Increasingly, automotive interior noise levels and the "acoustic signature" of a vehicle -- the way the car sounds to its occupants under various operating conditions -- are becoming competitive differentiators for automakers. A sports car driver wants to hear the throaty roar of a powerful engine under heavy acceleration, for example. And for high-end vehicles, interior acoustics that enable easy conversation between front seat and back seat passengers can provide a higher perception of quality and luxury.

"As basic issues such as reliability are better addressed by all of the car makers, then companies start looking for other ways to differentiate their products, and trying to give the consumer an unusually quiet or pleasing product acoustically is one of those," notes Bob Baker, staff development engineer responsible for acoustics at the General Motors Technical Center (Warren, MI).

New materials
With this in mind, car manufacturers and their interior systems suppliers have in recent years shelled out large capital expenditures for sophisticated acoustical testing systems and laboratories. Tier One suppliers, in particular, have ramped up efforts to develop new automotive materials that can not only help improve acoustic performance, but also meet the added goals of reduced weight, lower cost and improved recyclability.

"They're all striving to come out with the perfect, cost-effective materials that have the biggest bang for the buck in helping control acoustic problems inside vehicles," observes Scott Sumerton, an applications engineer at Bruel & Kjaer North America Inc. (Livonia, MI), a supplier of sound measuring equipment.

Besides developing lighter-weight alternatives, suppliers are also focusing on methods to effectively "tune" automotive materials and interior systems to enhance the perception of quality among car buyers. "We're hearing more and more buzz in the industry about perceived quality," says Michael Dinsmore, director of new product strategy at Johnson Controls Inc. (Holland, MI), an interior systems supplier.

Focus group studies have shown that even though sensitive instrumentation may measure overall noise levels to be equal within a vehicle equipped with different acoustic materials packages, consumers will often prefer one over another. The mix of high and low frequencies and spectral content within a vehicle can create an impression that can affect a consumer's feelings about the quality of the vehicle, researchers say.

"Every vehicle has a special acoustic signature, and most vehicles also have a frequency that we can identify as being the major problem inside the vehicle. So what we have to do is design the materials to absorb the frequencies that are causing the problem," explains Graham Tompson, senior vice president, Global Product Development Div., at Collins & Aikman Corp. (Troy, MI), another interior systems supplier. By making slight alterations to a material's formulation, the material can be tuned to attenuate objectionable frequencies, he says.

Much current industry effort is aimed at tuned materials and interior systems that can eliminate extraneous noise in the 300 to 3,400 hertz voice band of human speech, as a way to make in-car conversation easier, say industry sources. Greater use of cellular telephones is also driving the trend. "With cell phones, you often have difficulty hearing people when you're in a quiet room, much less in a moving car," says Dinsmore. "So if people want to use a cell phone and they're making a vehicle purchasing decision, they're going to pick the quieter car."

Draining the swamp
Not surprisingly, luxury vehicles typically feature the best acoustic performance. But interior noise levels in all classes of vehicles have declined dramatically over the last couple of decades. The average sound level in a vehicle traveling at 55 mph dropped from 72 decibels in 1980 to 66 decibels in 2000, says Christopher Griffen, senior director, acoustic R&D North America, at Collins & Aikman. Because the decibel is a logarithmic unit, that's around a 30% improvement, Griffen says, and about equivalent to the noise reduction that comes with turning an indoor room air conditioner from high speed to low.

These lower vehicle noise levels can produce a generally more pleasing interior environment for consumers. But they often also present new challenges for the car makers, in that sounds and noises that were previously masked can become more audible when background noise levels decline.

"It's like the proverbial stump in the swamp. As you keep draining the swamp down, you get new stumps popping up," quips Paul Riehle, engineering director at Roush Anitrol (Livonia, MI), a division of Roush Industries Inc. that provides noise and vibration engineering services.

