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Vehicle Interior NVH

Vehicle interior NVH includes the noise inside the vehicle from road, body, chassis and components.

Road Noise is one of the most annoying interior noises in a vehicle. Road excitation that contributes to the interior noise both structure borne and airborne. Target setting based on subjective evaluation and experience, vehicle verification typically by analysis of SPL, third octave band spectra, narrow band spectra, AI at different vehicle speed asphalt type. Targets include noise level and frequency balance, tyre cavity resonance levels (ca 230 Hz) and thread noise.

External road noise is also becoming more and more critical for fulfilling pass-by noise targets, even if the typical real life problem often is related to constant speed driving close to living areas.

The sound package is affecting most of the NVH areas. The first step is to make sure that the body is sealed as much as possible. This reduces high frequency leakage and increases the overall NVH performance a lot. Structural damping and isolation mats with heavy layer needs to be optimized for best performance and lowest weight and cost. Finally, acoustic absorption material at critical locations is used to improve the acoustic interior comfort.

Component noise is concerning all additional systems like HVAC system, power windows etcetera. Mostly electrical or driven by the engine belt but sometimes hydraulically.

Correlation between component/system/vehicle tests/CAE is complicated.
Cooperation with the design/test team and sometime the supplier for the component/system is essential in order to be able to perform a valid test – wrong test conditions cannot be saved by good test equipment.

Typical flow for components, some tests are handled by suppliers:

  1. Component test - each component in a generic or preferable correct load situation.
    SPL / Sound power / Frequency analysis as appropriate to detect the noise corresponding to the component. Test in anechoic or semi-anechoic test cell. Used for acceptance test, production test or CAE correlation.
  2. System test – complete system like a complete electric sunroof module. Test similar as for component testing, but now definitely at correct load conditions but probably not correct boundary conditions.
  3. Vehicle test – System during typical usage, but isolated (no other sounds). Test method similar to system test and correlated to subjective evaluation. Final verification of project specification and if the target is not fulfilled for further development testing.

It is important to develop validated test procedures that still are fast and easy to perform and preferable with standardized transducer positions in order to execute several tests efficiently. These procedures should also be as generic as possible to allow variation of system design. The engineers need to stay updated on new concepts and perform benchmarking.

Body and chassis design has a big impact on vehicle NVH performance, especially for powertrain and road noise. CAE correlation tests are essential.

Brake squeal is a critical chassis related noise, a very disturbing noise causing a lot of customer complaints. Brake squeal requires a lot of component, system and vehicle testing.

Typical challenges within Vehicle Interior NVH:

  • Testing time has to be short since the access to prototypes is limited.
  • Instrumentation must be fast and easy in a way to record repeatable results.
  • The microphones must be installed to minimize structure borne sound.
  • Microphone holders and cables shall not introduce any rattle noise.
  • The installation shall be safe for the test engineer during vehicle test.
  • Calibration verification must be easy to perform.

Reliable equipment is critical since testing often is performed at a test track offsite. Rattle free installation is extremely important to be able to evaluate high frequency content that is not contributing to SPL but to sound quality. Microphone field type is important since this noise is often analysed above 5 kHz, and time data is sometimes also used for later playback for subjective noise comparison.

Rear end rush (road noise coming from rear air evacuation, rear window, C-pillars, luggage compartment) is important. Difficult to extract from measurement results with omnidirectional measurement microphones.

Development of validated test procedure, still fast and easy to perform and preferable with standardized transducer positions in order to execute several tests efficiently.

  • Available test time is often short.
  • Fast setup of equipment, easy and repeatable installation of transducers.
  • Minimize influence of structure borne noise to microphones.
  • Safe installation for the driver.

Microphones used for Vehicle interior noise testing shall be robust and also able to handle dust and humidity. Low noise performance and full audible frequency range is also required. Free-field or random incidence microphones are most often used and the choice depends on test procedures.

Total interior noise

The total interior noise is a way of testing the general experience of the acoustic comfort of a vehicle. It’s a good basic benchmarking measurement and the noise level testing is also often used in by automotive magazines for comparison of different car models. It concerns the overall noise level inside the vehicle at its full speed range and sometimes also Articulation Index as a good index for isolation and seals as well as speech intelligibility.

Challenges:
• Testing time has to be short since the access to prototypes is limited.
• Instrumentation must be fast and easy in a way to record repeatable results.
• The microphones must be installed to minimize structure borne sound.
• Microphone holders and cables shall not introduce any rattle noise.
• The installation shall be safe for the test engineer during vehicle test.
• Calibration verification must be easy to perform.

The transducers must be of rugged design to withstand daily usage and in different environmental conditions. At worst by accident dropped to the floor.

Example:
This test is performed on a smooth road test track while driving from low to high speed and with microphones at ear positions.

Road noise
Sound package
Component noise
Body and chassis acoustics