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Concert hall in a Cadillac

Marlborough, MA--It was a Bose engineer's dream: to have a carmaker ask for a world-class audio system and then be willing to alter the car's structure and materials to help make the system the best it could be.

The dream came true. "Cadillac came to us with their desire to have a vehicle that exceeded the best in the world and gave us free reign to design the audio system first," says Finn Arnold, chief engineer for the Bose OEM Div. (Framingham, MA).

For other car music systems, Bose engineers faced the natural constraints of car design and economy: Carmakers aren't usually willing or able to move a window mechanism, for example, or change a structural member to improve the performance of a music system.

But for the Seville, Cadillac went to great lengths to fit the vehicle around the Bose 4.0 audio system. Result: even music distribution throughout the whole cabin and extended low-frequency performance, which delivers a level of bass that is rarely experienced in any stereo system.

The first step was to make the interior of the car as quiet as possible so ambient noise wouldn't interfere with the audio system. Second, engineers had to figure out optimal speaker placement and how to mount the speakers without compromising noise performance. Third, Bose developed such leading-edge technologies as Automatic Volume Control (AVC), which makes the volume seem constant regardless of wind or traffic noise, opening and closing windows, or pounding rain.

"It was a difficult high-wire act to have both excellent noise performance and a superior sound system," says Scott Reilly, noise integration engineer for Cadillac's luxury car division.

Trials and tribulations. The system comprises eight speakers, which essentially behave as two-way sound ports. The loudspeaker in the rear package shelf is 12 inches in diameter. "So we started off by telling Cadillac we wanted to put a 12-inch-diameter hole through the rear package shelf of the car," says Arnold. The problem is that the assembly would allow transmission of noise, such as wind and tire rumblings, from the trunk into the passenger compartment.

Typically, some kind of box sits behind each speaker to block the noise that exists inside the door cavities and the trunk. But that would disturb and hamper the performance of the Bose speaker system. Instead, Cadillac decided to design the interior of the trunk and door cavities to be nearly as quiet as the passenger compartment itself.

The noise-blocking strategy: dampening, absorption, and barriers. "First," says Reilly, "we used a damping material so that the sheet-metal panels that make up the structure are dampened when they try to vibrate. Then elastic material changes the vibration energy into heat and kills some of the oscillating behavior." Cadillac also treated the trim with a cotton fiber or "shoddy," and in some areas of the car used 3M's Thinsulate material to save mass.

The hole for the 12-inch speaker also caused another problem: package-shelf vibration. So they had to change the materials and the rigidity of the shelf to have it be stiff enough to handle the speaker's output. The change added cost and mass.

"We were unhappy about the 12-inch hole in the trunk," says Reilly. "That was a big deal. But to tell you the truth, the door speakers were even worse for us because they're so close to the tires." Reilly was especially worried about tire noise--specifically a "sizzle" that happens at 800 to 1,000 Hz.

The solution: a pad of Thinsulate material that goes into the door behind the speaker and helps port the noise behind the area of the door speaker.

Fine-tuning. "Speaker placement was just the beginning for us," says Arnold. "We had to make sure we could create the kinds of sound fields in the car that would enable us to give thrilling performance." So Bose set about tailoring their system to the Seville's interior.

Because most sounds reflect thoroughly from interior surfaces, different musical frequencies reproduced within a car instantly pile up into zones of relatively high pressure (louder) and low pressure (softer). Other obstacles to superior sound are acoustic shadows caused by seats and other interior features, absorption by upholstery, and losses induced by less-than-ideal speaker locations.

Bose's active equalization technology returns the reproduced sound in the car to its original musical balance by compensating electronically for the unavoidable acoustic impact of the car's interior. Digital signal processing algorithms resulting from years of research make this possible.

Speed and sound. Ever-changing ambient noise can impact the character of music; quiet passages can be masked and lyrics can become muffled and unintelligible.

"The first-cut solution," says Arnold, "is called speed-compensated volume, where the system turns the level up as a function of how fast you're driving." But if you had the system quite loud in the first place and the outside noise wasn't really affecting you, the sound would still increase--for seemingly no reason. The other problem is that noise quantity isn't purely based on speed.

Bose wanted the system to always sound the same--no matter what's happening during driving.

To determine the noise level in the car, Bose uses the headliner-mounted cell phone microphone. Digital signal processing separates the music from the noise. Using this data, the system's volume and frequency response automatically adjust several times per second--fast enough to restore the quietest passages without affecting the overall perceived volume.

Can't stop the music. This project was truly a labor of love for Bose. One night four engineers were working late fine-tuning the Seville for a public demonstration. They worked in a garage bay that has a 10-ft projection video screen in front that they use as a "drive-in" computer display. When controlling the signal processing from inside the car, the screen lets everyone see the results.

They'd been working until 2 am, and had finally hit a level of performance so perfect that--instead of going home--the four sat in the car, switched the display's input to videodisc, and watched and listened to Pink Floyd's The Wall.

Bose says the 4.0 audio system sounds as good as or even better than a home-based system because it was tailored for the Seville environment by some of the best in the business. The home equivalent, says Arnold, would be having Steven Speilberg help set up your home theater system.

What this means to you

- Working closely with suppliers and learning their concerns can result in a superior end product

- A secondary feature, such as a sound system in a car, can make a product a stand-out

- Digital signal processing does better in one chip what it takes many analog components to do

- Thinsulate--a woven fiber with metallic strands--is not only a great insulator for heat, but also for sound

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