by Jim Kerr

Interior noise levels in passenger vehicles have been decreasing for years. Hop into most new vehicles and the first impression will be how comfortable the ride is. Part of this comfort is provided by the quiet ride. Achieving this quiet ride is not easy. Any time an object passes through the air and rolls on a surface, noise will be generated. The trick is to allow only noises that are not objectionable into the passenger compartment.

Tire noise can be annoying, especially on some of the rough pavement roads around the country. Engineers design suspension bushings to match the tires characteristics used on the vehicle. Tires with higher sidewalls are more flexible so they can dampen out road noise better. The low sidewalls on performance tires gives a stiffer ride and better handling but at the expense of more road noise. By changing suspension bushing composition, much of this noise can be dampened out.

Frame design is also used to lower vehicle interior noise levels. Full frames, such as used on trucks, offer the best isolation of the passenger compartment from the tires. A pure unibody construction where the frame and body are one unit is lighter but transfers the most noise. Many passenger vehicles are using “sub frames” - small frame sections bolted to a unibody shell to provide low noise transmission and lighter vehicle weight.

Tire tread design has a huge effect on noise levels. Some tread patterns will “sing” as they are driven down the highway. Others may cause a rumble or vibration. Most tires provide reasonably low noise levels but some are better than others. You can’t tell (at least I can’t tell!) by looking at the tread design. Sometimes open patterns are very quiet while smoother tread patterns make more noise. Sometimes it is the other way around.

Road conditions also make a difference. While testing Yokohama’s GeoLander truck tires a few years ago, I found their All Terrain tires much quieter on gravel roads than their Highway model. But if I was driving on pavement most of the time, I would prefer the highway tire.

Wind is another source of noise. Any vehicle in motion will has wind rush. This is caused as air passes over the vehicle body. Even a glider pilot experiences wind rush in their streamlined machine. Outside mirrors are a major contributor to wind rush on a vehicle. Take a closer look at mirror design and you will find fancy shapes, added fins and strategically placed holes to allow air to flow around the mirror smoothly.

About 90% of all wind noise complaints come from disturbances in airflow from the base of the windshield to the door post behind the driver. This isn’t surprising, as this is the area where there are a lot of body angle changes, many openings, and in close proximity to our ears!

Lexus, when they designed the 2004 RX 330 paid a lot of attention to detail in this area to make the RX 330 SUV a very quiet vehicle. Windshield wipers were placed below the main airstream off the hood to eliminate air turbulence. Roof rack crossbows have a “lift-neutral” design for smooth airflow. The windshield pillars are angled to direct air smoothly around the body. Extra seals are positioned between the hood and fender and between the front and rear door opening to nearly eliminate noise at these points. According to Chief Engineer, Yukihro Okane, achieving low interior noise levels was an important aspect of attaining Lexus luxury.

Wind noise can also be caused by misaligned body panels. There is a slight “shingling” effect used on automobiles to reduce wind noise. Body panels are slightly higher before an opening and lower behind an opening. This causes the air to flow over the opening rather than swirl into a protruding gap. You can feel this effect by sliding your hand over body panel gaps. If they are not higher at the front, usually the panels can be adjusted to reduce the noise.

As vehicle interior noise becomes lower, the product planners can then add sound to the vehicle that gives it some personality. A slight exhaust rumble on acceleration gives that feeling of power. High-end sound systems can play music with concert hall quality, undisturbed by outside vehicle noises. And for those without children, the sound of air slipping gently over the vehicle’s body may be all the sound you need to make motoring relaxing and enjoyable.