Kamyonlarda kullanılan gürültü kalkanlarında  malzeme ve tasarımın etkisi
Effect of material and design in noise shields used in trucks
Yazar: PARS BATUHAN METİNOĞLU
Danışman: YRD. DOÇ. DR. VEDAT TEMİZ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Akustik = Acoustic ; Akustik emisyon yöntemi = Acoustic emission method ; Akustik impedans testleri = Acoustic impedance tests ; Akustik özellikler = Acoustic properties ; Motor gürültüsü = Motor noise

Özet:
Binek ya da ticari araçların çevreye ve sürücülere karşı yerine getirmeleri gereken bazı yasal hükümleri vardır. Homologasyon adı altında belirtilen bu hükümlerden biri de araçların çevreye yaydığı ses'in şiddetidir. Bu şiddet belli bir "dB" olarak motor sınıfına göre belirlenip iki tür halinde sunulur; İç ses ve Dış ses. Bu ses seviyelerini yakalamak araçta birincil olarak ses kaynağı olduğu için motorda, ikincil olarak ise blokaj parçlarına düşmektedir.
Çalışma için genel anlamıyla ses ve ses algısı üzerinde durulmuştur. Ses'in dalgalanması ve ses'in frekans değerlerinin etkisi incelenmiştir. Sesleri incelemek için kullanılan test metodları ve cihazları hakkında araştırma yapılmıştır. Çalışma ses kaynağın olan motorda bir değişiklik yapılamayacağı düşünülerek sadece izolasyon parçalarını baz almıştır. Ses'in incelenmesi ile igilenen ekibin çalışma  prensipleri ve tasarım öncesi dikkat edilmesi gereken kısımlardan bahseilmişir.
Blokaj parçaları ise aracın tasarımındaki ses yollarının kapatılması ve gelen ses'in sönümlenmesini sağlar. Ses yollarının kapatılması bazı yapışkanlı padler ile yeterli olurken, en fazla katkıyı Gürültü Kalkanları sağlar. Gürültü kalkanları İç ses'in sınırını yakalamak için farklı özelliklerde istenirken (yumuşak ve gözenekli), Dış Ses'in sınırını yakalamak için farklı özelliklerde istenmektedir. (dış yüzeyi sert ve sesi yansıtan gibi.) 
Çalışma sırasında iç ses'i etkileyecek olan blokaj parçaları yerine yasal yükümlükleri olan dış gürültü kalkanlarını ele almaktadır. Etkili bir dış gürültü kalkanı için aranan temel nitelikler içerisinde parçanın sönümleme performansı ve sonrasında da araç üzerindeki performansı karşılanmaldır. Çalışma sırasında seçilen farklı malzememlerden oluşan 3 farklı set incelenmiştir.
Parçaların tasarımı sırasında malzemelerin ses sönümleme katsayıları, maliyetleri, testlerdeki başarısı gibi pekçok kriter önemli bir faktör oynamaktadır. Faktörler arasından en iyi seçimi yapabilmek için sistematik bir eleme yapılıp kısıtlı sürede en uygun seçimin yapılması sağlanır. Çalışma sonunda performans açısından uygun olan parça seti maliyet analizi açısından da değerlendirimelidir. Çalışmanın sonucunda izolasyon malzemesinin kullanılmadığı gürülüt kalkanı seti maliyet olarak fazla çıkması sebebi ile elenerek PU kaplı RIM parça setinin en ideal gürültü kalkanı olmasına yol açmıştır

