MOTOR Magazine

A MOTOR Magazine Newsletter
September 18, 2017

Contributed by Bob Chabot
Seeing Noise

Sound quality is a vehicle-wide design consideration

If you’ve been in a new car lately, you know it’s about more than just performance and fuel economy. Automakers are also focused on reducing noise inside cabins from sources such as wind, tires, the engine and other vehicle systems. With the focus on reducing noise, new acoustic challenges have emerged, as drivers and passengers notice noise from other systems more.

Drivers and passengers now notice noise more readily than ever before. A false note sounds as wrong to the customer as their favorite song being sung off-key. Likewise, the sound produced by an automobile’s exhaust still serves as the audible signature of the brand. And in recent years, noise produced by automobile HVAC systems has become a top area of complaints.

Current Acoustical Quality Technological Challenges Abound
Sound quality is a trending topic in many automotive applications. Considerations typically include the reproduction, transmission, and reception of sound signals under diverse conditions. Sound signals, for example, interact within structures, porous materials, flow and even the transducers involved in their generation and detection.

To address the complex physics involved in automobile acoustical systems today — from in-cabin infotainment to vehicle exhaust/muffler noise — acoustical engineers (aka acousticians) are increasingly using modeling and simulation. Simply put, both offer computational tools that can accelerate design tasks, while simultaneously reducing the need for costly and time-consuming physical prototypes. In addition to time and cost benefits, modeling and simulation also facilitate an increased understanding of a design, which leads to better-informed decisions and higher-quality products.

In some cases, the sound outcome must resonate with crystal clarity; in others, only certain sounds are desired; and in others, noise must be minimized, if not masked entirely. Each represents a set of multiphysics problems that acousticians have to consider for the efficient and optimized development of new products and technologies.

For example, they each require a detailed understanding of sound propagation, transducer behavior and the other factors necessary to optimize each system. Clever digital signal processing is not enough to make systems behave and sound good. This places a critical requirement on the modeling software in terms of the ability to couple physics effects relevant to the full system.

Sound quality control is now a critical part of the design of any vehicular system producing noise, ranging from the reproduction of sound inside car cabins (above) to the output from the exhaust/muffler systems. The acoustics simulation of a sedan interior above includes sound sources at the typical loudspeaker locations and the resultant acoustic pressure field inside the cabin. (Image — COMSOL Multiphysics)

Optimization Requires Detailed Software and Numerical Analysis
Cabin sound has transitioned from automakers slapping an off-the-shelf sound system into a vehicle to designing more precise and pleasing sounds. Gone are the “trial and error” days, where abundant vehicle and road noise negated any reason or possibility of having clean pure sound. Driven by consumer demand, drivers and occupants, together with regulators, have made decibel quality marketable.

Fortunately, digital software modeling, such as that provided by COMSOL Multiphysics’ Acoustics Module, is ideally suited for modeling the many frequencies and variables involved in modern acoustics design and management, ranging from infrasound to ultrasound, as well as the multiscale nature of acoustics when dealing with thermoviscous (heat), aeroacoustics (flow) or other loss mechanisms. Many multiphysics scenarios can be set up seamlessly between the different physics. In addition, a library of many specialized formulations of the governing equations of acoustics is integrated into the software. And even for the most complex modeling scenarios, users can change design parameters and quickly analyze results with respect to industry standards and customer-specific requirements.

Miniature loudspeaker and infotainment systems are now driven at such high sound pressure levels that distortion and attenuation due to nonlinearities, some produced by the hardware itself, must be considered. The delicate multiphysics balance between electrostatics sensitivity, structural membranes, and thermo-viscous acoustics is essential to the modeling and development of modern condenser microphones and speakers used in sound systems today.

Heat is another issue, specifically its effect on sound caused by the pressure/temperature fields created by electronic devices. Computers, sensor, circuit boards and chips have become prolific, more powerful, and subsequently generators of increasing amounts of heat. If not adequately dissipated and managed, this can distort sound, impair the performance and even reduce the lifespans of electronic hardware devices.

