Taşıtlarda İç Gürültü Değerlendirmesi

Abstract

Günümüzde yaşanan teknolojik gelişmelerin en çok etkilediği sektörlerden biri de otomotiv endüstrisidir. Bu teknolojik gelişmeler ile beraber müşterilerin beklentileri yükselmiştir. Bu beklentileri karşılamak için üreticiler birçok konunun üzerinde çalışmalar yapmaktadır. Bunlara örnek vermek gerekirse; yakıt ekonomisi, güvenlik, konfor vb… Konfor, müşterilerin bir aracı seçme kriterlerinin üst sıralarında yer almaktadır. Konforun da en önemli parametrelerinden biri, belki de en önemlisi taşıt içi akustiğidir. Bunun farkında olan üreticiler taşıt içi akustiğinin iyileştirilmesi için ciddi harcamalar ve araştırmalar yapmaktadır. Taşıt üzerinde bulunan ve gerçekleşen birçok sistem taşıt içi akustiğine etki etmektedir. Bu karmaşık olayları incelemek ve taşıt içi akustiğin geliştirilmesi için ciddi bir zaman ve kaynağa gerek vardır. Ayrıca taşıtın sınırlı bir büyülüğe sahip olması, uygulanacak bir takım modifikasyonların ağırlığı artıracağından dolayı yakıt tüketiminin artması gibi zorluklarda karşımıza çıkmaktadır. Özellikle kamyon, otobüs ve büyük ticari gibi yüksek tork ve güce sahip araçlarda bulunan hava kompresörü, soğutma fanı, şaft freni gibi elemanlar da taşıt içi akustiğine ciddi bir biçimde olumsuz etki yaratmaktadır. Kötü bir kabin içi akustiğine sahip araçlar, içinde uzun süre bulunan sürücü ve yolcularda sağlık sorunlarına yol açabilmekte ve iyi bir sürüş kalitesi sağlanamadığı gibi güvenlik açısından da sorunlar yaratabilmektedir. Taşıt içi için yapılacak olan akustik iyileştirme öncelikle gürültünün kaynaklarının belirlenmesi, bu kaynakların yarattığı gürültünün seviyesini ve hangi frekans aralıklarında oluştuklarının bilinmesi gerekmektedir. Tüm bu işlemlerin yapılması düşünülenin aksine kolay değildir. Ölçümlerde karşımıza birçok engel çıkmaktadır. Örneğin, motora bağlı olan soğutma fanın etkisini incelemek için hem fan çalışır iken hem de devrede olmadığı durumda ölçümler yapılması gerekir. İç gürültü ölçüm standardı (ISO 5128) de belirtilen motorun rejim sıcaklığına getirilmesi hususu, günümüz motor soğutma fanlarının devreye girme ve çeşitli hızlarda çalışması lojiklerini tam olarak kapsamamaktadır. Motor performans ve emisyonlarının daha iyi kontrol edilebilmesi için motor devrinden bağımsız çalışan sistemler kullanılabilmektedir. Bu durum iç gürültü ölçümleri yapılırken soğutma fanının etkilerinin kontrolünü zorlaştırmakta ve ölçüm sonuçlarını etkilemektedir. Yardımcı donanımların iç gürültüye etkilerinin belirlenmesi amacıyla, bu donanımlar kontrollü olarak devreye sokulup çıkarılarak ölçümler yapılmış ve bu etkiler frekans analizlerinde irdelenmiştir. Bu irdelemelerde sübjektif değerlendirmeler de önem taşır. Yardımcı donanımların devreye girdiğinde iç gürültüde meydana gelen değişimler (ses seviyesi, frekans içeriği ve algılama) irdelenmeye çalışılacaktır. Bu tezde ilk olarak temel ses bilgisi ele alınacaktır. Sonrasında taşıtlarda motorlu taşıtlar iç gürültü ölçümü standardı anlatılacaktır. Ardından taşıtlarda iç gürültü hakkında istatistik bilgilere yer verilecektir. İstatistiksel bilgilerden sonra taşıtlar iç gürültünün önemine değinilecektir. Son olarak da bu tez için bir otobüste çeşitli durumlarda yapılan iç ses seviyesi ölçümlerinden ve sonuçlarından bahsedilecektir.

