VEHICLE NVH DEVELOPMENT PROCESS AND · PDF fileICSV21, Beijing, China, 13-17 July 2014 1 The 21st International Congress on Sound and Vibration 13-17 July, 2014, Beijing/China VEHICLE

ICSV21, Beijing, China, 13-17 July 2014 1

The 21st International Congress on Sound and Vibration

13-17 July, 2014, Beijing/China

VEHICLE NVH DEVELOPMENT PROCESS AND TECHNOLOGIES

Shaobo Young, Ph.D ChangAn Ford Automotive Inc, Chongqing, China 401122

e-mail: [email protected]

NVH (noise, vibration and harshness) performance directly affects a customer's perception of vehicles. It directly impacts vehicle's sales, durability, warranty costs and customer driving comfort. A good vehicle NVH design needs to start from a well thought NVH development process, and to follow system engineering principles. It needs to balance many different attributes such as vehicle dynamics, vehicle brand image, vehicle market position, target customer groups. It needs to be designed based on the above parameters, plus having a NVH further reserve to make sure the vehicle still meets its engineering targets when it is delivered to market. Engineering considerations on target cascading, separation of structural modes, separation of different sound and vibration transmission paths, vehicle hardware design principles for NVH, and finally NVH vehicle level deliveries are all of great interest to automotive industries. Advanced CAE and testing facility and methods are necessary enablers for good vehicle NVH to be delivered. The author hopes that this lecture presentation will provide congress participants with a clear and brief review of automotive NVH design principles and methods.

1. NVH development process

1.1 “V” system engineer model Vehicle is a complicated system that involves many components. An averaged modem vehicle

has about 2000 components, in which an engine alone could have as many as 300 components. To develop a vehicle, we have to rely on a system that takes care of every aspect of vehicle development process. This system is called vehicle production system.

Figure 1. “V” representation of system engineering

21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014


Every car OEM has a vehicle development system, such as MPDS for Mazda, TPDS for Toyota and so on. In Ford vehicle production development system involved from the WCP (Word Class Process) prior to 1995 to the FPDS (Ford Product Development System) in 1995 and then the GPDS (Global Product Development System) in 2004.

The fundamental of the FPDS is the “V” concept derived from system engineering as illustrated in Figure 1. The V-Model provides a system engineering point view on “what we do?” on the left side to “how well are we doing?” on the right side of the “V”. The V-model is a graphical representation of the systems development lifecycle. It summarizes the main steps to be taken with the corresponding deliverables within a system validation framework. The “V” represents the sequence of steps in a project life cycle development. It describes the activities to be performed and the results that have to be produced during product development. The left side of the "V" represents the decomposition of requirements, and creation of system specifications. The right side of the V represents integration of parts and their validation. The V-Model provides guidance for the planning and realization of projects. It improves project transparency and project control by specifying standardized approaches and describing the corresponding results and responsible roles. It permits an early recognition of planning deviations and risks and improves process management, thus reducing the project risk.

1.2 Target cascading processes To execute the left hand side of the “V” model, we need to specify “what we do?” in details.

The process involves generation of vehicle targets, cascading target to system and components. This target generation and cascading can be described as below: 1. Corporate cross functional senior management decides on vehicle program intentions, vehicle

scale and timing; 2. Program assumption defined, and then benchmark drive and focus groups work starts; 3. The total vehicle quality function development (QFD) team is established to identify important

customer desires that relates to NVH, such as translate customer desires from “strong body” to joint stiffness and body/chassis NVH;

4. Develop NVH functional subjective image targets, such as “boom” needs to be better than certain image vehicle, etc.

5. Covert subjective functional image targets to objective engineering metrics to define vehicle level NVH targets. This is done by benchmark study and projected NVH improvement provision based on historical data. These vehicle level targets include road noise targets ( sound pressure level targets for front seat and rear seat; seat track vibration and steering wheel vibration for impacts, front floor and steering wheel vibration for tire imbalance, and road noise on proving ground); wind noise targets for 80 and 100 mph; PT NVH targets (sound pressure level for wide open throttle, sound pressure level, steering wheel vibration, seat track, brake pedal and toe board vibration at engine idle; vibration dose values (VDVs) of transmission shifts, start-up shudder, as well as interior sound and vibration during high speed cruises; as well as climate control NVH targets (sound pressure level at each blower setting, etc)

