Case Study: NVH Development During Conversion of a Sports Convertible into a Coupe

MIRA worked in collaboration with a design house on a project to develop a coupe from an existing convertible variant, for a vehicle OEM.  The specific NVH issues requiring particular consideration were:

• Exhaust related noise
  • Interior noise levels
  • Interior and exterior noise quality
  • Legislation (pass-by noise

• Road related noise
  • Broadband and narrow band characteristics
  • Wheel size increase
  • Tyre profile change

• Aerodynamic noise contribution
  • Body style change

The exhaust sound quality of the pre-identified competitor/target vehicle was deemed be of a “sporty” nature:

• 3rd & 1.5th order dominate
• Split exhaust system
• 2 x 3 cylinder sources

The initial prototype vehicle was considered to have a harsh exhaust related characteristic:

• 3rd & 6th order dominate
• 1.5 m common system
• 1 x 6 cylinder source

In the 100Hz to 1kHz band, tyre noise dominates overall noise level, being structurally transmitted from the rear axle.  Tyres and wheels were tuned to optimise interior noise performance whilst maintaining Pass-by noise characteristics within legislative limits.

Body changes resulted in a “boomy” boot cavity response.  MIRA were able to optimise parcel shelf area to reduce the impact of this on interior noise, ensuring that the final coupe derivative achieved the required NVH targets.