车辆总工程师并不满意他在最近一次试驾中听到的车内的噪声。

他原本正在仔细复核一款新的变速箱，并打算把它用在公司最具竞争力的跑车系列上。然而，当车搭载上这款变速器后，车内产生了很大噪音，效果令人难以接受。对此，首席工程师显然很不满意，而据设计师和制造工程师解释，这款新的变速箱的最大特色就是其纤薄轻巧的铝制外壳。然而，这款轻巧的外壳正是制造出巨大噪音的“罪魁祸首”。

公司已经投入资金做了这个部件的预定，项目截止日期也不断逼近，一通电话被拨到了Pranab Saha的办公室。

“他们请我帮他们设计一个解决方案，”Saha这么说道。Saha是密歇根州 Waterford 地区咨询公司 Kolano & Saha 的注册噪声控制工程师、声学专家。他告诉我们，一开始，由于铝箔衬背绝缘材料的缘故，这款变速箱的噪音问题得以掩盖。然而，这块绝缘材料上必须开几个孔，这样才能不影响连接和布线。而问题就在于这些切口会给噪声的传播制造通道。

对此，Saha的解决方案是：与其在变速器外壳采用绝缘材料，不如将绝缘材料放在变速器通道下方。工程师将重新审视自己的 CAD 图纸设计，并进行相应的修改。接下来，他们将进行噪声分析，验证新解决方案的有效性。

Saha指出，“这只是我们帮客户解决噪声问题的案例之一：该从哪里入手优化 NVH 性能呢？”

目前，分析、衰减和消除车辆 NVH 影响，已经成为汽车研发中不可分割的一部分。NVH 工程团队正在开展“每天降噪一分贝”的项目，要像降低车辆重量或提高车辆燃料经济性一样，努力降低车舱内的噪声。

“噪音和震动（N&V）这个领域很特殊，因为它的发展原动力来自于市场竞争，而非政府规定，”Saha解释道，“如今，我们会听到有些OEM吹嘘，说自己某款车的车内声效比市面上的某一款好，噪音也更小。而随着汽缸钝化、混合动力以及电动汽车等技术被引入市场，降噪减震会变得愈发重要。”

Cooper-Standard全球产品战略副总裁兼防震系统开发组主管Joe Emmi告诉我们，随着自动驾驶汽车所搭载的全新人机交互界面（HMI）走向市场，车厢更安静和驾车更平顺已经成为了人们的一种诉求。

目前，OEMs和一级供应商已经投入了数十亿资金，用于开发新的 NVH 软件、设施、材料及支付 Saha 等其他声学专家咨询公司的咨询费。此外，吸音材料对保证车辆的 NVH 性能非常关键，因此吸音材料制造商的发展也迈入上升趋势。根据 2017 年《大观研究》(Grand View Research) 报告，2021 年之前，消声产品的全球市场规模的年均复合增长率将达 6%，在 2021 年达到 135.2 亿美元。

在此背景之下，聪明的供应商也开始顺应市场趋势，推销自己的消声材料，比如：Material Sciences Corp. 用于仪表盘的“无声钢（Quiet Steel）”，日企Technol Polymer Co. Ltd的“Hushlloy”热塑性消声塑料，以及Sika Automotive公司可用于抑制车身共振的共挤型合成橡胶“Sika Damp”。

此外，为了响应汽车行业对 NVH 性能的重视，市场中还涌现了一批专用NVH 性能优化软件，包括：SCS 公司的 V-Path 传播路径分析软件；HEAD Acoustics 公司的 Artemis NVH 测试软件；Genesis 公司的 LEA；Altair Engineering 公司的 HyperWorks 套装，及达索系统 CATIA V6/3DS 内的嵌入式NVH 工具箱及Siemens PLM 等等。

SAE International一直以来都致力于为NVH工程师提供一个专业的交流、沟通的平台，学会每两年举办一次（今年会议与展览将在密歇根州的Grand Rapids地区举办）的“噪音与震动技术大会与展览（Noise and Vibration Conference and Exhibition）”。

 “控制变量法”