6. Cascading vehicle level targets to system and component level NVH targets, this involves two scenarios:

a) When the new vehicle is a derivative of an existing vehicle – in this case the new vehicle is only slightly different form the baseline vehicle, the PT and chassis are basically carryover with some modifications and improvements. The body structure will have some new styling and some modifications. CAE models from the baseline vehicle can then be used after some minor modifications to cascade vehicle level NVH targets to system and component level NVT targets. In parallel baseline vehicle hardware can also be slightly modified to aid this target cascading through means of hardware testing and also used to correlate to the CAE model. Once reasonable CAE /



test correlation is achieved, CAE model can then be optimized to achieve the required NVH improvements and weight efficiency. Body system targets in terms of overall static stiffness, modal frequencies, modal alignments, body sensitivity and body attachment nobilities can be cascaded from body system model, and similarly body component targets in terms of section properties, joint stiffness, moment of inertia, torsional stiffness can be obtained by local body CAE model or vehicle testing.

b) In the case of a brand new vehicle development, no CAE model is available, and no hardware prototype is available for target cascading. The process of cascading vehicle level NVH target to system level targets and component level NVH target becomes an iterative process. In this case vehicle system NVH targets setting is based on a team consensus. This system level consensus can be achieved by studies on benchmark results from surrogate vehicles, corresponding component level NVH performance can also be used as initial set of NVH targets. These initial system level and component NVH targets are subject to further modification in parallel to CAE modal development. These processes are more iterative due to the fact that the vehicles based on which these initial targets were development can be very different from the target vehicle in design philosophy. Time involved in the development of a verified CAE model is thus longer that the previous case. Though an iterative process, a validated CAE model is finally developed and used to generate system and component NVH target for the new program.

2. Vehicle NVH basics NVH stand for Noise, vibration and harshness. Noise is unwanted sound in the whole audible

range of 20-20kHz, vibration in vehicle; however is usually a topic of vehicle tactile response of up to 100Hz. Among the three, harshness is the only one that is not well recognized by other industries. Harshness is a term that describe human’s subjective feeling on an event. Harshness is created when a person’s subjective feeling is not in agreement with his/her anticipation. For an example, when a customer sees a smooth road surface on road, he/she would have an anticipated low level of noise and vibration level when he or she drives the vehicle on that road surface. But if the vehicle has more than anticipated level of noise and vibration, harshness feeling is created in the mind of the driver.

In vehicle NVH we use sound pressure level (dB, dBA, sometimes dB(B) or dB(C)) when describing an engineering noise phenomena, such as engine noise, road noise, sound transmission etc; We use psychoacoustics terms such as loudness, sharpness, roughness, articulation index, etc when we describe items that relate to customer perception of a noise. For the vehicle’s tactile vibration, due to the fact human’s perception is flat in frequency with velocity, velocity is often used as the primary metric for vibration when dealing with items that relates to customer perception and use acceleration for general engineering items.

Vehicle NVH is usually studied into two categories in air borne and structure borne NVH. It is usually believed that 300-500Hz is the cuff off frequency, below which structure born NVH is dominant and above which air born NVH is more dominant. Both experiments and CAE analysis have proven that being a good frequency separating point for the structure and air borne NVH.

