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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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
Interior noise Exterior noise
Vehicle acoustics: minimization
Vehicle acoustics: design
2.4 Benchmarked vehicle noise [4].
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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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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 25km/h
Shudder 60 to 100km/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 40km/h Noise and Vibration Rating at 60km/h Noise and Vibration Rating at 80km/h Noise and Vibration Rating at 100km/h Noise and Vibration Rating at 120km/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
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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 55–60dBL/N. The design target for the structural mobility function is set as 0.312mm/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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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 4000rpm 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 90 80 70 60 50 40 30 20 10 0
RPM
Articulation (%)
1500 2000
Competitor vehicle Structural updates Sound package updates Baseline vehicle
2500 3000 3500 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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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.
80 70 60 50 40 30 20 10 0
Frequency
Attenuation (dB) 80 100 125 160 200 250 315 400 500 630 800 1000 2000 4000 5000 63002500 31501250 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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