To better manage the advancing electronic technology, the design of heat sink geometries has become a critical consideration. Heat sinks are a combination of materials and airflows used to provide cooling capacity and dissipate heat from electronic circuitry. When modeling heat transfer in these systems, it’s important to accurately and dynamically measure the temperature of electronic components to determine the cooling capacity of various heat sinks.

This modeling case (illustrated below) consists of an electronic chip, aluminum heat sink and a thin layer of various thermal greases between the two. Conduction and convection are the main forms of thermal energy transport that dissipates heat away from the electronic component. Not only can the optimum thermal grease be determined, the modeling can be configured to assess different gauges of aluminum or alternate materials used to construct the heat sink. (Image — COMSOL Multiphysics)

Heat sinks have become critical to the cooling of electronic devices that have proliferated in the modern automobile, including the management of sound. To that end, materials, lubricants, location and other factors must be optimized. Digital modeling has greatly reduced the time and costs involved; it has also enabled better results. (Image — COMSOL Multiphysics)

Seeking the Sound Sweetspot
Noise mitigation is central to vehicle safety, comfort and compliance. That’s why acousticians seek to locate and control not just engine noise, but noise from as many other sources as possible. Today, we know noise emanating from an exhaust system comes primarily from two main sources: (1) Acoustic pulses generated by periodic combustion pressures in the cylinders, and (2) Flow-generated noise created within the exhaust system by complex geometries and airflow paths.

Consider loud exhaust systems: Despite finely detailed geometry and complex airflow patterns, they are one of the most common culprits for undesired noise. In the past, loud car exhaust usually wasn’t detected until prototype testing, too late to efficiently identify the source and correct the problem.

Even recently, the attenuation of acoustic pulses from cylinders was estimated using 1-D acoustic simulations or simple semi-empirical tools that provided some guidance to exhaust tuners. But the shortcoming of these methods has been their inability to predict and identify accurately the source of the self-noise produced within the exhaust system itself, let alone do so before manufacture.

These key problems have been addressed by “digital aeroacoustics simulations,” new 3-D visualization technologies that add dimension and insight pertaining to acoustic pulses and also the second major source — flow-generated noise issues. These 3-D simulations essentially visualize the transient flow throughout complex exhaust system geometries to help engineers improve the mitigation of noise pollution, reduce exhaust noise inside cars and meet exhaust regulations.

In addition, simulation and visualization facilitate locating and correcting the sources of unwanted sounds in noisy car exhaust systems before a physical prototype is built. This allows designers to try out design changes to find the best solution faster — long before prototype testing begins — and ensure vehicle development meets its timeline, budget, and noise targets.

Turn up your volume and watch this exhaust noise simulation, complete with a detailed technical description of flow visualizations, which was produced using Exa Corp.’s PowerFLOW solution. (Video — Exa Corp.)

Diagnostics Pinpoint, Quantify and Qualify Air Flow Noise
Exa Corp.’s PowerFLOW technology is an example of the solutions used by manufacturers to engineer out undesirable noise, while retaining desirable noise. Not only does the technology accurately predict the turbulent airflow driven by the intricate geometry details in the exhaust system, it simultaneously identifies how to capture and propagate the desired noise generated from inside and outside the exhaust system.

In addition, engineers can listen to sound generated by the simulated exhaust system long before a prototype is built. This technology makes it possible for engineers to quickly investigate the performance of different exhaust designs and architectures to achieve the right sound signature and meet noise and fuel economy standards with fewer design and prototyping iterations.

The technology lets you hear and visualize transient flows throughout complex 3D geometry to improve the mitigation of noise pollution, reduce exhaust noise inside cars and meet exhaust regulations. The sources of unwanted sounds in noisy car exhaust systems can be found and corrected by hearing vehicle noise before a physical prototype is built. Designers can try out changes to find the best solution faster — long before prototype testing begins — and ensure vehicle development meets its timeline, budget, and noise targets.

