Passive Noise Reduction Possibilities for Trucks

Validation of rolling-stock is a time and cost expensive task during the development of train. Replacing type tests by simulation (virtual validation) that fits the need of the complete railway industry in terms of accuracy and uncertainty, is possible in some cases. Simulation is widely used by train manufacturers during development of new products with good accuracy of prediction in a controlled environment as free field and ballasted track. Methodologies available are able to predict correctly in controlled conditions (ISO 3095/3381) interior and exterior noise for different operating conditions. In this paper, simulation cases will be presented for a standard metro product operating in controlled conditions. Simulations will be compared with experimental data for interior and exterior noise. Exterior noise modelling and propagation is done with analytical tools while interior noise is modeled using an experimental-numerical (beam-tracing) approach. Input data for simulations corresponds to numerical models of rolling noise and measured sound power level of equipment. Validation of the models are discussed and analyzed to confirm the reliability of virtual validation for rolling stock operating at controlled conditions. A proposal of a virtual validation approach based on uncertainties of the simulation is also discussed.

<div class="htmlview paragraph">The Idaho National Engineering Laboratory (INEL) evaluates EV propulsion systems and components for the U. S. Department of Energy (DOE) Electric and Hybrid Vehicle (EHV) Program. This paper describes an INEL study which compares the exterior and interior acoustic noise levels of an electric vehicle to its internal combustion engine (ICE) counterpart base vehicle, under various operating conditions.</div> <div class="htmlview paragraph">The electric vehicle was a converted 1988 Ford Escort station wagon, retrofitted with a DC electric powertrain developed by Soleq Corporation. A comparably-equipped gasoline-fueled ICE-powered Ford Escort station wagon provided the baseline acoustic noise levels with which to compare the electric vehicle. Measurements of the interior and exterior noise levels were obtained using a Bruel and Kjaer (B&K) Type 2231 Modular Precision Sound Level Meter. The tests were conducted in accordance with applicable Society of Automotive Engineer's (SAE) standard practices (<span class="xref">1</span>, <span class="xref">2</span>, <span class="xref">3</span>, <span class="xref">4</span>, <span class="xref">5</span>, <span class="xref">6</span>, <span class="xref">7</span> and <span class="xref">8</span>)<span class="xref">*</span> at Chrysler's Arizona Proving Grounds in Wittmann, Arizona.</div> <div class="htmlview paragraph">The results indicate that radiated interior and exterior acoustic noise levels of the electric vehicle were noticeably quieter under acceleration and idle conditions. However, under constant speed operation the electric and the ICE exhibited essentially equivalent interior and exterior noise levels.</div>

<div class="section abstract"><div class="htmlview paragraph">Transfer path analysis is a powerful tool to support the vehicle NVH development. On the one hand it is a fast method to gain an overview of the complex interplay in the vehicle noise generation process. On the other hand it can be used to identify critical noise paths and vehicle components responsible for specific noise phenomena.</div><div class="htmlview paragraph">FEV has developed several tools, which are adapted to the considered noise phenomena: Powertrain induced interior noise and vibration is analyzed by VINS (Vehicle Interior Noise Simulation), which allows the deduction of improvement measures fast enough for application in the accelerated vehicle development process. Further on vehicle/powertrain combinations not realized in hardware can be evaluated by virtual installation of the powertrain in the vehicle, which is especially interesting in the context of engine downsizing from four to three or six to four cylinders. Road induced interior noise is investigated by “Chassis-VINS” or “Fast-VINS”, depending on the required level of detailing and project timing. With the upcoming change of pass-by noise legislation VENS (Vehicle Exterior Noise Simulation) supports the target oriented component development to fulfill the legal limits.</div><div class="htmlview paragraph">The application of these methods on more than one hundred vehicles - from microcar to heavy duty truck - in the last decade allows for rating vehicle noise shares relative to the state of the art. Selected examples of different vehicles show possible distributions of several airborne and structure borne noise paths.</div></div>

