Taşıt Akustiği

Abstract

Gürültü en basit tanımıyla istenmeyen sesdir. Günümüzde gürültü, insanlan sevdikleri işleri rahat ve huzur içinde gerçekleştirmekten alıkoyan, boş zamanlarını verimli bir şekilde değerlendirmelerini engelleyen, onlan uykularından alıkoyan, sinir sistemlerini etkileyerek onlarda psikolojik rahatsızlıklara kadar varabilen etkiler yaratan, işitme organlannda geçici ve en kötüsü kalıcı hasarlar meydana getirebilen, toplum sağlığını tehdit eden bir unsurdur. Son zamanlarda gürültü kirliliği toplumda en az hava ve su kirlilikleri kadar tepki toplayan bir problem haline gelmiştir. Çevreyi koruma bilincinin toplumda giderek artan oranda kabul görmesi, büyük kentlerde gürültünün de insan sağlığı ve rahatını tehdit eder boyutlara ulaşması, gürültü konusunun önemini ön plana çıkarmıştır. Günlük yaşantımızın her anında, evde, işyerinde ve sokakta, çeşitli kaynaklardan yayılan gürültünün etkisini sürekli üzerimizde hissederiz. Çevremizde gürültü kirliliği yaratan kaynaklardan bir tanesi de motorlu taşıtlann etrafa yaydığı gürültüdür. Kentlerde şehiriçi ve şehirlerarası ulaşım gereksinimimizden kaynaklanan trafik gürültüsü, her gün artan sayıda trafiğe çıkan araçlardan dolayı sürekli artmaktadır. Sağlığımızı giderek ciddi bir şekilde tehdit eden trafik gürültüsünü önlemek, en azından kontrol altına almak yine bizim elimizdedir. Bu çalışmada ilk önce akustik konusunun temel kavramlan incelenmiştir. Daha sonra taşıt gürültüsü ve taşıt gürültüsünde etkili olan faktörler araştınlmıştır. Motorlu taşıtlar birbirleriyle bir arada çalışan bir çok parçanın biraraya gelmesi sonucu imal edilmektedirler. Sonuçta taşıt gürültüsü olarak karşımıza çıkan gürültü, birbiriyle bağımlı veya bağımsız olarak çalışan elemanlann herbirinin çevreye yaydığı gürültünün logaritmik toplamıdır. Bu nedenle araç içinde ve dışında yapılan ölçümlerle, çeşitli elemanlann toplam araç gürültüsüne etkisi araştırılmış, kullanılan çeşitli izolasyonlarla bu gürültünün önlenmesine yönelik çalışmalar yapılmıştır.