2.1 Structure borne NVH For the structure borne NVH the focuses are:



2.1.1 Shake Shake is a low frequency vibration above the vehicle ride motion frequency and usually in the

frequency range of 5-40Hz. The scope of shake includes: a) Steering wheel shake b) Engine idle shake c) Vehicle jiggle d) Mid car shake e) Road induced shake f) Road wheel shake g) Seat shake Vehicle shake forces usually are from: a) 1st order imbalance related (suspension peak (10-15Hz), tire imbalances (10-20Hz)) and

tend to increase with vehicle speed. b) Road irregularity induced shake. It is also affected by suspension resonances and peaks

(10-15Hz) c) Engine idle firing induced. This is mainly for shake from 20-35Hz, and more sever with

auto transmission vehicles. Shake is strongly affected by the powerplant (engine, transmission, and others installed) rigid

body modes (7-15Hz, includes bounce, pitch, yaw, roll etc modes) and the vehicle suspension modes (12-15Hz incudes hop and tramp modes), as well as vehicle global modes (1st bending, 1st torsional, floor panel modes, and steering column modes) therefore the modal alignment strategy needs to be enforced to avoid resonances. Current practice is to design vehicle’s 1st bending and 1st torsional modes to above at least 20Hz. At the same time, engineering balance also need to be considered in terms of increased body stiffness will also create a new set of higher frequency harshness issues (noise radiation efficiency goes up and vehicle become more rigid and abrupt and harsh). 2.1.2 Boom

Boom is low frequency sound in the range of 20-100Hz. The scope of boom in vehicle usually involves:

a) Body boom b) Driveline boom (30-80Hz boom usually caused by powertrain, driveline inputs) c) Exhaust boom (boom caused by exhaust system and resonances) d) Idle boom (boom caused by engine idle firing) e) Impact boom (boom caused by road impacts and resonated by body panel and cavity

modes) f) Lugging boom (boom due to engine firing force pulsation when the engine is operating at

high torque and low PRM. Boom forces usually come from: a) Engine idle firing, usually worse with an automatic transmission vehicle b) Engine lugging, usually in the range of 0-50Hz range c) Road irregularities, usually in the range of 20-100Hz d) Exhaust inputs, usually in the range of 20-100Hz e) Driveline imbalances, usually in the frequency of 30-80Hz and getting worse with

driveshaft RPM



Similar to shake, boom is also strongly affected by powertrain and suspension modes and resonances. These includes powertrain rigid modes (idle, lugging etc) and suspension modes ( hop and tramp modes), vehicle body modes in overall 1st bending, floor 1st bending, roof and other major panel 1st bending also affect vehicle boom performance, together with vehicle cavity modes. Therefore a clever design of those models, structure properties are necessary for a good vehicle boom performance. 2.1.3 Moan

Moan is a sound in the 80-120Hz region consisting of a small number of pure tones. Moan is usually results from engine firing force and generated from powertrain system and assemblies. Moan is largely affected by powertrain resonances, and vehicle suspension resonances. Moan can also be due to local panel resonances and local modes of vehicle structure. Because of its frequency range, vehicle acoustic cavity modes can amplify moan in a vehicle.

2.2 Air borne NVH Air borne noise is a term that refers to all sound which does not result from forces which

reach the body via mechanical routes. In a vehicle air borne noise is usually above 400Hz. 2.2.1 Airborne Powertrain noise:

Powertrain noise is the noise radiated from the powertrain and transmitted into vehicle cabin in a frequency range of 400-10kHz. A vehicle powertrain noise includes the followings:

i. Engine noise ii. Exhaust noise

iii. Gear noise iv. Induction noise v. Other miscellaneous noise

Powertrain noise is mainly generated by powertrain system and relates to the rotation of the

engine. Therefore it usually has a definite order contents and order relationship. This often times gives great convenience to identify the origin of a powertrain NVH issue. 2.2.2 Wind noise

Wind noise is the noise generated by air passing through vehicles and pressure differentials due to aerodynamics. Wind noise include air rush noise, aspiration noise and whistle, as well as cavity noise. Usually wind noise total level increase 5-8 dB for a speed change from 70mph to 100kph. a) Air rush noise is due to flow separation and eddy generation. When air passing through an

obstacle, it will overpasses the object and a small space immediately behind the object before it flow joints again. As a result, a turbulent vortex motion is generated. This vortex motion and pattern can be easily seen by rain drops on side glass and backlit window. In this category, separation of the air flow at the vehicle A-pillar is of particular importance because it is close to the driver’s ear location and the glass has relatively high transmissibility. A regular automotive side glass has a very poor sound transmission loss performance at around 2-4 kHz wave matching coincident frequency.

