Hush: Improving NVH through improved material

Gary S. Vasilash

How making structures with a specially engineered steel laminate can result in quieter cars.

Gregory M. Goetchius comes into the conference room with a red, steel toolbox. The sort of thing that you might have in your basement or garage. About the size of a tackle box. Goetchius also has a Dremel with him. It’s part of the equipment he’s using to give a demonstration of what “noise” is. He knows more than a little something about the subject, as he is the NVH Technical Manager at Material Sciences Corp. (MSC), Engineered Materials and Solutions Group (Farmington Hills, MI). For those who are not NVH specialists, he simply says, “Noise is a generic term for measuring sound pressure level. Sound pressure level is often reported as ‘db’ or ‘decibel.’ Decibel is a logarithmic reduction of something our ears hear.”

“Noise” is something that he’s about to create with that Dremel and the toolbox. In the place of a tool, there’s a bent paper clip in the Dremel’s chuck.

Goetchius says that the toolbox can be thought of the engine box in a car. The Dremel is the engine. “What I like about this Dremel is that as it spins, it has several different sources of noise and vibration.” (Remember, he’s an NVH engineer.) There’s the sound of that bent paper clip, which he says is analogous to the imbalance of a piston-driven engine. There’s a high frequency whining from the fan motor. The magnetic windings generate a lot of noise, as well. And when he puts it in the box, there’s even more noise. The steel panels vibrate; noise radiates from the steel. That’s what’s known as “structure-borne” noise. There’s also a high-frequency sound from the motor. That’s “air-borne” noise.

He provides an example of some of the countermeasures that NVH engineers could use. He puts the Dremel on a hard rubber mount. This changes the sound of the noise. Reduces some of the structural noise so that it becomes a moan as the rubber absorbs a measure of the energy. He takes a piece of soft plastic and replaces the rubber mount. More of the noise is quieted. Goetchius points out that while softer may be better so far as isolating mechanical vibrations, which give rise to structure-borne noise, when it comes to working in, say, a real engine box, softer isn’t particularly good because of durability issues (which could lead to the whole engine bouncing around).

He demonstrates how more noise can be attenuated by putting magnetic patches around the outside of the toolbox. There’s still a “woooo” sound from the Dremel. The pieces of magnet—doing what’s known in the business as “constrained layer damping”—are helpful, but not sufficient. There’s still the high frequency, air-borne noise. Goetchius suggests that you think of a house that’s completely empty. No furniture. Nor carpet. It echoes far more than a house with sofas and rugs. They absorb the noise. So one thing that NVH engineers do to cars is to add absorption pads, such as inside the firewall and blankets below the hood. He puts pads inside the box. There’s still the moan from the structure-borne portion. He takes a heavy mat and covers the box with it. The noise is isolated still further.

“This is the way that people chip away at noise in the auto industry,” he says.

Here’s an important thing to think about from Goetchius: “Quiet cars don’t happen by accident. It takes a huge amount of effort to make them quiet.”

Here’s a philosophical thing to think about that he proffers: “Noise and vibration are sort of like entropy in the universe. Left to themselves, noise and vibration in a car would always be worse.”

Mark Gresser, director of Automotive Marketing at MSC, points out that when it comes to structures, the body structure people are more concerned with things like stiffness and crashworthiness than they are concerned with NVH. They make body structures that work. The people who are most concerned with having a quiet cabin are the interior people. Yet, as Goetchius’s demonstration indicates, a lot of the noise comes from the box—the structure—itself.

So the root cause of noise may be the structure.

Which brings us to what MSC has developed to deal with noise in cars and trucks. A different kind of steel. A laminate. A material that they’ve trademarked as “Quiet Steel.”

How is noise typically dealt with in cars and trucks? Well, there are a variety of mastics. Spray it on. Stick die-cut steel with mastic backing onto various areas, such as the dash panel or floor pan. These approaches help reduce the structure-borne noise. There are cotton shoddy mats and foams and absorbers. These help absorb the air-borne noise.

While all of these are helpful, MSC engineers believe that a better way to deal with the noise that’s created as large panels vibrate is to keep the panels from vibrating. Which is what their material is all about. Well, there is some vibration. Consider a piece like a cowl plenum between the engine and the passenger compartment. (The people at DaimlerChrysler thought of it and made a running change for the 2003 Chrysler Town & Country, Voyager, and Dodge Caravan minivans: from a mastic patch to a Quiet Steelcomponent.) There are three layers: two pieces of steel with an engineered viscoelastic layer in between. The layer of steel that is on the engine side vibrates. But the vibrations are absorbed by the middle layer (technically, there’s a “micro-shear deformation” that leads to the dissipation of the vibration). Which means that the structure-borne sound is significantly eliminated. Instead of adding something to an object to make it quieter, the object itself is made so that it is quieter. (This, incidentally, can actually reduce mass in a vehicle.)

Quiet Steel is more expensive than solid steel. That’s obvious. People who are looking at piece costs, or price per pound, are not going to be too excited about it,” Gresser admits. People, say, in purchasing. Or people who are res-ponsible for making individual components and who are looking at their discrete costs.

But for those who take a step back and recognize what they’re trying to accomplish—assuming that this includes making a quieter vehicle—then, Gresser suggests, the laminate can have a compelling value proposition, especially when the costs associated with adding various and sundry sound-absorbers are taken into account.

Ford was something of a pioneer, as it had the first application of an automotive body panel made with the material: the dash panel for the 2001 Ford Explorer Sport Trac. Various other applications have followed, including the dash panel and the oil pan of the 2003 Lincoln Navigator.

One concern that some people may have is that this is a different material. A different steel. But it is both the same as that which is typically used. And different. That is, the two layers of steel are likely to be the same type of steel that’s used for the given application. They’re just each half the thickness of what’s ordinarily used. That coil steel is sent to MSC, which then transforms it into a laminate at one of its plants. The coil is then shipped to the appropriate stamping plant.

Some people in stamping plants might be concerned with the fact that they’re dealing with something that they haven’t had to deal with before. Gresser says that in terms of stamping, they’ve had several reports that the material actually stamps better than solid. When stamping tests were run for the dash panel that’s being used in the 2003 Cadillac CTS (for which MSC was awarded a 2003 PACE Award), they were run in the tooling that was used for the conventional steel panel.

Apparently, one of the problems with laminates in the past was associated with spot welding. Simply, there were “cold” welds. Welds that didn’t make it through from one side to the other because of the material in the middle. According to Gresser, this has been resolved by MSC chemists and engineers, and that they’ve shown that it is even possible to spot weld Quiet Steel to Quiet Steel (think of a dash panel to a floor pan), which means four layers.

One of the things they do at MSC is to conduct a predictive analysis of a vehicle architecture to determine where the material might be best deployed. It isn’t a one-size-fits-all sort of thing. There are issues of both strategic positioning and type of material to be deployed. Gresser admits, “It doesn’t solve every problem.” But he adds, “There are very few vehicles that we’ve come across that wouldn’t benefit from it.”