To understand why and to reduce the unwanted high noise levels, we performed analysis on the vehicle. To do this, we instrumented the vehicle to measure the acoustics in the passenger compartment and the vibration levels at very specific locations. With these measurements, and the technical data of the vehicle provided by the manufacturer, we could highlight the noise source.
By continuing the analyses, we noticed that an element of the structure entered in resonance because it was mechanically linked to this source of excitation. In other words, the excitation of an eigenmode of the structure was taking place.
To solve our customer’s problem, we had to modify the frequency of the eigenmode in order to avoid that the element of the structure enters in resonance, which consequently would induce amplified noise levels inside the passenger compartment. Several solutions were investigated to modify the unwanted resonance frequency and thus to reduce these noise levels inside the vehicle:
- Modification of the point mass of the vibrating element,
- Modification of its stiffness,
- Correctly dimensioned damping of the structure.
At the end of the work, our customer was able to strongly reduce noise levels inside the vehicle cockpit
If the problem had been identified in the early stage of the project (i;e. before testing on a real vehicle) Metravib could also have assisted the customer’s technical team by dimensioning decoupling elements (or anti-vibration pads) adapted between the noise source (the drive shaft) and the vehicle body. In this case, the siren noise levels would have been completely masked by other noise sources in the cabin.