Today, automotive is the one of the industry that mostly influced by technological improvements. At the same time this improvements change customer’s expectations. As a result of changed customer’s expectations, automakers have been investigating lots of subjects that are related to automotive and human. For example; fuel economy, safety, comfort etc… Comfort takes first place customer’s checklist to buy a new car. Besides interior acoustic of vehicle is the most important factor of the comfort level in customer’s point of view. Therefore automakers have been making investments and researches for improving vehicle’s interior acoustic’s level. As you will see in this thesis, acoustic in vehicle has lots of complex systems. To make improvement in acoustic levels, researchers need lots of time and money. Some modifications on vehicle for improving interior acoustic level can affect fuel economy and some other factors in negative way due to adding mass to the limited volume of vehicle. Some parts such as air compressor, cooling fan, retarder affect have an important affect in heavy commercial vehicles and busses’ interior acoustic levels. Health problems and safety issues could be occured during long driving times due to acoustic levels that is not good enough in a vehicle. Before studies for improving acoustic level, we need to clarify sourcing, level and frequency range of noise. But these steps not easy to clarfiy as it seems. There are lots of difficulties in measurements of interor acoustic levels. For example; due to ISO 5128 when we need to investigate affects of measure cooling fan that is integrated to engine, to vehicle interior acoustic level. But ISO 5128 doesn’t contain engine operation, cooling fan operation condition detailed informantion. Today, cooling fans’ operation condition doesn’t related to engine speed for controlling engine performance and emissons. For these reasons, it is too difficult measure interior acoustic level for investigate the affect of cooling fan. During mesurements regarding to investigate affects auxiliary units on interior acoustic level, this units are actived and deactived for specific time period. As a result, auxiliary units affects could be determined. Also, subjective assessments are important for these measurements and investigations. Firstly, basic sound information with details has been explained in this thesis. Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing. Sound is a sequence of waves of pressure that propagates through compressible media such as air or water. (Sound can propagate through solids as well, but there are additional modes of propagation). Sound that is perceptible by humans has frequencies from about 20 Hz to 20,000 Hz. Vibrations that are felt are under 200 Hz, vibrations between 20 Hz - 20,000 Hz are audible by humans, vibrations over 20,000 Hz are ultrasonic and not audible by humans. In air at standard temperature and pressure, the corresponding wavelengths of sound waves range from 17 m to 17 mm. During propagation, waves can be reflected, refracted, or attenuated by the medium. Noise and noise in vehicles ( especially interior noise ) has been investigated and explained after basic information of sound. In relation to sound, noise is not necessarily random. Sounds, particularly loud ones, that disturb people or make it difficult to hear wanted sounds, are noise. Noise in vehiceles can be classified as interior and exterior noise. There are some regulations about exterior noise that is caused by a vehicle. If a car doesn’t meet regulations, it can’t get ok to sell. But there is no regulation about interior noise of a car. Vehicle interior noise is a combination of engine noise, road noise, intake noise, exhaust noise, aerodynamic noise, noise from components and ancillaries, brake noise. squeaks, rattles and tizzes. Engine noise results from gas forces in the cylinders applied to the structure of the engine, causing vibration to occur which is then radiated as noise. Road noise that affects interior noise of a vehicle resulting from the contact between the tyres and the road, being transmitted to the interior by both airborne and structure-borne paths. Aerodynamic noise is caused through a variety of mechanisms; aerodynamic excitation of the glasswork and the roof panel that is causing structure-radiated noise in the interior, airflow over the underside of the vehicle causing transmission of airborne sound to the interior, noise transmission through door and glass seals due to leakage, vortex passing over sharp edges in the bodywork, cavity flows through partially open windows and sunroofs. Brake noise is the pairing of friction surfaces at the pad and rotor disk. Brake noise is radiated by the brake components themselves. Squeaks, rattles and tizzes influenced by material choice, surface finish, frequency of excitation, amplitude of excitation, interference levels between the two materials, normal loads,temperature, humidity. Apart from squeaks, rattles and tizzes that occur inside the passenger