b) Aspiration noise is of high frequency of 2-5 kHz, it is usually due to local leaks in the vehicle seal systems. Due to its high directivity, aspiration noise is very annoy to customers. Despite of its low energy level, customer can easily detect aspiration noise and its location. Customer will call his/her vehicle to be broken when hearing aspiration noise, but will many times call general overall air rush noise as if he or she was driving in a windy day.



c) Whistle noise is a pure tone noise, usually associated with a certain definite geometrical size, such as a hole, a gap, an antenna or a plate. For an example an antenna, the dominant frequency content is given by f = 0.202*v/d., thus a 3 mm diameter antenna would generate a pure tone noise of approximately 1.8kHz at about 60mph.

d) Cavity noise is due to flow in channels, such as in tubes or in particular in between different seals. The gap between the door B-pillar in between front door and rear door seals, the gap in between vehicle primary seal and secondary seals are all examples of cavity noise. Since cavity noise propagates inside a small channel, it’s energy does not spread out and always contained within the channels, so the cavity noise can propagate very efficiently and affect customer, so cavity noise must be blocked or treated in vehicles.

2.2.3 Road noise Road noise is the noise generated by road irregularities. The road noise frequency range is

from 20-1000Hz. Below 400Hz, the road noise is structure borne dominant and above 400Hz road noise is air borne dominant. Usually road noise level increase 6 dB for a speed doubling. Road noise consists of drumming, boom, rumble, tire and open hole noise. Table 1 shows a rough classification of the vehicle road noise.

Table 1. Vehicle road noise classifications

Road noise excitations come from two categories, a) force due to discrete impacts and b)

force due to road random irregularity input. Force due to discrete impact will result in input force at vehicle spindle at both vertical and fore and aft direction. A SEA paper by B.G Kao in 1990 showed that a semi-circular tar strip of 21mm diameter can result in a vehicle impact force at spindles for as much as 1000N in vertical direction and 400N in fore and aft direction. Force due to impact can be further thought of force due to summation of a series sinusoidal forces. Therefore the tire impact force can be simply done by study of a single sinusoidal force input.

Road random irregularities input can also be thought of infinite number of sinusoidal inputs but with a random amplitude and random phase. In this case power spectrum density (PSD) function of the road surface is often used. The idea is still trying to split road profile into a set of random sinusoidal for very meter length of the road surface. Then the tire spindle force can then be calculated by the transfer function of the tire.

Spindle vertical input force enters the body via the shocks and struts and cause body structure to radiate road noise. Spindle fore and aft force gets into body via transverse load paths such as A



control arms and certain suspension types (McPherson strut) to cause vehicle lateral motion and torsional vibration.

Resonant modes in tire and suspension can also amplify road input forces. Tire and suspension acts as filter to vehicle so that the vehicle road induced NVH can be classified into three frequency bands [2]:

a) Below 30Hz: In this frequency range, tire is acting as a pure spring, tire vibration of the tire is

negligible and force transmitted into vehicle mainly depends on the tire radial stiffness. b) From 30-250Hz: In this range tire modes are active, include tire radial modes, tire transverse

modes, and tire tangential modes. These tire modes respond to road inputs and amplify and transmit forces into vehicle body suspension.

c) Above 250Hz: In this range tire global modal vibrations are strongly damped. Local tire vibrations occur in front of and at the rear end of the tire patch.

Depend on the type and wavelengths of the road irregularity, road induced NVH can be perceived as a jolt, impact or continuous road noise. A brief bread breakdown can be roughly as followings:

2.2.3.1 Large road irregularity wavelength For road irregularity wave length 0.5 ~ 50 meters and 5mm to a few centimetres in height,

vehicle gives a primarily a jolt feel. Table 2 gives frequency vs. different vehicle speed and different road irregularity wavelength. Jolt below 1Hz usually cause motion sickness where frequencies above 1Hz to 20Hz gives ride roughness.