Exterior helicopter noise is often perceived as loud and unpleasant by people boarding the helicopter or people living in urban areas or close to airports where rotorcraft is flying. Excessive cabin noise in the helicopter is also an issue for passengers or crew flying the helicopter, especially for long duration flights. The objective of the paper is to investigate commercially available techniques for measuring and analyzing helicopter noise and ultimately help design engineers get insight into noise mechanisms in view of potential noise improvements. Exterior noise measurements were performed on a MD902 helicopter equipped with a Notar (No TAil Rotor) system. State-of-theart focalization and deconvolution techniques are presented to localize and rank dominant noise sources and identify aeroacoustic phenomena or mechanical devices responsible for the helicopter noise signature. Primary and secondary sources are clearly identified which provides guidance for possible noise reduction measures. Main rotor noise is found to be clearly dominating the overall noise spectra whereas Notar, engine exhaust and cooling system constitute secondary sources. Interior noise is also investigated for various flight conditions and dominant noise sources are localized in the cabin and some possible noise control treatments are suggested. It is shown here that the acoustic array technique is a very efficient and well suited tool to analyze both interior and exterior helicopter noise.

Beech 1900D flight test to characterize propeller noise on the fuselage exterior

<div class="section abstract"><div class="htmlview paragraph">An Air intake system (AIS) is a duct system which leads the airflow going into the internal combustion engine. Combustion requires oxygen, and the more oxygen is provided into the combustion process the more power it will produce. The lower the air temperature, the higher its density, and hence there is more oxygen in a unit volume. The quality of air entering engine can be measured with the air temperature.</div><div class="htmlview paragraph">AIS design and routing influence the air charge temperature (ACT) at intake manifold runners and ACT is normally measured at AIS throttle body in reality. Higher ACT lead to inefficient combustion and can lead to spark retard. Optimization of AIS designs and reduction of ACT can improve engine performance and vehicle fuel economy.</div><div class="htmlview paragraph">High ACT can be a result of two different phenomena: <ul class="list disc"><li class="list-item"><div class="htmlview paragraph">Recirculation - Hot air from the underhood environment ingested into the dirty side of the air intake system. This is the result of the AIS snorkel not being isolated or sealed from the engine compartment hot air.</div></li><li class="list-item"><div class="htmlview paragraph">Underhood temperature rise - Air inside AIS is heated by the ducts walls because it is routed through a hot underhood environment. The longer the length and larger the diameter of the air induction system, and the higher duct skin temperatures, the worse the temperature rise.</div></li></ul></div><div class="htmlview paragraph">In this paper, it is planned to simulate and study the effects of following factors on ACT with CFD using a FCA production vehicle: <ul class="list disc"><li class="list-item"><div class="htmlview paragraph">Snorkel location: snorkel sucking fresh air at side air guide or snorkel sucking underhood hot recirculation air.</div></li><li class="list-item"><div class="htmlview paragraph">Exhaust hot end at front of an east-west engine or at the rear of the engine; these two arrangements will result in different underhood air temperatures and radiation to AIS surfaces.</div></li><li class="list-item"><div class="htmlview paragraph">Heat shield effects on ACT: heat shield blocks radiation from exhaust system to AIS surfaces, and hence reduces charge air heat pick from AIS duct walls.</div></li><li class="list-item"><div class="htmlview paragraph">The effects of other measures such as cutting holes on hood or fenders to lower engine compartment air temperatures.</div></li></ul></div><div class="htmlview paragraph">Although the airflow in vehicle AIS duct is transient in nature, steady CFD simulations will be performed to study the above cases for simplification, and CFD approach will be discussed in details. Vehicle test data will be used to validate the CFD models.</div><div class="htmlview paragraph">Finally, general practices of AIS development will be discussed, and future work on this topic will be outlined.</div></div>

<div class="htmlview paragraph">Dividing a vehicle pass-by test into a series of short analysis intervals and calculating a spectra for each of these intervals can provide meaningful information to help identify individual noise sources throughout the pass-by test. Due to the continuously changing instantaneous frequency and amplitude of each of the tonal components in pass-by noise data inaccuracies can occur in the spectra calculated in this manner. The questions which are addressed in this report are the selection of the frequency resolution and block size for FFT processing of pass-by test data to provide a power spectrum with acceptable accuracy for noise source identification.</div>

