October
5, 1998 Design News
ENGINEERING NEWS
Concert hall in a Cadillac
A 'clean-sheet' collaboration
between Bose acousticians and Cadillac engineers created
a quiet Seville with a top-notch audio system
by Julie Anne Schofield, Senior Editor
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
Cherokee transmission encompasses materials
potpourri
Auburn Hills, MI--As the trend toward lighter-weight
more energy-efficient vehicle designs continues, materials
will play an increasingly critical role. The new power
train for the 1999 Jeep Grand Cherokee is a good example.
Chrysler's newest sport utility vehicle (SUV) is larger
than its earlier models, but gets 8% more mpg on average,
while reducing emissions some 30%. Behind these noteworthy
statistics lies the Cherokee's new 4.7l V8 engine's
automatic 5-speed 45RFE fully electronic automatic transmission
and transfer case. In addition to the lower fuel and
emissions figures, the power train weighs 50 lb less
than its predecessor, according to Mike Kirk, Jeep power
train project manager.
How did this come about? Kirk reports that the results
stem from the use of a potpourri of advanced materials.
They include: aluminum cylinder heads and an all-aluminum
front cover; powder-forged connecting rods; intake manifold
made of 35% glass-filled nylon 66 molded with the lost-core
process; magnesium cam covers for less weight, more
strength, and less space compared to aluminum designs;
and an "industry first" compacted graphite-iron
bedplate for more robustness during foundry and machining
operations.
Chrysler believes that the $2.6 billion spent on the
Grand Cherokee design is worth it. Why? Because predicted
annual demand for SUVs will grow to 3.3 million by 2005.
That's up from 2.4 million in 1997 and an estimated
2.7 million for the 1999 model year.
CFD cuts windshield nozzle design time
Columbia, MD--Bowles Fluidics Corp.'s windshield
washer nozzles are complex fluidic devices that rely
on flow instabilities to oscillate the jet. The most
critical design elements are the fan angle and cold
weather performance required to meet automobile manufacturers'
specifications for cleaning performance. In the past,
meeting those requirements involved a lengthy trial-and-error
process plus two days to build each prototype. Now Bowles
uses computational fluid dynamics (CFD) to evaluate
nozzle designs in a fraction of the time previously
needed.
Initially, Bowles engineers analyzed an existing washer
nozzle to determine the accuracy of CFD simulations.
The results achieved with 2- and 3-D turbulent models
matched those attained
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