Wang X. Vehicle Noise and Vibration Refinement

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20 Vehicle noise and vibration reﬁ nement

Setting vehicle noise and vibration targets is important for the successful

operation of a vehicle development program, as it ensures that planned

resources and efforts are directed towards better vehicle noise and vibra-

tion performance than that of competitors in order to satisfy customers at

the beginning of the development process. Without it, individual system

suppliers would determine their own interpretation of an appropriate level

of noise and vibration. The ﬁ nal vehicle would most likely be truly reﬁ ned

only in some aspects and not in others. Excessive noise and vibration caused

by one component or subsystem in the intended production vehicle would

cause its design validation test to fail, lead to a large cost increase and time

delay for the program, and jeopardize the program targets. The purpose of

vehicle noise and vibration target setting is to ensure that the newly devel-

oped vehicle has no noise and vibration complaint issues and that it has

superior noise and vibration performance when released to market.

2.1.2 Scope of vehicle noise and vibration target setting

and benchmarking

Vehicle noise and vibration targets consist of interior and exterior targets,

subjective and objective targets, noise level and sound quality targets.

Exterior noise targets include whole-vehicle exterior targets and single-

component exterior noise targets. The exterior pass-by noise targets must

conform to the national design rules and standards. Interior targets include

the whole-vehicle and single-component noise targets inside the vehicle

cabin and ride quality (vibration) targets at idle, constant speeds and slow

Determine what group of customers

the vehicle is targeting

Benchmarking & market analysis

Determine competitor best-in-class

systems and subsystems

Complete the overall vehicle

specifications (OVS)

Complete the vehicle system/

subsystem targets

Determine the customer wants

for this group

Determine the competition vehicles

Analyse the competition vehicles

using the matrix provided

2.3 Vehicle target setting process [2].

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Target setting and benchmarking for vehicle noise 21

accelerations, wide open throttle run-up and overrun/coast-down driving

conditions.

The noise and vibration targets for the whole vehicle system, individual

components and subsystems are documented in the overall vehicle speciﬁ -

cation and system speciﬁ cations by the brand holder, and adherence to

them has become a condition of contract for any suppliers who implement

an advanced product quality planning process.

Corresponding to vehicle noise and vibration targets, a benchmark study

needs to be conducted on competitors’ vehicles for noise and vibration

performance of the interior and exterior, both subjectively and objectively,

of the whole vehicle and of components/subsystems, and of the level and

quality in all vehicle operating conditions.

2.2 Benchmarking of vehicle noise and vibration

Benchmark vehicles are selected according to similar style/platform capac-

ity, cost, weight and targeted market segment agreed by the program team.

They are tested for exterior noise including pass-by and idle; for interior

disturbing noise including rattle/squeak, resonance noise, road noise, pow-

ertrain noise, auxiliaries, load reversal and gear noise; for communication

and audio acoustics including hi/ﬁ qualiﬁ cation and articulation index; for

interior actuation noise including servo actuators and door closing; and for

interior driving noise in conditions of acceleration and constant speed. The

interior and exterior disturbing noise will need to be minimized, while

interior driving noise, actuation noise and communication/audio acoustics

can be designed as shown in Fig. 2.4. The vibration and ride quality are also

tested in all vehicle operating conditions.

Subjective evaluation is an important part of vehicle development

because its results are directly related to customers’ feelings. Subjective

evaluations are conducted in a group of people (more than three) and the

results are produced from the statistical average of the group. Table 2.1

shows a vehicle subjective evaluation rating scale and rules. Table 2.2 shows

a typical vehicle NVH subjective evaluation form.

2.3 Target setting for vehicle noise and vibration

Both subjective and objective vehicle evaluation tests must be conducted

on the selected benchmark vehicles. The test results plus previous vehicle

model test data and service warranty data of released vehicles are analysed

to set vehicle noise and vibration targets. According to the vehicle noise

and vibration subjective rating scale shown in Table 2.1, the noise and

vibration subjective evaluation target rating should be typically set as R8

for a future vehicle development model. The engine combustion order

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22 Vehicle noise and vibration reﬁ nement

tracking boundary lines of the second gear slow acceleration test data, the

ﬁ rst gear wide open throttle acceleration test data and overrun coast-down

test data will be powertrain noise and vibration target lines, and the con-

stant speed spectrum boundary lines will be target lines of the whole vehicle

tyre/road noise, wind noise, driveline and wheel-induced noise and vibra-

tion as well as idle noise and vibration.