<div class="section abstract"><div class="htmlview paragraph">The passby noise is the net effect of the noise from the engine, tires, vehicle panels, the fan and the exhaust silencer. Exhaust noise is probably the loudest component of the passby noise in diesel trucks. The passby noise is measured when the vehicle is in transient condition according to the standards. A technique is developed to simulate the transient noise from the exhaust using 1-d simulation software AVL-Boost. It considers thermodynamics of combustion in the engine including the turbocharger, inertia of the engine and the vehicle. The technique helps in optimizing exhaust silencer, acceleration and in-cylinder parameters in transient conditions and the turbocharger fatigue.</div><div class="htmlview paragraph">A 165 kW 6-cylinder turbocharged and intercooled engine is simulated. The transient performance of the engine in the vehicle is validated by comparing the predicted results with the observed fuel consumption on road, time to accelerate to 60 km h<sup>−1</sup> and the fluctuations in the turbocharger speed. The Linear Acoustics model of the silencer in conjunction with this transient model of the engine is used to predict the insertion loss of the silencer and the vehicle passby noise. In the first step, the silencer model is validated with test results by measuring the insertion loss in an actual vehicle. In the second step, the vehicle is modeled with the silencer by using load signals, fuelling and injection timing maps. The predicted noise is compared with the experimentally measured passby noise and the results are encouraging.</div></div>

Vibration isolation of machines, machinery, technological and civil structures has become an inseparable part of their design. Machines can generate energy with very strong low-frequency vibrations which can affect nearby neighbouring machinery, structures and humans. The transmission of vibration energy is highly dependent on isolation of the frame support and/or usage of resilient foundation mountings. On the other hand, improper vibration isolation of a particular system may results in negative dynamic effects occurring in connected/surrounding technological and civil structures, including working spaces of the machine. Well-designed vibration isolation of potential sources can not only effectively reduce vibration energy into supporting and surrounding structures, but also improve sound energy transmission and fatigue characteristics. The presented paper investigates the conditions for reducing transmission of low frequency vibration energy from a recirculation ventilator and air-conditioning unit employing different types of vibration isolators. The solution of this problem is based on theoretical knowledge and existing methodology for transmission of longitudinal and transverse waves including measurement of the structures involved. Theoretical results were compared with vibration measurements for different conditions and configurations of vibro-isolating elements. Application of specific dynamic sources with respect to blocking forces and wave transmission where also performed to determine their effects on the machine foundation and transmission of energy to its surroundings. Finally, the paper suggests some measures that can be taken which will effectively reduce unwanted vibration energy occurring on the foundation of the machine as well as the machine itself and its immediate surroundings.

<div class="section abstract"><div class="htmlview paragraph">One of the practical consequences of the development of low CO₂ emission cars is that many of the traditional NVH sound engineering processes no longer apply and must be revisited.</div><div class="htmlview paragraph">Different and new sound sources, new constraints on vehicle body design (e.g., due to weight) and new sound perception characteristics make that the NVH knowledge built on generations of internal combustion-powered vehicles cannot be simply transferred to Hybrid and Electric Vehicles (HEV). Hence, the applicability of tools must be reviewed and extensions need to be developed where necessary.</div><div class="htmlview paragraph">This paper focuses on sound synthesis tools as developed for ICE-powered vehicles. Because of the missing masking effect and the missing intake and exhaust noise of the Internal Combustion Engine (ICE) in electric vehicles, on one hand electric vehicles are quieter than traditional vehicles. On the other hand, other components such as HVAC system, alternator, vacuum pump, power-steering pump, cooling systems, transmission systems, inverter noise, etc., become more important and generate new complex sound signatures. Typically, their interior noise is well-defined by high-frequency noise components which can be subjectively experienced as annoying, hence the increased importance put to sound quality aspects and perceptual-relevant NVH studies.</div><div class="htmlview paragraph">In the paper, a Virtual Car Sound (VCS) synthesis technique for HEV is designed. This approach turns out to be a very helpful engineering tool to design a brand specific sound and allows reproducing typical sound features of HEV during real-time driving simulation based on a Sound Quality Equivalent Model (SQE). Several SQE models were constructed for different powertrains of HEV in various engine conditions. Finally, the results of the sound metrics and listening tests indicate a comparison between the synthesized sounds and the original sound recordings.</div></div>

Interior noise characteristics, source identification and its quantification contribution of 400 km/h high-speed train in different operating environments