A squeak or a rattle that might previously have gone unnoticed, for example, can suddenly become a problem when the overall vehicle noise level goes down. And it can negatively impact a consumer's opinion of vehicle quality. "Some of these noise issues today are primarily a perceived quality issue, where it's not so much the general noise level that's there, but you've got this specific thing that's an irritant to you, and it happens only under certain conditions," Riehle observes.

Barry Wyerman concurs. "In some vehicles, there are particular sounds. It can be anything from a certain growl in the engine to a whine in the transmission at a certain operating speed," says Wyerman, director of advanced flooring and acoustical engineering at Lear Corp.'s Interior Systems Div. (Southfield, MI). "The reality is that this sound was probably there all along. It's just that the vehicle has gotten a little bit quieter, and all of the sudden, people notice it."

Testing chambers
It is these kinds of problems that Tier One interior systems suppliers such as Collins & Aikman, Johnson Controls, Lear and Magna Corp., among others, are attacking with a growing array of sophisticated acoustics testing systems and materials development tools.

Johnson Controls, for example, opened a new, 10,500-square-foot automotive acoustics laboratory in late 2000 that incorporates multiple, large-scale reverberation and anechoic testing chambers. Collins & Aikman, likewise, completed a major expansion of its Plymouth, MI, vibro-acoustics testing facility early this year. And Lear completed an upgrade of its acoustics testing facilities about three years ago.

The Lear facility includes a hemi-anechoic chamber with a four-wheel chassis dynomometer that can simulate acoustic performance at varying speeds and load conditions. "The walls are completely lined with sound absorbing materials, so we can do very detailed studies of a vehicle and actually pinpoint some of the sources of noise, so we can go after the root causes," Wyerman explains.

The Collins & Aikman lab includes a similar facility, and also includes a range of other sophisticated gear. A portable, laser-based imaging system in one lab, for example, can be used to optically measure and map vibration in sheet metal structures. Compared to conventional methods requiring sensors placed on the material, the system cuts test Arial by a factor of 10 and also provides a visual map of vibration hot spots, Griffen says.

Car makers and suppliers are also relying more heavily on software simulation tools for predictive acoustic modeling that can dramatically cut the time required to develop and tune acoustic automotive materials and systems. Collins & Aikman uses a proprietary package called Comet, for example. By reducing the need for physical material modification and testing, the package can cut materials development times by some 75%, according to Tompson.

Similar time savings can be achieved at the full-vehicle level through a noise and vibration modeling technique known as statistical energy analysis (SEA). "Using SEA, we can build a computer model of a vehicle before the first vehicle is ever produced," says Lear's Wyerman. "Then we can ask a lot of 'what if' questions, such as, 'What if I change the thickness of this material?' or 'What if I put a fiber here instead of a foam?' By doing lots of predictive studies, we can really help our customers [the automakers] reduce their development times and the number of iterative steps that they have to go through," Wyerman explains.

SEA technology from two software suppliers -- Cambridge Collaborative Inc. (Cambridge, MA) and Vibro-Acoustic Sciences Inc. (San Diego, CA) -- is now being widely adopted throughout the automotive industry, executives say. GM, for one, is currently working with its interior systems suppliers to develop a set of standard conventions for use with SEA models, says Baker. "By agreeing on the details of the model, we will be able to talk to all of our several suppliers in much the same way," he notes. "It provides a good device for communicating our requirements to the suppliers and for better understanding each other."

Barriers fall
In line with general industry trends, the automotive OEMs are turning over more up-front, interior system design responsibility to their Tier One suppliers. And they are setting more aggressive targets not only for acoustic performance, but also for lighter weight and cost. One major trend, consequently, is the development and use of lighter-weight acoustic materials.

Traditional automotive designs have relied upon heavy barrier materials, particularly in the floor and in the dash area behind the engine, as a way to block out road and power train noise. So-called mass-backed floor carpeting that carries a heavy layer of plastic barrier material has been common in vehicles since the mid-1970s, for instance.