Pass-by

Idle

Squeak/rattle

Resonance effect

Road noise

Auxiliaries

Load reversal

HiFi qualification

Articulation index

Servo actuators

Door closing

Acceleration

Constant speed

Driving noise

Actuation noise

Communication

audio/acoustics

Gearbox noise

Disturbing noise

Interior noise

Exterior noise

Vehicle acoustics: minimization

Vehicle acoustics: design

2.4 Benchmarked vehicle noise [4].

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Target setting and benchmarking for vehicle noise 23

Acoustic target setting and hybrid simulation assist in engineering the

sound characteristics of vehicles. State-of-the-art CAE technology and pro-

cesses are used to combine test data on existing components with virtual

models of new parts to accurately represent entire vehicles and set acoustic

targets up-front during development. In this way, the whole-vehicle noise

and vibration targets can be cascaded into vehicle subsystem and compo-

nent targets, and vehicle development can be achieved by predicting and

tuning passenger compartment sound early in the conceptual stage, even

before the detailed design of the vehicle is started.

Transfer Path Analysis (TPA) cascades system-level noise and vibration

targets down to subsystem level targets (Fig. 2.5). In the early stages of a

vehicle design program, targeted vehicles for the new vehicle are selected

based on their subjective noise, vibration and harshness (NVH) perfor-

mance. A reference vehicle for the new product will be selected which will

be used as a baseline vehicle for the whole vehicle program. Noise and

vibration measurements will be taken on both the reference and targeted

vehicles under multiple load conditions. The simulation target for the new

product will be derived from the measurements of the reference vehicle,

measurements of the targeted vehicle, and the simulation of the reference

vehicle model. Reverse Transfer Path Analysis tools will be used to quan-

tify the subsystem targets for the new vehicle based on the simulation

targets and design intent simulation models of new products.

The stiffness design of hard points between chassis and body structure

plays an important role in vibration isolation and noise reduction. The

hard points are engine mounting brackets, engine sub-frame/cross-

member, transmission cross-member, shaker towers, etc. Structural

Table 2.1 Vehicle subjective rating scale (courtesy of General Motors

Holden Ltd, 1999)

No. in scale Criterion

Commercial

10 Not noticed even by trained evaluators

9 Noticeable only by trained evaluators

8 Noticeable only by critical customers

7 Noticeable by all customers

6 Rated disturbing by some customers

5 Rated disturbing by all customers (border

line)

Non-commercial

4 Rated as failure by all customers

3 Complained as bad failure by all customers

2 Limited operation

1 Non-operation

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24 Vehicle noise and vibration reﬁ nement

Table 2.2 Vehicle NVH subjective evaluation form (courtesy of General Motors

Holden Ltd, 1999)

1. ENGINE CRANKING

Engine cranking noise

Engine cranking vibration

2. IDLE

Idle sound and vibration

Fan noise

A/C noise

Alternator whine inside vehicle

Power steering noise – when turning the steering

wheel from left to right full lock

Fuel pump noise inside the cabin

Exterior engine noise level & quality

Radiator or condenser fan noise

Exhaust tail pipe sound

3. ACCELERATION

Off idle boom

Quiver 25 km/h

Shudder 60 to 100 km/h

2nd gear slow acceleration noise

1st gear wide open throttle noise

Gear shift noise and vibration (auto transmission)

in normal acceleration

Gear shift noise and vibration (auto transmission)

in wide open throttle

4. CRUISING

Noise and Vibration Rating at 40 km/h

Noise and Vibration Rating at 60 km/h

Noise and Vibration Rating at 80 km/h

Noise and Vibration Rating at 100 km/h

Noise and Vibration Rating at 120 km/h

Doors, windows, pillars sealing

Mirror vibration

Road impact

Shake, throttle tip-in tip-out

Torque converter lock-up boom

5. OVERRUN

Noise when decelerating with the 2nd gear engaged

Noise when decelerating with the neutral gear

engaged and engine off

6. BRAKING

Noise when braking

Vibration felt when braking

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Target setting and benchmarking for vehicle noise 25

mobility functions are used to evaluate their stiffness. A structural mobility

function is deﬁ ned as a transfer function or frequency response function

between a force and the response velocity. An acoustic mobility function

is deﬁ ned as a frequency response function or a transfer function between

a force and the acoustic pressure response at the driver’s ear. The design

target for the acoustic mobility functions at the hard points is set as

55–60 dBL/N. The design target for the structural mobility function is set

as 0.312 mm/s/N. Finite element analysis and frequency response testing are

used to verify the design targets. Structural and acoustic mobility functions

at the hard points are measured and analysed, and the mobility function

values at the hard points are reasonably distributed and designed to achieve

a performance compromise of NVH and ride handling. This will be further

illustrated in the following chapters.

Statistical Energy Analysis (SEA) is an established technique for predict-

ing high frequency vehicle noise and vibration performance. SEA is more

sensitive to certain parameters such as material properties, damping,

absorption and treatment thickness and coverage than to ﬁ ne details of

geometry. Using SEA is especially practical and it can be particularly

advantageous in the early design phase of a vehicle development program

to set subassembly noise and vibration targets.