compartment,noise or vibration usually originates from outside, interacting with the vehicle structurein some way and then producing radiatedsound inside the compartment. The interaction with the structure can be either as; an airborne noise path - airborne noise from outside the passenger compartment leaking in to cause airborne noise inside. A structure-borne noise path - vibration from outside causing the surfaces of the passenger compartment to vibrate and radiate noise. After investigating noise, ISO 5128 – Acoustics, Measurement of noise inside motor vehicles was explained with all details including nature of tests, measured quantities, measuring equipment, test road conditions, vehicle conditons, microphone conditions, etc... Interior noise levels has been reducing since 1970s due to technical improvements about devices in vehicles such as engines, intake – exhaust systems, isolation. Nearly 400 vehicle’s interior noise levels during driven in specific speeds can be found. Some analyses was done with nearly 400 vehicle’s noise levels and other attributes. In these analyses we consider the vehicles’ fuel type, mass, maximum torque, maksimum power and noise level. Extended values of these analyses and information can be found additional section of this thesis. Secondary systems that are reasons of the interior noise too has become important these years. We can mention two main reasons for this statement. Firstly, technical improvements about noise and vibration in primary systems such as tyres, engines, intake-exhaust systems. Secondly, electrical and hybride cars. As you know there are lots of electrical and hybride cars that haven’t got internal combustion engine and gearbox( main sources of interor noise of vehicles.) in the traffic. Secondry systems powered by engine serpentine belt and secondry systems in electical vehicles were investigated. These are the accessories driven by the engine through a serpentine belt. For example; alternator, power steering pump, air conditioning compressor. The sound quality attributes induced in the vehicle cabin are tonal components derived from the fundamental frequencies of the prime movers (the compressor for the HVAC system,the power steering pump for the hydraulic-assist steering system,etc.). The climate control system however has another sound quality attribute, which is air rush, or noise due to the blower and the ventilation ducts. Also seat adjusters, mirrors, pedals, sunroof, window lifters that are powered by direct current motor cause undiserable noise inside of a vehicle. For electrical cars, electrical motor is the main reason of interior noise. It would be wrong to assume that electric motors are completely quiet. Depending on the design of the motor, the electromagnetic pulses and corresponding torque pulses from the motor can be very strong. These can be radiated as noise directly from the motor housing and can also be transmitted structurally to the support structure through the motor mounts. Also, the noise radiated directly from the motor is generally quite high in frequency (>1000 Hz), which means that conventional acoustical materials are highly effective at blocking and absorbing this airborne noise energy. So while it is true that electric motors present a lesser challenge for noise control engineers as compared to internal combustion engines, strategies for mechanical and acoustic isolation of the motor must still be effectively executed. In addition to the motor itself, other parts of the electromotive system create noise, in particular, the gearbox and the power electronics unit. Also there are some ancillary systems that can cause noise inside of a electrical vehicles. For example; vacuum pump to power brake booster, heat exchanger cooling fan for motor and battery cooling, fluid pumps for system cooling, compressor for cabin cooling, anti-lock braking system’s pump and module, electric steering rack. All these mechanisms and systems have become important for interior acoustic in vehicles. After all, a study about interior noise has been done in a bus. Five microphones and five accelerometers were placed proper areas inside of the vehicle. Then these devices connection has been provided to the workstation. After the test setup, we ran the tests in stationary and nonstationary status in different conditions. While we were making test, we collected all datas from microphones, accelerometers in workstation for analyze. After the tests, noise maps has been created from datas that we collected during test by analyze program. Then assumpiton and reviews has been done in this thesis according to noise maps and datas. In summary, basic sound has been explained. Then mesaurement of noise inside motor vehicle standard ( ISO 5128 ) was desribed. After this section statistics about vehicle interior acoustic levels were take a part. Afterward statictical information, importance of interior acoustic has been explained. In the end, there is a study of interior acoustic level measurement and analyze of a bus for this thesis.