Table 2. Jolt frequency at different road wavelength and vehicle speed

2.2.3.2 Discrete road irregularity For road that has discrete obstacles of 5-30mm in height and a few millimetres to a few

centimetres in length, the tire is subject to experiencing shocks or impacts. Due to its nature of impacts, a broad band excitation can excite multiple tire modes, causing noise and vibration up to a 250Hz. This noise and vibration can last longer or shorter, stronger or less depends on the tire structure and vehicle response. In vehicle NVH this is called impact harshness.

2.2.3.3 Small road irregularity wavelength Road surface itself usually has aggregates that sizes from a few millimetres to up to 2

centimetres. By definition, a surface is called rough when the sizes of its aggregates are of size of more than 1 mm. When tire rolls over a rough surface, force introduced by road roughness generates structure borne and air borne noise and that get filtered by vehicle suspension and sound package, as a result, road noise perceived inside a vehicle cabin is limited to below 1000Hz.



Besides vibration and noise generated by road irregularity, imperfection in tire also generates

force impact to vehicles. These includes: a) Tire radial stiffness variations b) Mas distribution variations c) Shape distribution variations These non-uniformity in stiffness, mass and shape usually cause vehicle vibration up to 40Hz

and also noise up to 300Hz. When road irregularity excites tire, it does not only excite the tire structure to vibrate it also

excites the air inside the tire to generate so called “tire ring”. Tire ring can be quite easily detected when vehicle is going over a pavement joints or when the vehicle is driving on a smooth road surface. The frequency of the tire ring is given by c/2πR, where c is the speed of sound, and R is the radius of the tire. For normal vehicles the tire ring frequency is at around 200-250Hz.

In order to minimize tire resonances and tire noise, it is very important that tire tread pattern

does not concentrate energy in certain band, this is done by randomize tire pattern to dilute excitation energy into broad energy distributions.

3. Vehicle NVH design principles

3.1 To strategically place modal frequencies to avoid resonances 3.1.1 Basic idea

Decouple frequency of excitation forces from frequencies of vehicle structures or acoustic cavities. This is done by designing the vehicle so that the resonances of the vehicle, systems and subsystems are separated in frequencies. 3.1.2 Things to consider Excitations 1. PT excitations

a) Engine idle (cold, and hot) b) Lugging

2) Road excitations a) Tire/wheel imbalances

3) Frequencies 4) PT rigid modes 5) Body modes 6) Global bending (1st bending) 7) Global torsional (1st torsional) 8) Local modes in (front door, spare tire, dash panel, steering column, IP ) 3.1.3 Design principles

1) Avoid excitation/resonances as much as possible 2) Avoid body bending with idle excitations 3) Avoid cavity modes coinciding with panel resonances 4) Avoid body torsion mode coinciding with suspension tramp mode 5) Avoid body bending coinciding with tire order at common driving speed



3.1.4 Frequency separation principles 1) Body modes >3Hz 2) Steering column modes > 5Hz 3) Body modes to steering column modes > 3Hz 4) Suspension modes to body modes > 4Hz 5) Suspension modes to PT rigid modes > 3

Three important stationary rpm settings (cold idle, hot idle and lugging speed) are defined

by program office. For lugging only consider main firing frequency (2nd for I4 engine). For idle, need to consider both 1st and 2nd order firing frequencies, those firing frequencies cannot be placed coincide with vehicle modes, then place PT, suspension, body, steering column, IP, floor, roof, cavity modes on the chart according to above separation criteria. By applying above modal separation strategy, major vehicle mode are placed strategically to avoid harmful modal resonances and such a modal placement chart is usually used in automotive industry and is called modal separation chart as illustrated in Figure 2.

