Despite all the science that is applied to engineering vehicle systems, the realm of automotive exhaust remains an area that lacks true methodology - not so much how it works but breaking down the sound that resonates from the tailpipe.

Anyone with a keen ear can identify sounds they like and dislike, but quantifying what exactly creates those sounds and how they can be controlled and tweaked is something more elusive.

So automotive exhaust suppliers, encouraged by their OEM customers, have been compiling vast databases to include every exhaust sound imaginable, from a purring Lexus to a roaring Viper to the diesel thump of a Super Duty pickup.

When complete exhaust suppliers such as Arvin Industries and Tenneco Automotive will be able to analyze the sounds much more precisely and attach digital values to them, which would finally bring objectivity to a process that for years has relied on the subjective ears of engineers.

Ultimately, suppliers will be able to create whatever sound the customer wants for a particular vehicle.

Tenneco, for instance, is moving ahead with its "active muffler," a concept that has been in the works for several years and was recently applied to a prototype Plymouth Prowler, says Robert Parmann, Tenneco's vice president of engineering-exhaust.

The standard passive muffler was replaced with a "noise cancellation" system, as well as speakers (generally one per tailpipe) to create a more sporty sound. In addition, the system reduces exhaust back-pressure, which improves engine performance and boosts horsepower.

Although the prototype package has not seen production, it did earn Tenneco a Technology Role Model Award from DaimlerChrysler Corp. last year. In addition, it opened the door for Tenneco to develop a similar system for a higher-volume DaimlerChrysler platform. That system is now in the works.

Tenneco also is developing an aftermarket package for customized exhaust sound, perhaps as early as '01, Mr. Parmann says. It's too early to discuss price, but this stuff won't be cheap initially.

To motorheads, exhaust sound is everything, whether in cars or motorcycles. Harley-Davidson Motor Co. has been trying in recent years to obtain a government trademark for the famous low rumble of its motorcycles at idle, partly because competitors were working hard to copy it. (Honestly, you could record someone repeating the word "potato" and not be far off.)

Like Tenneco, Arvin is serious about quantifying sound, so serious that it tapped the music industry for an expert with a sensitive ear and educational background to apply true science to analyzing sound quality.

Arvin hired Kurt Zwanzig, who had made a name for himself in the recording business. He had spent hundreds of hours in studios with musicians (no, he didn't "discover" anyone big like the Goo Goo Dolls) and teaching professional recording techniques to students. In his last job, Mr. Zwanzig was living in Medina, OH, and designing and building recording studios.

He wasn't looking for a new job two years ago when a headhunter called and said Arvin wanted an expert who could help the company better define the sounds created by its exhaust systems and then share that information with Arvin's engineers.

Arvin had a joint research program with the U.S. Air Force Research Laboratory at Wright-Patterson AFB in Ohio (see WAW, Feb. '98, p. 60) to create a computer program for quantifying sound quality, but that project has since been discontinued. Mr. Zwanzig is continuing some of that research.

What attracted Arvin to the 32-year-old Illinois native were both his experience and educational credentials. He has a physics degree, with an emphasis in acoustics, from Northern Illinois University.

He has a lot of questions to answer in his job. "Are customers seriously interested in sound quality, and how far will they go to make it a priority? It will cost you to improve it," Mr. Zwanzig says. "Are the sounds they want even possible?"

For years, the mission of exhaust suppliers was primarily to eliminate bad sounds. But today, cars and light trucks are much quieter than they used to be, largely due to improved sound insulation. Some luxury cars are so quiet it's hard to tell they're even running. So the sounds that do enter the cabin have to be further refined.

"We got rid of the bad sound, and now we want to make what's left more pleasant," Mr. Zwanzig says. "If you put the pedal down, you want to hear the engine rev. If you hear and feel it, you have a great hand-shaking going on. If you don't hear it, you might not like it if you're in a muscle car. It's just like a musical instrument in a band."

In a luxury car, he estimates that 10% of interior sound comes from the exhaust system, while in a muscle car it's more like 30% to 40%.