N&V工程师研究的领域包含了信息源、路径（包括实体架构和空气）和接收器（即噪音和震动的目标物）。这个领域中，越早确定问题所在并着手解决，成功的可能就越大。在一定程度主观判断的基础之上，这些工程师把重点放在量化目标参数之间的关系上，比如：通过某种数据获取方式精确比对声音和频率的测量值。

终端客户输入也是分析的一部分。对于很多客户而言，提高音效和减少噪音及降低其分贝同样重要。心理声学专家对此十分认同。声音的音量、刺耳度和清晰度指数（一种在噪音环境下聆听讲话的清晰度测试）都是主观参数。

接受本文采访的专家都认同说，与钢结构相比，轻薄的铝结构在众数分析中会展现出更多不同的N&V特性，并且有不少特性令专家相当犯难。要控制好震动就需要更结实的车身面板和附加的抑制材料，如LASD底部填补剂和洞孔填充泡沫。

有些应用程序采用了主动降噪系统（详见Honda补充报道，将于近期即将在微信平台推送）。这类应对方案常被冠以“创可贴”的称呼，原因就在于它们并不直击问题根源——也就是基本设计框架的问题，比如：气缸体谐振过度等结构性问题。此外，这种解决方案还会增加车身重量。

Altair Engineering汽车部的高级副总裁Richard Yen观察后指出，“很多公司在NVH方面都做得不错，但它们的根本问题是总要等到汽车研发的尾声阶段才进行各种测试。也就是说他们把大量的钱花在了纠错上。”

如果选择使用铝制材料，工程师往往会添加更多用料，以达到和钢结构同样的降噪减震效果。在NVH专家Saha看来，这样做的结果就是“实际上在汽车内部‘购置空间’来安放噪震吸收材料，而不是直击车体基本结构的问题。而他们对此的解释就是吸收材料比较轻。”

缓冲垫已经成为了将发动机噪音屏蔽在客厢外的一种战略性“城防工事”。缓冲垫像“防火墙”一样覆盖在隔板两边。最初，缓冲垫是充当屏障和解耦装置的，而如今，它的作用无疑是充当吸收材料。而作为一种“耗散性”材料，显然，缓冲垫会占据更多剖面空间。

如今，为了追求更高的燃料经济性，越来越多的OEM开始将目光放在气缸钝化等提升能源效率的技术上，而由于发动机在开启/关闭部分气缸时（比如 6 缸切换至 4 缸或 3 缸）会产生不均匀燃烧，造成发动机低频震爆（具体水平取决于传播路径有所不同），从而带来噪声。为了应对这一独特需求，相关 NVH 创新技术更是层出不穷。

气缸间歇式发动机会带来一些不同以往的NVH问题，缓解这些问题的有效方案有很多，如：控制扭矩转换离合器打滑时，采用发动机主动制动控制系统（AEB）和变速箱支撑胶垫（效果好但成本高），以及使用车内音响系统主动降噪等等。[更多详情请参阅SAE技术论文 2014-01-1675，由Tula Technologies发布]合成材料会在一些扭矩反作用应用上产生吸收震动的效果。基于此，Cooper-Standard正在一款欧洲车型上使用连续纤维的热塑性塑料发动机支架。

整车建模

NVH方面的建模已经相当成熟了。其核心就是统计能量分析（SEA）工具，包括Cambridge Collaborative Inc.研发的SEAM 3D，以研究潜水艇的结构噪音。自此，汽车业就将其作为了一种有效的CAE设计工具，用来研发NVH声音工具，为轻巧型车型设计服务。通过使用SEA，工程师能评估在中频和高频时复杂结构的动态反应，并弥补轻量化设计带来的NVH方面的不足。

SEA“能让声学工程师采用‘控制变量法’做研发，”Pranab Saha说，“比如说，某个组件的质量减轻后，噪音水平会怎么变？”

通用汽车最近开始使用一种内部开发的建模工具，叫做DISPET。DISPET能让工程师把NVH工具包在不同车型中的作用发挥到最大。在开始时录入车身设计和动力系统噪音后，就能具体指定所需的NVH材料。“就给定的厚度和表面密度，用户只要输入测得的声音数据即可，”一位熟悉DISPET的工程师告诉我们，“系统会告诉你是否达到预期的降噪目标。而如果你自己不知道目标数值是多少，DISPET会告诉你，在某个频率下，数值是否高了或低了一分贝。”