Figure 2. Typical modal separation chart

3.2 Control appropriate body sensitivities Vehicle NVH level depends on vehicle sensitivity and the excitation. In cases where multiple

paths exist for a given NVH concern, the total NVH level is the summation of all contributing paths, i.e,

= ∑ = ∑ ∗ (1) In early design stage where the phasing information of each path is unknown, it is often convenient to assume all paths are independent so that the total NVH becomes:

= ∑ ∗ = ∑ ( ) = ∑ ( ∗ ) (2)

In this case, the path with the largest contribution dominates the total NVH performance. Targets setting, although dependent on vehicle classification, is often believed that the following ball park body sensitivity numbers are reasonable target starting points for a good NVH vehicle:



Forcetothebody: 10 Velocitysensitivity: . Acousitcsensitivity: 0.01 / (3)

3.3 Decouple body from PT and suspension systems A body is said decoupled from PT or suspension system if the force to the body does not

change much (within 10%) if the body attachment point stiffness becomes infinitely rigid. When decoupling is occurred, body modal behaviour such as modal frequency and shapes can be changed without affecting rigid PT modes or suspension modes. This can really benefit NVH tuning and easy the task of achieve a good modal alignment strategy. For unitized body vehicle, it is often believed that the vehicle body is fairly well decoupled from suspension and PT rigid modes. If we look at a simplified suspension model in the following sketch shown in Figure 3:

Figure 3. Quarter car suspension model

By using mechanical – circuitry analogy, it is easy to equalize above mechanical system to a simple circuitry:

By solving this circuitry for the force going into body, one can get: = ∙ ∙ ∙ (4)

It is easy to see that when Zb >> Zs, vb ->0 and then the force goes into the body Fb is no longer dependent on body and Fb = vs x Zs. Thus the decoupling occurs.

3.4 Using appropriate ways to reducing body sensitivity Most ways of reducing body sensitivities involve increasing body stiffness, damping, adding

mass as well as the use of dynamic absorber.



3.4.1 Stiffness effect: To increase panel stiffness will reduce body displacement, and as a result to reduce panel

vibration level. But increasing panel stiffness will also upshift panel resonance modes and increase panel acoustic radiation efficiency. Therefore caution needs to be placed when deciding the trade-offs between reducing vibration level and increasing sound radiation efficiency. Especially when the body is subject to a massive vibration source input, in this case body will be forced into a constant velocity vibration regardless of the change of the panel stiffness. 3.4.2 Damping effect

Damping effect: Damping will provide energy dissipation by converting sound vibration energy into heat and thus reducing body vibration. Caution needs to be placed to the fact that damping will increase force transmissibility and degrade the isolation. A trade off study is needed between energy dissipation and reduced isolation for a given NVH issue. General design considerations for damping treatments are:

1) Apply damping where maximum displacement occurs 2) Use low density material as much as possible 3) Use high stiffness material as possible 4) Damping effect increases with the square root of thickness 5) Use as high as possible loss factor material 6) Constrained damping layer material is always better than the free surface damping

material 7) Normal damping material has a limited temperature range of 0 ~ 80 Celsius degrees.

Damping performance quickly diminishes outside its working temperature range. 3.4.3 Mass effect:

Adding a mass sometimes will result in a reduction in a vibration for a given excitation force. But its efficiency is often not optimal and is not recommended for its negative impact on the total vehicle weight. 3.4.4 Dynamic absorber

Dynamic absorber is usually developed for very specific NVH issue. Due to its cost, package and weight impacts, dynamic absorber should be generally avoided. It is interesting that in vehicle certain components are actually designed to work as a dynamic absorber. Radiator, for an example, is tuned with its supporting rubber grommet to work as a dynamic absorber for cancelling 1st order engine idle shake or can be designed to cancel 1st vehicle bending resonance.

3.5 Proper body structure design for NVH Vehicle NVH depends very much of vehicle body structure. At low frequency noise and

vibration issue is mainly caused by so called “structure borne” paths, i.e. powertrain, road and wind induced sound and vibrations get transmitted by body structure via body attachment points, and then propagates along body structures and excites body panels to generate sound and cause body structure to vibrate. The rigidity of vehicle structure determines how much a vehicle structure responds to external force and vibration excitation and therefore is crucial for the NVH performance of the vehicle.

The integrity of vehicle body structure mains involves in body joint stiffness and sectional property of the major body load bearing elements. Figure 4 shows a sketch of the major body joints and load bearing elements which are crucial to vehicle NVH performance.