Sound is such a common part of everyday life that we rarely appreciate all of its functions. It provides enjoyable experiences such as listening to music or to the singing of birds. It enables spoken communication with family and friends. It can alert or warn us, with the ringing of the telephone, or wailing siren. Sound also permits us to make quality evaluations and diagnoses about the chattering valves of a car, a squeaking wheel, or a heart murmur. Yet, too often in our modern society, sound annoys us. Many sounds are unpleasant or unwanted. These are called noise. However, the level of annoyance depands not only on the quality of the sound, but also our attitude towards it. The sound of his new jet aircraft taking off may be music to the ears of the design engineer, but will be ear-splitting agony for the people living near the end of the runway. But sound doesn't need to be loud to annoy. A creaking floor, a scratch on a record, or a dripping tap can be just as annoying as loud thunder. Worst of all sound can damage and destroy. A sonic boom can shatter windows and shake plaster off walls. But the most unfortunate case is when sound damages the delicate mechanism designed to receive it, the human ear. Noise, like air and water pollution (and it is a form of air pollution), is thus receiving more attention at the community, industrial, and governmental levels. Within this study an overall picture of the acoustical fundamentals are described that are involved in the study of noise. And then, an introduction to the analysis of sound and its applications in the automotive industry is examined. And focused on vehicle pass-by noise and studied on an insulation package on a vehicle. Sound may be defined as any pressure vibrations that the human ear can detect. The number of pressure variations per second is called the frequency of the sound and is measured in Hertz (Hz). The frequency of sound produces its distinctive tone. The normal range of hearing for a healty young person extends from approximately 20 Hz. up to 20000 Hz. Sound requires a source, a medium for transmission, and a receiver. The source is simply an object which is caused to vibrate by some external energy source. The medium is the substance which carries the sound energy from one place to an other. In this context, the medium will generally be air for air-borne noise and body of the vehicle for structural-borne noise. The receiver is a transducer which reacts the same way as an ear react to the pressure variations in the medium. It is a microphone connected to a sound level xii meter. The microphone converts the sound signal to an equivalent electrical signal. The most suitable type of microphone for sound level meters is the condenser microphone, which combines precision with stability and reliability. The electrical signal produced by the microphone is quite small and so it is amplified by a preamplifier before being processed. Several different types of processing may be performed on the signal. The signal pass through a weighting network. Most common used weightings are "A", "B" and "C" weightings. The "A" weighting network weights a signal in a manner which approximates to an inverted equal loudness contour at low SPLs, the "B" network corresponds to a contour at medium SPLs and "C" network to an equal loudness contour at high SPLs. A specialized characteristic, the "D" weighting, has also been standardized for aircraft noise measurements. Within this context, all measurements are taken in "A" weighting as described in standart for measuring vehicle interior and exterior sound levels. When more detailed information about a complex sound is required, the frequency range from 20 Hz. to 20 kHz. can be divided up into sections or bands. This is done by electrical filters or crystal filters which reject all sound with frequencies outside the selected band. These bands usually have a bandwidth of either one octave or one third octave. An octave is a frequency band where the highest frequency is twice the lowest frequency. A third octave covers a range where the highest frequency is 1.26 times the lowest frequency. The process of thus dividing a complex sound is termed frequency analysis and the results are presented on charts called spectogram. After the signal has been weighted or divided into frequency bands the resultant signal is amplified, and the root mean square (RMS) value determined. The RMS is a special kind of mathematical average value that is directly related to the amount of energy in the sound being measured. When measuring sound, measurements are to be taken in the free field area to eliminate the effects of reflected sound that occurs because of objects within the environment. In a free field the SPL decreases by 6 dB for each doubling of distance. The sources of vehicle noise have been identified as the power unit (engine, air inlet and exhaust), cooling fan, transmission (gearbox and rear axle), rolling noise, brakes, body rattles and load. In general, sources related to the power unit and transmission up to the driveshaft are refered to as power train noise and all other sources are termed rolling noise. The relative importance of these sources depands on the type of vehicle and the operating conditions. With light vehicles the engine is dominant at low road speeds in low gears, at higher road speeds in the top gear tyre rolling noise is likely to be of the same order or higher than power train noise. However, with heavy diesel engined lorries the engine, exhaust and cooling fan noise are dominant sources under most operating conditions. Internal vehicle noise is also a problem with the commercial vehicle and with passenger cars as the competition in the market is getting tougher. The sound of vehicle tells more about the quality and comfort to the customers. XIII Because the separate noise sources combine logarithmically to produce overall vehicle noise, it is imperative that all main sources are reduced together since little or no noticable improvement is made if, for example, exhaust noise is reduced by half when the engine noise is left untreated. Consequently, any workable programme of vehicle noise reduction has to take into account every important noise source. In the internal combustion engine, either spark or compression ignition, the duration of the applied force on the piston resulting from combustion in a cylinder is relatively short, typically about l/SO* of the total cycle time and this transient nature of the force gives rise to the vibration and noise which are familiar characteristics of the internal combustion engine. Since all the noise generated from either mechanical or combustion processes within the engine is radiated by the external surfaces of the engine. Noise is produced by the gas loads applied to the engine structure by the combustion process. This is known as combustion noise. Vibrations applied to the structure by the impacts from the pistons (piston slap), by the timing gears driving the camshaft and fuel pump, by the bearings and valve gear is classed under the general heading of mechanical noise. Combustion noise as a major source is primarily restricted to diesel engines with thr present design objectives of lean mix, advance ignition timing and higher compression ratios to achive high power output with economy, display noticable combustion noise. This means that high pressures are reached in the combustion chambers in very short times, the resulting steep pressure wave fronts causing the impacts which are transmitted through the piston, connecting rod, cylinder head and wall and crankcase to reach the outside air as noise. In summary, fuel is injected into the cylinder near the top of the compression stroke and when mixed with air at a high enough temperature and pressure the mixture ignites. The preiod between the start of injection and ignition is known as ignition delay period and this quantity has an important bearing on combustion noise. After ignition the injected fuel burns very rapidly causing a large and sudden release of heat and rise in combustion chamber pressure. The steep rise in cylinder pressure near top dead center differs in different type of engines. The rate of steep pressure rise is high in direct injection naturally aspirated engine with its generally long ignition delay period. The turbo charged Dl engine has a shorter delay period with a more gradual rise in pressure resulting in lower combustion noise. Mechanical noise can originate from many different sources on an engine and can be much more difficult to locate and quantify than combustion noise. The surfaces and components which are usually significant noise sources on a typical automotive diesel engine are crankcase, cylinder head, sump, rocker cover, front timing cover, fuel injection pump, crankshaft front pulley and damper, gearbox, intake and exhaust manifolds. The rank order of noise levels from these sources varies for different engines but the most prominent sources are usually the crankcase, sump and front timing cover. Usuallly acoustic intensity measurment method is used to rank the individual sources. xiv The noise generated by the action of tyres rolling on road surfaces by the moving vehicles are known as tyre (rolling) noise can have a considerable influence on the total noise level of the vehicle. In this study, in one of the cases, by applying an insulation package and replacing Dl natural aspirated engine by a Dl turbo engine, an attenuation about 5 dB in interior vehicle noise level is reached.