But in a growing number of new designs, these barriers are being replaced by sound absorbing materials that provide equivalent acoustic performance at much lower weight. Collins & Aikman, for example, was able to knock 23 pounds out of the interior system design for a forthcoming GM sport utility vehicle, in part through the use of a new, lightweight, sound-absorbing floor carpeting material, says Griffen. And at Lear, Wyerman says his firm cut the dash weight by about 50% in one vehicle by replacing a barrier material with a sound-absorbing foam.

"With the trend toward lightweight technology, we're looking at specialized fiber and foam constructions that provide more sound absorption for the interior, while giving up some of the transmission loss barrier performance. So it's sort of a trade-off," says Wyerman. "It's a new approach to providing quiet in the vehicle."

In many cases, the new lightweight materials are also acoustically tunable, and can include multiple components that can work together as a system. For instance, Collins & Aikman's AcTfiber materials (for Acoustically Tunable fiber) can be configured as a three-part floor carpeting system consisting of underliner, carpet and floor mats, with each of the components tuned to attenuate different frequencies.

This same acoustical integration approach can be carried over to all of the components in the interior package, such as seating systems, headliners and door panels. "If you give us just one component to acoustically optimize, there's only so much that we can do. But if you give us the entire system to optimize, we can achieve a lot more, because we can look at tradeoffs, we can look at part elimination and we can look at using more efficient materials in one area to remove the necessity for having materials in other areas," says Johnson Control's Dinsmore.

Fix it, please
Despite today's increased focus on acoustic performance during the early stages of a vehicle design, some problems do slip through. "As good as the car companies are, there are times when they have designed a system and started tooling up a vehicle that does have some kind of noise problem," Dinsmore says.

In some cases, these relate to mechanical design issues that can be fixed with relative ease. Roush Anitrol's Riehle was recently asked to help solve a noise problem on a vehicle that was nearly ready for launch. Upon analysis, Riehle found that at certain vehicle speeds, pressure pulsations from the power steering pump were causing vibration in a power steering line that was attached to the side rail of the vehicle frame, which created a vibration sound path to the passenger compartment.

Riehle was able to eliminate the problem by rerouting the power steering line and changing the mounting method. "In the previous model year of that vehicle, you would never have heard that noise at all, but since everything else had been made quieter in the vehicle, now you were hearing this problem," Riehle observes.

In other cases, the solution may not be as straightforward. "You might see problems related to a powertrain that's making a little more noise than it's supposed to, or some kind of sound leakage in the body," says Dinsmore. Because these kinds of mechanical systems problems would be too expensive to correct at a late stage in vehicle development, OEMs often turn to their interior systems suppliers for help, Dinsmore says. "Some of our tooling tends to be tooled up later in the program, after a problem is discovered, so that we can make some changes to our parts."

In these cases, the use of larger pads on the back of a door trim panel, or in a headliner, for example, might do the trick. "There are a number of things that we can do to help block noise if the car company has, in fact, not met its acoustical targets," Dinsmore adds.

In the end, say industry sources, the competition in automotive acoustics is getting tougher, as interior noise levels for all classes of vehicles are reduced. By one estimate, the difference between the noisiest and the quietest vehicles on the road has dropped from 4 decibels 20 years ago to just 2 decibels today.

As that gap narrows, car buyers are becoming more discerning about interior sound quality, says Collins & Aikman's Tompson. "That's why having the right materials technology, the right testing equipment and the sophistication to understand the subtle differences from one frequency to another," Tompson concludes, "is becoming a lot more important."

• Automotive interior noise levels declined from an average of 72 decibels in 1980 in a vehicle traveling at 55 mph to 66 decibels in 2000.
  
• Light weight, sound absorbing fibers and foams are replacing traditional, heavy sound barrier materials in automotive floor and dash systems.
  
• OEMs and suppliers are relying heavily on software simulation tools for predictive acoustic modeling that cuts the time required to develop new acoustic materials and systems.
  
• Major effort is aimed at tuning interior systems to eliminate extraneous noise in the 300 to 3,400 hertz voice band of human speech, as a way to make in-vehicle conversation easier.