Abstract:
Commercial or Passenger vehicles have some legal requirements that they need to obliege towards the enviroment and the passengers. One of the  requirements that are also known as "Homologation" is about the sound level that is emitted to the enviroment. The sound level is determined according to the engine classifications and shown in "dB" as either Interior Noise or Exterior Noise. To achieve the required sound level is initially the engines target as it is the primary sound source. The secondary target falls upon the blockage parts throughout the vehicle containning isolative material.
In this study general concept of sound and it's perception is briefly mentioned. The effects of sound waves and frequencies over the sound is also investigated. A research was done on the test method and procedures as well as testing equipments used on the sound measurement is mentioned in the study. This study focuses on the effects of the isolation parts rather than the engine due to the fact that it has less possibility to change according to just sound limitations alone.
During the study of the matter the supportive department's methodology and principles are also given.The isolation parts aim to block or damper the sound effect through the sound paths that start from the source and follows until the end receiver in the vehicle. While some isolative pads alone work to block the sound paths the most effect comes from the Noise Shields surrounding the engine. For interior noise these Noise Shields are required to more porous and soft to absorb more sound. For exteiror noise Noise Shields has to resist to the harsh enviromental effects and perform just as the same requiring rough outer shell. 

Interior noise is important as it is directly effecting the driver's concentration and comfort during the voyage. The issues that cause the spikes in the interiror noise often is due to poor fits and manufacturing problems. But in some cases a flawed design can be the source as well.This study ignores the interior noise effects because there are no legal requirements for it. The interior noise only is for the vehicles quality metrics and a sign of product value. 
Exterior noise on the other hand is regulative and has to be obligied. The effectiveness of the Noise Shield for the exterior sound is the part's absorption performance as well as it's vehicle performance. Any aspect of the vehicle from the powertrain parts to the shape of the mirror housing effects the vehicle performance in exteriror noise point of view. 
Exterior performance involves many engineering approaches such as aerodynamics, fluid dynamics and metalurgy. Using almost all of the engineering knowledge. Due to this it is almost impossible to pre-determine the vehicle performance via a method of analysis. But from the material as well as initial vehicle data a curve of the vehicle performance can be found. 
This study is about the noise shields that have the most effect accustically. During the testing all the variables are kept to a minimum to see just the design and material effect. The geometry of the part plays a vital role as it can be used to suit the needs of the vehicle. For instance if the vehicles noise output is close to legal limits the design of the noise shield can be used block the excess noise. However if the vehicle is in critical noise level noise shield design should be such that maximum amount of isolative material can be used to absorb as much noise as possible.
To see the material and design effect in a vehicle level 3 sets ( PU , Thinsulate and PP based ) of different noise shields are compared and examined. These materials are chosen to represent different isolative parts approach throughout the years.
Thinsulate material represents the off-the-shelf approach that utilises an industrial level of expertise. The company responsable of this particular isolative material has perfected the absorbtion performance keeping the heat levels as the core starting point. These isolative materials involves many ingrdiants and mixtures to withstand harsh exterior effects. However it's design flaw is that it is hard to shape as it can not be molded.
PU material represents the classical approach that is commonly used in almost any field requiring isolation. The reason of still using the PU after all the years back from it has been found, is due to it being managable in almost any way. The effectiveness of the material can be tailormade to fit to ordinay usages. Not only that it is moldable to easy to medium diffulty shapes. However it is not resistant as almost all the other isolative maietrials to water and heat. It's pourous composition is the reason why it is easly effected by the outside effects. To overcome this external layers are used. The most common approach is to lace the PU with hydrophobic layers to disable water ever reaching to PU.
Lastly PP material represent the latest design approach in the isolation parts.The usage of isolation materials while being effective also comes with additional problems such as covering large amount of package space and additional weight to the vehicle. Therefore engineering knowledge that is gathered over the years aims to use simple parts that can function as the isolation material laced noise shields. One example to these new part is using blowmolded PP noise shields. The design is in such way to create air cushions on the interior surface while having an aerodynamic design on the outer surface. The main disadvantage is the accustic performance limit of these parts. At the end of this study all the parts will be compared to see their effects.
The part's absorption coefficient , cost and performeances in the vehicle tests all play an important role in the final decision. A sistematic decision and the key values comparison is used to find the optimal design. In the end of the tests a cost analisis is done to check the feasibility of the design. In the end of the study the noise shield set that passes all tests is the PU laced RIM Noise Shield. This resultshows that the best performance is obtained in the porous material isolation with the sound encapsulation outer shell combination. However the future development in the parts structure leans toward single composite materials that can be both absorptive and encapsulative enough to hold the sound levels as intented