Tech Briefs
Closing the window on NVH

Initial lamination stage of automotive glass production Sound reduction comparison between laminated and tempered side glass. The test was done on a diesel truck traveling at 105 km/h (65 mph).

The priorities of car design are arguably reliability (if it does not work everything else about it is irrelevant), safety (achieving, or exceeding, international standards), practicality (to meet the criteria of its role), mechanical efficiency (performance, economy, environmental capability), convenience (ease of operation, comfort), and style. "The appearance of a vehicle and its style and luxury are now just as important as its capability to get from A to B. A significant contributor to this feeling of comfort and quality are a smooth ride, easy handling, and a quiet environment inside the car," says Ray Cross, Product Manager with glass manufacturer Pilkington Automotive.

In recent years, vehicle manufacturers have invested significantly in research to reduce engine and road noise in an effort to improve driver and occupant comfort, and reduce driver fatigue. This has been driven by NVH (noise, vibration and harshness) research, which concentrates on the audible and tactile properties of a vehicle. An increasing proportion of the passenger area of a car is made up of glass, a trend most obvious in people carriers or MPVs (multi-purpose vehicles). Glass is only a few inches away from the occupant's ear and any noise or vibration is therefore transmitted directly to them.

The design and specification of glazings on windows is an increasingly important method of reducing noise and vibration and providing thermal insulation. In recent years, double-glazing has been used on some luxury models like the BMW 7-series, though the process adds both weight and cost. But regular glass also contributes to improved NVH levels. Cross explains that laminated glazing, when compared to standard toughened glass (monolithic glazing), is an extremely effective method of reducing the transmission of noise due to the bonding polymer material sandwiched between the two pieces of glass. Pilkington Automotive is now looking at ways to use this bonding material to alter acoustic properties. "Within five years it is predicted that a significant number of vehicles will use laminated glass in the side glazing, and this offers great potential to control vibration and noise," says Cross. "Currently, noise can be reduced by up to 5 dB by manipulating the bonding material—a level easily discernible to the human ear."

Sound reduction comparison between laminated and tempered side glass. The test was done on a diesel truck traveling at 105 km/h (65 mph).

The windshield and back windows significantly increase the torsional rigidity of the vehicle. These glass areas now form structural body panels that can vibrate like any other panel, such as door skins. Changing the shape and curvature of these glass panels will alter the natural resonance characteristics. This needs to be taken into account in vehicle design and is increasingly important as designers seek to reduce vehicle weight. Vibration through the tires or from within the body structure (e.g., the suspension or engine) can cause the glazing to vibrate in sympathy, making occupants extremely uncomfortable. "Pilkington Automotive is developing computer simulations that manipulate the thickness, shape, and curve of the glass to reduce vibrations," says Cross. "We're also looking at the modal frequency of glass vibration to make sure it does not coincide with other vehicle vibrations. The effect of these vibrations can be enormous, greatly affecting the way a vehicle is perceived by its occupants. This is obviously of vast importance to luxury vehicle manufacturers."

Laminated glazing also plays a role in occupant comfort by reducing what is known as 'cold shoulder'—the draft experienced from the driver-side window. The double layer of glass and the polymer bonding material effectively insulate the vehicle. Double-glazing also provides insulation and acoustic performance, but it is significantly thicker than laminated glass. Pilkington says its laminated glazing has a weight comparable to the 4 mm (0.16 in) monolithic glazing. The company also claims that laminated side glazings will reduce the risk of theft from the vehicle, improve personal security within the vehicle, and reduce the risk of occupants being ejected from the vehicle in the event of an accident.