Figure 4 Important vehicle structures for NVH

Table 3 shows how those body properties affect vehicle NVH performance. For a family

sedan the major body structure joints include A, B, and C pillars to roof rail, hinge pillar, B and C pillar to the rocker, rear rail to rocker, as well as shot gun to hinge pillar; the major load bearing element section properties include A, B, C pillar, roof rail, rocker and rear rail properties; important body panels such as windshield glass and backlit glass also contribute greatly toward the overall vehicle structure integrity.

Table 3. Vehicle structure effect on NVH

4. Practical considerations on vehicle NVH designs

4.1 Panel stiffness effect Generation of sound requires the body sheet metal to act as loudspeakers. Amount of sound

generation is proportional to the rms velocity of the panel and the area size of the panel. Acoustic mode of the vehicle amplifies the sound generation of the sheet metal. Based on sound radiation theory, sound radiation from a plate is W = | | ∙ , here W is the radiated sound power, v is the surface average panel vibration velocity, and σ is the radiation efficiency.



σ = , ≪0.45 , =1, ≫ (5)

And fc is the coincident frequency at which plate bending wave speed is equal to the speed of

sound. f = = 12 ( ) = 0.55 = (6)

Where k is a constant dependent on material type. Interesting enough for most materials we

used in automotive industry such as steel, aluminium, glass etc, the K constant is very close to 12. This is to say that the critical frequency of a plate is mostly dependent on the plate thickness. It is easy to see that to reduce sound radiation, one needs to increase the critical frequency by increase mass, and to reduce stiffness. So in automotive NVH control, stiffening panel, on one side will reduce panel displacement for a given force input, but it will lower the critical frequency and increase sound radiation efficiency, especially then the panel is subjective to a constant velocity input. Therefore caution needs to be taken when design panel stiffening measures.

4.2 Acoustic cavity volumetric effect Vehicle interior cavity has a volumetric mode where sound pressure on any point of the

interior surface is changing in phase, sound pressure is acting like a pulsating bloom. This mode is similar to the rigid body mode of a mechanical system. Usually this mode has a frequency of 0 Hz, much lower than the first longitudinal frequency of the vehicle interior cavity. Due to the pulsating pressure change from this volumetric mode, low frequency (below the first cavity mode) air pressure inside the vehicle cavity can result in quite big level of sound and contribute to the vehicle interior response.

4.3 Vehicle rigid body motion effect Air in inside a vehicle has a mass. As the result with an infinite rigid vehicle body structure,

when a vehicle is driving on road, the air particles inside the vehicle are subjective an inertia force when the whole vehicle body moves as a rigid body. This motion of air particle will cause air pressure to change and generate sound. In this case stiffening vehicle panels will not improve the situation, rather it will produce more sound inside the vehicle. When a vehicle is jumping on road, vehicle floor and roof will act as two rigid pistons radiating sound with a 180 degree out of phase. Sound at the mid height of the vehicle cavity is close to zero, but at place such as driver’s ear location or foot locations, sound generated due to this vehicle rigid body motion can be rather high. Figures 5 shows the sound generated by a unit body velocity in z direction.



Figure 5. Interior sound from rigid body motion

4.4 Sound quality effect In vehicle NVH, there are many cases where certain level of noise is required. Strictly

speaking we should call these as sound. Sometimes the term of sound and noise are co-existing in vehicle NVH. When we are dealing with an acoustic perception that is required for vehicle design, we call that sound; while we are calling non-wanted acoustic perception in a vehicle as noise. For that reason in vehicle NVH, we have powertrain sound, door closing sound, wiper sound, seat operating sound; at the same time we would normally only use terms of road noise, wind noise because these are not what customer wants in a vehicle. For sound in a vehicle, we have to deal with its sound quality. When we talk about sound quality, it is desirable to understand customer’s preference towards the sound they are experiencing. Such understanding of what customer wants are critical for an automotive OEM to design their products to meet customer wants, and to design a product that has its own brand sound “DNA” characteristics. All automotive OEMs are now using sound listening room to conduct such studies. 4.4.1 Consonant sound and dissonant sound

Good sound harmony normally requires sound to be consonant (harmonious, pure, stable in time, pleasant to ears). As this mainly dealing with human perception to a given sound, we need to first understand what consonant sound is. A consonant sound gives a harmonious, pure, pleasant and time stable sound perception to human ear, on the contrary, a disharmonious, time unstable, impure and disturbing sound is called dissonant sound. From music acoustics, Figure 6 shows the key board combinations for consonant and dissonant sound. Figure 7 shows the frequencies associated with each key strike on a piano.