整车NVH模拟技术在业界有有很强的竞争优势，因此，各家OEM对其内部程序的具体运作守口如瓶。大约五年前，Altair Engineering在这一技术的商业化方面处于领先。Altair的NVH模拟器样子很像电脑游戏的手柄，你的面前有一块巨大的液晶屏，驾驶员就在屏幕里的“路上”。

“有了整车模拟器后，我们又建了一个相关的模型，可以利用测试数据模拟出在某一噪音和震动环境下驾驶汽车的场景。”Altair降噪减震的项目总监Jiamin Guan解释道，“这一模拟需要汽车在不同环境条件下驾驶时的数据，同时，也会让你有身临其境的噪音感受。”

Altair与著名的专业仪表公司Brüel &Kjaer合作，共同开发出了这套能尽享“实车”体验的模型。模型是在某一汽车项目着手之初就建好的，也就是在设定标杆和具体规格时。Altair的第一步就是想象有一个虚拟的目标客户，然后怎样找到一款符合其要求的车。之后建模团队给出各种可行性较高的方案，进行包装并增加功能需求，接着让客户来聆听与感受这些测量出的数据。

整车模型“可以让社会中不同层次的客户群体来体验各种我们想要确定采用的噪音和震动水平，并让我们知道人们是否认可这些数值。”Guan谈到，“回放这些噪音和震动水平后，我们可以在某些地方进行精修。我们可以了解人们喜欢的是什么，不能接受的什么，以及更看重怎样的‘音效’。一旦我们有了具体的目标数值，无论是对于我们还是客户而言，接下来要考虑的就是如何实现它们了。”

Guan称，整车建模工具让工程师能够在汽车重量和成本一定的前提下，自由地进行改进，并评估这些概念性的想法。“如果没有整车建模，很多东西我们就无法确定。比如说，相比钢结构的车身，铝材质对于震动的影响大小有多少。有些构思可以通过整车建模来评估，无论最终是否能够达标。”

大多数的NVH讨论会似乎都绕不开汽车电动化这个话题。对于声学以及N&V方面的专家而言，这一趋势会让他们的工作变简单还是变得更难呢？“我们接到电话，让我们从声学和密封性两方面给电动汽车的隔音系统拿主意。”Pranab Saha说，“尽管我们还没有感受到很大的变化，但显然供应面转向电动汽车应该是大势所趋。”Saha和其他专家都提到，尽管电动汽车的整体噪音减小了，但此前被内燃机掩盖的其他噪音则现了原形。

Saha表示，“NVH专家仍在努力调试新的隔音系统，以应对今后电动汽车频段上的新挑战。”

The vehicle chief engineer did not like what he was hearing on this recent evaluation ride.

He was reviewing a new transmission scheduled for production in his company’s iconic sporty car. But the marriage of transmission and platform was generating unacceptably high cabin-noise levels. The CE wasn’t pleased. His design and manufacturing engineers explained that the new gearbox featured a thin-wall aluminum case for reduced mass. However, the lightweight case was the noise transmitter.

The tooling investment had been made and the program clock was ticking. So a phone call went out to Pranab Saha.

“They called me in to help them devise a solution,” said Saha, an acoustics expert and board-certified noise control engineer with consultants Kolano & Saha in Waterford, MI. He said the client’s original design blanketed the lightweight transmission case with a foil-backed insulation material. Problem was, the blanket included multiple cut-outs so that linkages and wiring could pass through. The cut-outs were built-in noise paths.

Saha’s suggestion: Rather than place the insulation material on the transmission case, put it on the underside of the transmission tunnel. The engineers would revisit their CAD drawings and revised the parts. Their next step is to launch noise study to prove the new solution’s effectiveness.

“That’s one of many examples of us getting called in to solve a problem—where is the best place to install NVH countermeasures?” he noted.

