exterior noise levels. Some methods used to reduce engine-related vehicle noise include improved intake, exhaust, and cooling system designs, engine compartment treatment, improved engine vibration isolation, and engine component treatment or redesign.
NOISE EVALUATION DURING ACTUAL RUNNING (TRIAS 20 METHOD)
Measuring layout for acceleration noise (TRIAS 20 Method)
In the case of a motorcycle which has a manual transmission with 5 speeds or more and with a displacement of 251 cm3 and over, the running test procedure is that the test motorcycle enters point A in 4th gear, at a speed of 50km/h or 0.75s (s means engine rpm at maximum horse power) whichever is a lower speed, then the rider opens the throttle fully at point A and closes it completely acceleration, maximum
sound pressure is measured to the left side of the vehicle at a distance 7.5 m perpendicular to the running pass.
Noise Control Legislation
Prior to 1970, most surface vehicles noise regulations where legislated at the state or local level. Recently however, several federal programs have also been implemented in an effort to minimize a level of environmental noise in our society.
STATE NOISE CONTROL PROGRAMMS
MOTOR VEHICLE NOISE REGULATIONS
Most common form of state-enacted noise legislation applies to motor vehicles. Historically, trucks have been the first vehicles regulated, with regulations for automobiles, buses and motor cycles following. Motor vehicle regulations generally established a quantitative sound limit at a specified distance from the vehicle.
FEDERAL NOISE CONTROL PROGRAMMES
OCCUPATIONAL SAFETY AND HEALTH ACT (OSHA)
OSHA was enacted in 1970 in an effort to ensure safer conditions for all workers. The act OSHA specifies the maximum noise level that a worker may be subjected to during a workday. The OSHA standard is based on a max: allowable steady-state level of 90dBA for an 8-hr day. When noise level exceed 90dBA, the permissible duration of noise exposure is reduced.
An OSHA noise does meter is an integrating sound level meter that weighs the level and duration of time-varying sounds in accordance with OSHA curve. If then displays a value that represents the time-varying sounds as an equivalent 8hr-steady state level. The noise dosimeter will also indicate if the time-varying sound exceeds a level of 115 dB (A) during analysis.
1, Stopping it at the source
Improving the engineering in many noisy objects has cut noise nearly by 30 decibels (i.e. snow mobiles)
Government has set up regulations to manufacturers such as GM and Mack truck to reduce vibration in heavy gears, axles and transmissions.
Reducing sound at the sources by an average of 10 decibel cuts soundness in half.
2. SHIELDING YOUR EARS
Without doubt, plugging up your ears is the cheapest and easiest method of noise control.
If you have to be around loud noise protecting yourself with earplugs is better than doing.
Excessive exposure to loud noise and or exposure to a quick sound noise could cause serious damage to your ears.
Interest in noise and its reduction pass become wide spread in many industrial advanced countries. Noise standards and legislations exists in such countries for protecting urban residents and industrial workers form hazards caused by excessive noise levels. It is high time that we also start thinking of such steps before the noise problem becomes too large and unwieldy. In mean while, as engineers we should try to control noise to the extend possible by properly designing machines and appliances by suitably locating machines and office spaces in industry by judicial location of residential areas and high ways by using proper noise control methods to reduce existing noise problems.
1) Noise cancellation systems by Jeffery .N Denenberg.
2) A study of noise reduction method on motorcycle- SAE 1999-01-3257 JSAE 9938012
3) Design strategies for low noise engine concepts by F.K Brandl, P.Wunsche
4) Noise control in IC engines â€œ BAXA
5) Diesel engine reference book- Bernard Challen