Sound Engineering--Computer engineering techniques help plastic battle metal

It's almost impossible to believe if you don't see it with your own eyes. The wind starts gusting, and suddenly the mighty suspension bridge starts twisting and squirming like a sick anaconda, tossing a car into the drink like a toy. It's a popular piece of historical footage on cable TV, and a stern reminder to engineers everywhere that you can't always overcome the force of the wind with mere steel and concrete.

The gusts were strong, but nothing near hurricane strength. The bridge's architect simply made a tragic error: he didn't design the bridge so air could rush through it easily. Instead winds pushed and pulled the mighty span, setting up powerful - and ultimately devastating - harmonic motions.

Powertrain engineers are struggling with these same forces on a microscopic scale. In this case, air rushing unpredictably through intake manifolds sets up harmonic vibrations within the manifold that sometimes make good engines sound bad. It may be only a minor raspy note; other times it's a problem that could ruin a car's reputation if it made it into showrooms.

In a marketplace where Harley Davidson is trying to patent its unique engine sound and Mazda Motor Corp. spent months re-engineering the exhaust note of the Mazda Miata because it initially ran "too quietly" noise is unacceptable. The problem is further amplified by the fact that just as engine noise is becoming a crucial issue to engineers and consumers, automakers are rapidly changing new intake manifolds from metal to plastic. New plastic manifold designs are lighter, cheaper and can even boost horsepower, but they can't damp unwanted sounds as easily as metal.

Fortunately, sophisticated computer engineering techniques are saving plastic's bacon - so to speak - by making it possible for engineers to precisely analyze how air rushes through the manifold and counter those vibrations with thicker walls or strategically placed stiffening ribs.

Today thermoplastics (essentially nylon 6/6 and nylon 6) have a 35% share of the global air-intake market. Aluminum accounts for most of what's left. Magnesium also has a small-but-growing share. Plastic is predicted to grab 50% of the global market by 2000, but the noise issue clearly has gotten BASF AG and DuPont Automotive - two of the world's biggest suppliers of nylon for intakes - a bit nervous.

Both gave presentations on acoustical optimization of manifolds at last winter's SAE conference touting their sophisticated testing and analysis capabilities in this area, including exotic-sounding technology such as acoustic holography. DuPont is playing up its new noise lab in Troy, MI, which is a mirror image of a facility it has in England. BASF counters by saying its sound engineering laboratory in Germany has been in use for 10 years. Both offer acoustical analysis free to their customers.

"The last couple of years, OEMs have become fanatical about air rush sounds," says Eric Sattler, an engineer specializing in sound attenuation at BASF. "In terms of acoustical insulating properties, plastics have general drawbacks, due to their lower mass density, compared to metals," Mr. Sattler continues. However, the noise emission from plastic components can be reduced to similar levels of metal parts by using computer-aided engineering (CAE) simulation programs to optimize vibro-acoustics behavior of a plastic part, says Mr. Sattler.

BASF and DuPont won't comment on each other's laboratories, but both seem to use similar - and horribly arcane - strategies of acquiring acoustical information, digitizing it, and then creating sophisticated computer sound-pressure maps and images that methodically isolate desirable and undesirable sounds.

Both typically record sounds coming from parts with a microphone and then digitize it with special software and computers. Digital filters then isolate and uncover specific undesirable noises. The digitized sound data shows noise levels and intensities that can be displayed in specific frequencies in various types of colorful sound pressure maps.

The next step of determining exactly where the noise comes from may include attaching an accelerometer to a fixed location while a specially instrumented hammer hits several specific locations to input vibration that covers all noise frequencies.

The latest gambit is using so-called acoustical holography or acoustical interferometry. Here the troublesome part is attached to a mechanical shaking device programmed to vibrate the part at frequencies that trigger the objectionable noises. A laser scans the part repeatedly to create a detailed three-dimensional map of its surface when it is making the objectionable sounds and when it isn't. Microscopic bulges and indentations between the two images are then used to pinpoint problem areas.

Both companies are becoming adept at using this technology to solve problems that otherwise would require costly and time-consuming trial-and-error methods using physical prototypes. DuPont boasts that it solved a serious noise problem for Porsche in just seven days.