The science of analyzing, attenuating and eliminating Noise, Vibration and Harshness has become an integral aspect of vehicle development. NVH engineering teams now push for “one dB [decibel] per day” reductions in cabin noise levels with the same verve that is devoted to vehicle mass reduction and greater fuel efficiency.

“The N&V [noise and vibration] discipline is unique because it’s driven by market competition, rather than by government legislation,” Saha explained. “The OEMs now brag about the superior interior sound quality and low noise levels of one model versus another. And this will certainly grow more important as technologies such as cylinder deactivation, hybrids and electric vehicles enter the market.”

Quieter and smoother are requisites for the new human-machine interfaces (HMI) being developed for automated vehicles, said Joe Emmi, vice president of global product strategy and head of the Anti-Vibration Systems group at Cooper-Standard. “Reduced cabin noise is particularly important for voice-recognition systems,” he noted.

OEMs and Tier 1s have invested billions in new NVH software, facilities, materials R&D and in the expertise of Saha and other acoustics specialists in the field. Makers of the absorptive materials that are vital to automotive NVH abatement also are riding a growth curve. The global market for their products is projected to reach $13.52 billion by 2021, at a compounded annual growth rate of 6% from 2016 to 2021, according to a 2017 study by Grand View Research.

Astute suppliers are branding their technologies to suit. The growing list includes Material Sciences Corp.’s “Quiet Steel” used in dash panels; Japan-based Technol Polymer Co. Ltd’s “Hushlloy” anti-squeak thermoplastics, and Sika Automotive’s “Sika Damp,” a co-extruded elastomer aimed at dampening body-panel resonance.

The industry’s commitment to NVH engineering has driven the market for dedicated software programs familiar to engineers: SCS’s V-Path for transfer path analysis; HEAD Acoustics’ Artemis; Genesis’ LEA; Altair Engineering’s HyperWorks suite, as well as NVH toolsets built into Dassault Systemes’ CATIA V6/3DS, Siemens PLM and many others.

SAE International has long served as a professional forum for NVH engineers, with its biennial Noise and Vibration Conference and Exhibition (held this month in Grand Rapids, MI) the premier technical event on the subject.

Playing the ‘what-if?’ game

N&V engineers live in a realm of Sources, Paths (structure-borne and airborne) and Receivers (the noise and vibration targets) where up-front problem identification and solving are critical to success. They focus on quantifying the relationship between objective parameters, such as precise sound and frequency measurements obtained through data-acquisition methods, with a degree of informed subjectivity.

End-customer input is part of the analysis process; to many of them enhancing sound quality is as important as reducing annoying noises and overall volume. Psycho-acoustics specialists agree. Loudness, sharpness and the articulation index (a measure of the intelligibility of hearing speech within a given noise environment) are all subjective parameters.

Experts interviewed for this article agree that lightweight aluminum body structures, in modal analysis, present different and often more challenging N&V characteristics than do steel structures. Keeping vibration in check requires greater stiffness in the body panels and additional damping materials such as LASD (liquid applied sprayable dampening) underbody mastics and cavity-filling foams.

In some applications, active noise control systems are employed (see Honda sidebar). Such solutions are sometimes dubbed “band aids” because they don’t address root causes that typically originate in a fundamental structure—an excessively resonant cylinder block, for example. And they add mass.

Observed Richard Yen, Senior VP Automotive at Altair Engineering: “A lot of companies do a good job in NVH but basically they’re applying a lot of testing at the end of the vehicle development cycle. They spend a lot of money trying to fix the problems.”

When working with aluminum, engineers often add more material to achieve the same amount of NVH reduction of a heavier steel structure. As a result, NVH specialists “see evidence of trying to ‘buy more space’ within the vehicle to make room for absorptive materials, instead of addressing the fundamental structure issues,” said Saha. “They rationalize that absorption materials don’t weigh as much.”

Dash mats have become a strategic bulwark in the battle to keep powertrain noise out of the passenger cabin. The mats cover both sides of the bulkhead formerly known as the firewall. They used to serve as a barrier and decoupler. Today, dash mates essentially play an absorptive role. Known as ‘dissipative’ materials, they require more cross-sectional space.