The digital aeroacoustics simulation approach — which uniquely combines flow properties and acoustic impact under realistic operating conditions — is critical to achieving a robust, efficient and economical design for systems earlier in the development stage. Of note, it allows changes to vehicle systems well before hardware prototypes are committed to, avoids setting back launch deadlines, and keeps part costs, weight and other consideration in check.

Furthermore, it avoids late-stage or post-production changes and their associated costly impacts (e.g. recalls). In short, digital aeroacoustics simulations re-create both functional and acoustical properties of 3D geometry, and replicate real-world conditions, enabling engineers to better understand and improve vehicle systems’ performance.

BMW Group used Exa Corp.’s PowerFLOW digital aeroacoustics simulation technology to design and evaluate noise levels in the new HVAC system components for its BMW 7 Series and Rolls-Royce Phantom models. The noise sources before (lower left) and after (lower right) are shown in red. (Image — Exa Corp.)

It’s Not Just Infotainment and Exhaust Anymore
When previously considering HVAC system design changes, the BMW Group relied on internal analysis of flows through standalone components such as ducts or blowers. This gave incomplete information on the acoustic performance of the whole system.

Not only was the former process time-consuming and expensive, it was incapable of optimizing multiple concerns. The automaker also found the methodology could be misleading when trying to decide which sub-system to improve. Furthermore, late-stage changes can’t account for potential impacts on related systems because it’s too late in the product development lifecycle to do so.

That unreliability prompted a change. With the HVAC system distributed throughout the vehicle, its components interact with many other systems. Because it’s so connected throughout the vehicle, changes to an HVAC system once a hardware prototype has been made is completed are difficult, costly and usually insufficient. When the automaker was preparing to develop the BMW 7 Series and Rolls-Royce Phantom, it wanted to avoid unwanted HVAC system noise, but do so in a more timely and cost-effective manner than its previous methods.

BMW then began collaborating with Exa Corp., and of note, began using Exa’s digital aeroacoustics simulation technology to perform evaluations of the new models’ complete HVAC systems. Design engineers from BMW and Exa reported on how they addressed these challenges in a 2015 paper at the 3rd International ATZ Conference in Zurich, titled Acoustic Source Detection for Climate Systems via Computational Fluid Dynamics for Improved Cabin Comfort.

“It was critical for BMW to achieve a robust design for the HVAC system early in the development stage in order to ensure a successful prototype test and avoid late-stage failures,” shared Dr.-Ing. Michael Spickenreuther, BMW Group Leader for Overall Vehicle Development. “Acoustic 3-D simulations and comparisons for characteristic sub-units of an HVAC system were completed under typical operating conditions, such as defrost, fresh air ventilation, and air recirculation mode — obtained by altering the flap positions and varying blower rotation speed or other controls. The noise sources were detailed as contributions per component, corrected by the acoustic transfer function between the source and a defined target point or target space, such as the driver’s ear and/or the passenger’s ear."

Simulation Removes the Guesswork
“We learned that only digital simulation can meet these needs, since it digitally re-creates both functional and acoustical properties of 3D geometry, and replicates real-world conditions so engineers can understand and improve vehicle systems’ performance," Spickenreuther noted. "We were so impressed with the noise-sourcing capability that BMW has now implemented the technology across all vehicle platforms to deliver quieter automobiles to customers.”

Automakers have long lamented that conventional fluid dynamics software and 1-D simulations are unable to accurately predict, identify and optimize the self-noise produced by automobile systems in a normal vehicle development cycle. But it's become clear to them that 3-D aeroacoustic simulations can.

Engineers now have the ability and tools to evaluate a wide range of possible designs simultaneously — sooner and less expensively, without having to build a prototype — to improve engine operation, fuel economy, sound quality or meet evolving government standards. That enables them to determine the ideal tradeoff between acoustic performance and flow resistance. In short, they can now get sound design right the first time.

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