<div class="section abstract"><div class="htmlview paragraph">While designing the exhaust system of passenger car on one hand there is stringent emission regulations, packaging constraints, high NVH performance requirement. On the other hand with lightweight vehicle design there is tremendous pressure on weight reduction of exhaust system while keep the same NVH performance levels. Exhaust system consist of muffler, bellows, pipes and hangers. For muffler design both acoustic (transmission loss, pass-by noise, tail pipe noise etc.) and non-acoustic (backpressure) parameters needs to be considered.</div><div class="htmlview paragraph">In the current paper, methodology for muffler design optimization using 1D acoustic simulation software is presented. The baseline exhaust design consist of two mufflers; main muffler and post muffler. Simulation methodology is developed to optimize main muffler design in order to eliminate post muffler requirement while achieving the same performance of baseline exhaust design. For this purpose a DOE study is conducted by varying primary muffler internal pipe dimensions, baffle location, perforation etc. to achieve transmission loss (TL) and backpressure (BP) of the baseline design.</div><div class="htmlview paragraph">Based on simulation study a physical prototype is made for optimized exhaust design consisting of only main muffler (with modified design compared to baseline main muffler). Transmission loss and backpressure is measured physically and confirmed that the optimized exhaust design is able to deliver the same NVH performance as that of the baseline exhaust design. Furthermore, a vehicle level physical test is conducted to compare the pass-by noise for optimized exhaust design and compared with baseline exhaust design. It is observed that the optimized exhaust design with one muffler is able to maintain the same pass-by noise level as that of the baseline exhaust design with two mufflers.</div></div>

<div class="htmlview paragraph">Research is demonstrating that it is possible to significantly reduce the in-cab noise levels of the Army's current truck fleet, with accompanying exterior noise reduction. Results are applicable to future Army trucks as well and are reported on a program utilizing a 5-Ton Army Truck. This type of truck was found generally to exceed the applicable limits established in the new MIL-STD 1474, “Noise Limits for Army Material.”</div> <div class="htmlview paragraph">The program identified major noise sources and important paths for sound to enter the cab and resulted in the selection of optimum sound absorption, sound barrier and vibration damping materials. The effect of a prototype exhaust system and its placement relative to the cab and the importance and optional means of controlling cooling fan and engine noise were also investigated.</div>

<div class="section abstract"><div class="htmlview paragraph">The influence of moisture adsorption, prior braking, and deceleration rate on the low-speed braking noise has been investigated, using copper-free disc pads on a passenger car. With increasing moisture adsorption time, decreasing severity of prior braking or increasing deceleration rate, the noise sound level increases for the air-borne exterior noise as well as for the structure-borne interior noise. The near-end stop noise and the zero-speed start-to-move noise show a good correlation. Also, a good correlation is found between the noise measured on a noise dynamometer and on a vehicle for the air-borne noise. All the variables need to be precisely controlled to achieve repeatable and reliable results for dynamometer and vehicle braking groan noise tests. It appears that the zero-speed start-to-move vehicle interior noise is caused by the pre-slip vibration of the brake: further research is needed.</div></div>

<div class="htmlview paragraph">Interior noise has become a significant performance attribute in modern passenger vehicles and this is extremely important in the luxury market segment where a quiet interior is the price of entry. With the elimination of early prototype vehicles to reduce development costs, high frequency analytical SEA models are used to design the vehicle sound package to meet targets for interior noise quality. This function is important before representative NVH prototypes are available, and later to support parameter variation investigations that would be cost prohibitive in a hardware test. This paper presents the application of an analytical full vehicle SEA model for the development of the acoustic package of a cross over luxury utility vehicle. The development concerns addressed were airborne powertrain noise and road noise. Power flow analysis was used to identify the major noise paths to the interior of the vehicle. The SEA model allowed the performance of alternative designs to be assessed, the reduction of the power input from the major noise paths to be quantified and optimization of the acoustic package for both road noise and powertrain noise to be completed. The SEA model was used to drive acoustic package design to meet not only acoustic targets but also the mass and costs targets and deliver an optimized package.</div> <div class="htmlview paragraph">The other major benefit of the full vehicle model that this paper will present are the impacts of not meeting sub-system requirements that are usually rolled down to the supplier. This is invaluable as it informs the vehicle design team what the impact is on the sound pressure level at the “drivers ear” of not meeting subsystem requirements, for example the “engine compartment absorption or front of dash transmission loss requirement”. It is concluded that the use of a detailed SEA model is a highly valuable design tool to support ‘A’ to ‘B’ design and material comparisons, to identify major noise paths, and to provide direction to the program.</div>