2.5 Vehicle noise and vibration target cascading (source: FEV

website).

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26 Vehicle noise and vibration reﬁ nement

Power plant sound pressure level (SPL) target setting is the ﬁ rst critical

step to develop an efﬁ cient NVH strategy that guides computer aided engi-

neering analysis and hardware research to achieve a desired goal in the

early stage of a program. Traditionally, speciﬁ cations have been set by

comparing a baseline power plant SPL average from several measurement

locations with its target; an effective method can be used to break down

the power plant SPL target into individual component levels at desired

frequencies quantitatively. The method is based on the inverse square law

that the reduction of sound power level equals the reduction of sound pres-

sure level at a ﬁ xed point in a free ﬁ eld. The SPL target could be test data

or theoretical calculations.

Figure 2.6 shows the articulation index of competitive vehicle, baseline

vehicle and development vehicles over an engine speed range from 1500 to

4000 rpm where the target of the articulation index can be set close to that

of the competitive vehicle. Figure 2.7 shows the noise reduction or engine

ﬁ rewall noise attenuation of baseline, prototype vehicles and the target line.

The engine noise attenuation of the baseline vehicle is less than that of the

target line. The engine noise attenuation of the development prototype has

reached the target line.

Figures 2.8 and 2.9 show a sound power contribution analysis of a vehicle

sound package using a window method where the vehicle was tested under

second gear slow acceleration on a chassis dynamometer in an anechoic

100

RPM

Articulation (%)

1500 2000

Competitor vehicle

Structural updates

Sound package updates

Baseline vehicle

2500 35003000 4000

2.6 Articulation index of sound pressure at the driver’s ear in the

centre for vehicles in ﬁ rst gear slow acceleration sweeps [5].

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Target setting and benchmarking for vehicle noise 27

room for the original condition as delivered, the minimum treatment, the

maximum treatment, and all individual contributing trim components on/

off. If the vehicle is the competitor best-in-class vehicle, then Fig. 2.9 pres-

ents the sound power targets for individual trim components in the vehicle

sound package.

Figure 2.10 shows typical SPL targets for different types of vehicle noise

sources. Figure 2.11 compares sound quality targets of a test vehicle.

Frequency

Attenuation (dB)

100

125

160

200

250

315

400

500

630

800

1000

2000

4000

5000

6300

2500

3150

1250

1600

Prototype actual

Target Baseline

2.7 Engine ﬁ rewall attenuation (noise reduction).

Original characteristics

(+ target definition)

Measure of the

minimum treatment

Measure of the

maximum treatment

Measure of all

contributing surfaces

Calculation of all sound

power contributions

2.8 Window method for sound power contribution analysis.

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28 Vehicle noise and vibration reﬁ nement

2.4 References and bibliography

1. Harrison, M. (2004), Vehicle Reﬁ nement – Controlling Noise and Vibration in

Road Vehicles, SAE International, Elsevier Butterworth-Heinemann.

Sound power contribution (dB)

Dash panel

Toe pan

Front footwell

Under front seat

Rear footwell

Heel kick plate

Front tunnel

Rear tunnel

Lower side

Under rear seat

Rear seat back

Parcel shelf

Rear bolster

ABC pillars

Upper structure

IP extension

Windows

Door panel

2.9 Sound power contributions (second gear slow acceleration sweep

from 500 rpm to 5000 rpm) from a vehicle sound package components

using window methods.

Overall interior SPL

65 dB

Engine noise

55 dB

Aerodynamic noise

60 dB

Road/tyre noise

58 dB

Exhaust noise

50 dB

Pleasantness

Well sounding

Luxury

Dynamics

Loudness

Sharpness

Target values

Test vehicle

Roughness

Powerfulness

Robustness

2.10 Targeted SPL for different types of vehicle noise [5].

2.11 Sound quality target comparison for a test vehicle [5].

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Target setting and benchmarking for vehicle noise 29

2. Wang, X. (2005), Introduction to Motor Vehicle Design, RMIT Publisher.

3. Happian-Smith, J. (2002), An Introduction to Modern Vehicle Design, SAE

International, Butterworth-Heinemann.

4. LMS Engineering Innovation (2007), ‘More than a quiet car: Acoustic target

setting supports BMW’s premium brand image’.

5. Gossler, J.V. (2007), ‘NVH benchmarking during vehicle development using

sound quality metrics’, Masters degree thesis, Department of Mechanical and

Mechatronic Engineering, Stellenbosch University, South Africa.

6. Zeng, P.W. (2003), ‘Target setting procedures for vehicle power plant noise

reduction’, Sound and Vibration, July 2003.

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