The growing number of OEMs who have embraced engine cylinder deactivation as a fuel-efficiency play is sparking innovations related to NVH created by the uneven firing when a 60° V6 switches to V4 and V3 operating mode, for example. Signature vibrations and acoustic patterns are created by these events, with powertrain excitation entering the cabin as low-frequency “boom,” depending on the leak paths.

Managing torque-converter clutch slip, adoption of active engine and transmission mounts (typically effective but costly) and active noise control using the vehicle’s audio system can be effective solutions for mitigating the unique NVH challenges of cylinder-deactivation engines. [SAE Technical Paper 2014-01-1675, published by Tula Technologies, offers more detail on this subject.] Composites offer vibration-absorbing benefits in some torque-reaction applications. For this reason Cooper-Standard is launching a new continuous-fiber thermoplastic engine mount on a European vehicle.

Full Vehicle Modeling

Modeling in the NVH arena is increasingly sophisticated. Core among them are SEA (Statistical Energy Analysis) packages including SEAM 3D, developed by Cambridge Collaborative Inc. to study structure-borne noise in submarines. It has since been adopted by the auto industry as an effective CAE design tool to develop NVH sound packages for use in lightweight vehicle design. Using SEA, engineers can evaluate the dynamic response of complex structures at mid and high frequencies and recover the NVH deficiency created by the lightweighting actions.

SEA “enables acoustics engineers to play the ‘what-if’ game,” said Pranab Saha. “For example, if mass is reduced in a subassembly, what then will be the noise level?”

General Motors recently began using a modeling tool developed in-house called DISPET that lets engineers optimize NVH packages across vehicle families. With primary inputs being vehicle body design and powertrain noise, they can specify the NVH materials needed. “Users only input the measured acoustic data for a given thickness and surface density,” says an engineer familiar with DISPET. “The system will tell you if it meets the target or not. And if you don’t know the target, DISPET will tell you if you’re 1 dB above or 1dB below (or not), at every frequency.”

Full-vehicle NVH simulation is viewed as a competitive advantage whose internal processes the OEMs guard closely. About five years ago, Altair Engineering took the lead on commercialization. Altair’s NVH Simulator looks like the console of a cool computer video game, with a big flatscreen in front of you puts the driver “on the road.”

“With the Full-Vehicle Simulator, we build a simulation model that uses testing data to drive a model of the vibration and acoustic environment in the vehicle,” explained Jiamin Guan, Altair’s Director of Vibration and Acoustic Solutions. “It takes data from vehicle operation under various conditions and allows you to listen to those as you would hear them in a real vehicle.”

Altair partners with noted instrumentation specialists Brüel & Kjaer to create the “real vehicle” experience. The model is built right at the beginning of a vehicle program, during benchmarking and setting specifications. Altair starts with an imagined target customer and how the vehicle fits into their usage. The model team then builds a competitive set, adds packaging and functional requirements, then brings in customers to listen to and experience the measured data.

The full-vehicle model “allows a broad spectrum of people to experience the noise and vibration levels we’re trying to target—and see if they agree with those targets,” Guan noted. “Based on the playback of the noise and vibration we hone in on specific areas. We see what people really prefer and object to and what “sound quality” means to them. Once we have the targets, this helps us and the client consider how we will achieve them.”

The tool, Guan claims, gives engineers the freedom to make changes and evaluate conceptual options under weight and cost constraints. “Without a full-vehicle model we wouldn’t be able to determine how aluminum, for example, factors into the vibration requirement versus a steel vehicle. These are constructs you can evaluate in the full-vehicle model, whether you can meet the targets or not.”

The subject of Vehicle Electrification seems to enter most conversations about NVH. Is it making acoustics and N&V experts’ jobs easier or more difficult? “We’re getting calls to help decide on sound-package materials—acoustics and seals—for new EV programs,” said Pranab Saha. “The supply base is moving on this trend although we haven’t seen dramatic changes yet.” He and other experts noted that although EVs’ overall sound level is low, other noises previously masked by the combustion engine are now more audible.

“NVH engineers are still working out the sound packages to address the new frequency ranges of future electrified vehicles,” he explained.