by Jim Kerr
Few of us ever take a vehicle apart, let alone cut one in half, but that is what Ford’s NVH (Noise, Vibration and Harshness) engineers did to demonstrate how they keep the interior quiet in Ford’s 2004 Freestar van. Jonathon Crocker, Freestar’s NVH manager, showed us that there is a wide range of sound dampening materials used but many are not very exotic. Placing the right ones in the right place is the key to building quiet vehicles.
Making the Freestar quiet wasn’t done with one big change. Improving NVH is accomplished in small steps using many different techniques. Special attention was paid to body and door sealing to reduce air leakage out of the passenger compartment. Expandable foam fills the cowl area to seal small leaks. Improved assembly techniques at Ford’s Oakville Assembly plant in Ontario (the only plant building the Freestar) ensure accurate weather-stripping and seal installations. The results are a 10% reduction in wind noise and an improvement in heater and air conditioning performance.
Expandable foam is also used at key body areas such as the top rear body corners to stiffen the body structure. Stamped ribs in the floor pan help strengthen the body structure and reduce body vibrations but don’t totally eliminate them. The most common material used by auto manufacturers to dampen the remaining vibrations is called mastic or melt sheets.
The Freestar uses mastic over most of the floor panels. The material is pliable, about 2 to 3 mm thick, and is applied to larger sheet metal surfaces. The rear portion of the floor was found to need more damping material so a special conformable constrained material was used in this area. The conformable constrained material uses mastic on the bottom and a plastic overlay sheet on top. When heated, the material conforms to the shape of the ribs stamped into the floor pan.
A special laminating technique is used for the Freestar’s firewall and rear wheel wells. A polymeric material is sandwiched between two steel panels to produce a structure that is still and “dead”. Tap on the panels and all you hear is a dull thud.
The laminated rear wheel wells, an industry first, were a special challenge. Their shape requires a “high draw” stamping that stretches the material. Ford engineers had to ensure they achieved proper material thickness in the sandwich construction during the construction process. Another difficulty to overcome was welding the composite wheel wells into the rest of the body structure. Because of the polymeric material, heat had to be controlled precisely and harmful welding vapours scrubbed from the assembly area. By using this compact wheel well construction, they were able to provide room for large wheels and tires, dampen out vibrations, and maximize interior space at the same time.
How do the engineers know where the vibrations and noise are coming from? One of the high tech methods used to locate vibrations is called material acoustic holography mapping. A shorter name is laser vibe. A laser is projected on the body panels as the vehicle is flooded with different frequency vibrations from large loudspeakers. Body panels excited by the projected vibrations show up clearly on the laser display. Placing the vehicle in a wind tunnel causes wind vortex that can excite panels. On the Freestar, the wind tunnel and laser vibe was used to locate a small area on the roof panel where the windshield vortex was causing a vibration. An acoustic pad applied only to this area damped the roof vibrations, meeting one of the NVH team’s objectives: maximum value with minimal material.
Computational fluid dynamics is another tool used to reduce noise and vibration sources. Both the mirror design and the roof rack were improved by computer simulation to reduce noise.
Powertrains come under the NVH engineer’s scrutiny as well. The Freestar has extra stiffening ribs cast into the transmission case, exhaust manifold and oil pan. Hydraulic motor mounts and a powertrain roll restrictor help reduce engine vibration transfer into the body. Exhaust heat shields also block radiated noise.
NVH improvements are not something that attracts us to new vehicles on the sales floor. They aren’t shiny or exciting, but riding in a quiet vehicle is much more comfortable and produces less driver fatigue. The next time you take a drive in a quiet vehicle, you are enjoying the labours of countless NVH engineers who work quietly in the background.