Vibration Damping on Body-panels needs in-depth analysis of vehicle NVH

Theory need be checked every time while applying automotive NVH solutions.

Whatever damping treatment is provided to thin structural panels of vehicle body or power-train related components will be totally ineffective if that structure has no self-resonating vibrations..

Below Test results are interesting for an Electric Car running on a road at high speeds:

When a damping pad of large area was pasted on its roof panel, and when this EV was driven on a normal road, there was absolutely no change in the In-cab sound. But when it was driven on the rough road at the same speed, there was a substantial benefit seen: Loudness level came down by 7 Sone.

The reason was that the rough road forces of mid frequency range 80 to 500 Hz passing thru' tyre-wheel-hubs and suspension pivots to the body could excite some of the roof resonances in bending and then the damping pads (applied to it ) could get a chance to reduce its vibration amplitude following a below famous graph and thereby, the structure-borne noise inside the cabin came down.

On smooth roads, electromagnetic and gear-meshing forces of higher frequencies from motor-train were too small to excite the roof panel resonances and hence, the damping sheets did not show any benefit.

Often, the damping treatment (like melt-sheets) for a car with a monocoque body showed a good reduction in the floor vibrations while passing over a rough road at 50 km/hr speed, but this could not give any benefit to the In-cab noise as shown below for an ICE powered car: a 270 Hz resonance of the front floor was almost killed by the melt-sheets (green curves), but the noise level at Driver's ear level did not get changed.

There are 2 reasons for such findings:

A. The structural resonances of the vibrations radiate sound inside the cabin of a car running over rough road. But this structure-bone noise could be attenuated by a thick acoustic insulation ( absorber + barrier - heavy layers) under the floor-carpet.

B. The air-borne noise inside the cabin could be more dominating over the structure-borne component (in the floor resonance freq. bands) as the car is driven at 50 km/hr on the rough road. Hence, the resultant noise level at Driver's ear may not get changed even though the best damping treatment is given on the body=panels.

Hence, a thorough analysis w.r.t. customer driving patterns & overall In-cab sound balance over a targeted frequency band is needed before a decision is taken to adopt a NVH-solution for mass-production of vehicles ...