Figure 6. Key combinations for consonant or dissonant sound

Figure 7. Piano key and frequencies

When two sound sources are generating sound simultaneously, the can be coupled in space.

Especially when they are close to each other or the sound generation of those sources are somewhat dependent on each other. This is usually called coherent sound sources. When two sources are not independent to each other, or the two sounds are coherent to each other to certain degree, the total sound in a receiving point will be modulating and to product frequencies not only original sound and their harmonics but also frequencies of n . These additional spectral contents will make sound “dirty” or “non-pure” and sometimes beating. But if the frequencies of the two sound are such that the frequency of sound A coincident with that of sound B or harmonics of sound B, then no new sound spectrum will be generated and sound quality is not deteriorated.

5. Vehicle NVH technologies

5.1 Advance CAE technologies In recent years with rapid development on personal computer and consolidation in CAE

industries, more and more previous unix based CAE computing software are merged into Microsoft Windows platforms and more and more software are now presented as integrated CAE software



suits. LMS’s virtual-Lab [3] and Altair’s Hyperworks are the two main CAE software packages that most of automotive OEMs use. All those software packages integrate all CAE software into one platform, including CAE mesh generation, CAE structure analysis, CAE acoustic analysis, multi body dynamics (MBD), and CAE optimization together with rich interfaces and data import/export capabilities to other software and test data formats. These provide a large degree of convenience for automotive engineers to conduct CAE analysis under one platform. Non-linear CAE modelling capability is also improving quite rapidly. However aero-acoustic CAE technology is still waiting for its breakthrough and CAE capability for vehicle wind noise prediction is still a long way to go.

5.2 Advance test technologies Similar to CAE technologies, advanced testing capability is also being developed quickly in

automotive applications. Advanced testing capabilities such as beam forming, near field acoustic holography, 3D laser vibrometry, inferred testing are all becoming commercially available. Such advancements in testing technology also include many OEM own developed test capabilities such as Ford’s virtual engine 5 dyno testing and sound intensity STL scanner shown in Figures 8 and 9.

Figure 8. Ford’s 5 dyno spin-torsional system Figure 9. CAF’s SI scanner system

5.3 Advanced Vehicle technologies More and more active NVH technologies have been used in today’s vehicles. Honda started to

use active engine mounts back in 2001 which electrically control engine mount that filled with magneto rheological fluid to change its stiffness for different engine operating modes. Since then active technologies have been widely used on vehicle for not only active noise and vibration cancellations, but also for active brand DNA enhancements, not only for single band control, but also for broad band active NVH control. Since 2003 Ford has been using vehicle entertainment system integrated DSP control board for its active noise and vibration control for its luxury Lincoln brand vehicles. Actively technologies have also been migrated to all aspects of automotive applications, EPAM (electoral polymer material) is being pioneered to be used as “smart” door seals that can adjust its size according to vehicle speeds; “smart” foam applications (foam with piezoelectric element) can adjust its thickness for best sound absorption; glass embeded with electro-chromatic characteristics can adjust its darkness with a simple change on its voltage supply; all these provide never thought-of new future for customers and automotive industries.

REFERENCES 1. C.L Magee and el, Vehicle body NVH guide, Ford internal publications 1990 2. Mechanical and acoustic comfort, Michelin 2002 3. Koen Vansant, LMS Virtual.Lab Noise and Vibration, 2009

Documents

VEHICLE NVH DEVELOPMENT PROCESS AND · PDF fileICSV21, Beijing, China, 13-17 July 2014 1 The 21st International Congress on Sound and Vibration 13-17 July, 2014, Beijing/China VEHICLE