Introduction

Our Project Car, the Corvette C5, is equipped with GM's premiere sound system. Its performance is well above your average car. However, it doesn't begin to compare to the quality aftermarket systems that are available.

Things to Remember

Here are a few things to keep in mind when comparing car audio and home audio:

• Quality: Just like in home theater, you get what you pay for. Don't expect a set of $30 speakers to sound good.

• Off Axis Response: 99% of the time in the car environment, you are sitting off axis (unless you drive sitting in the center of the front seat!), usually 30⁰ - 45⁰. This is something to consider. Off axis really affects material over 3 kHz, a range that humans are particularly sensitive to. We will measure and listen to all car audio speakers both on and off axis. The high end manufacturers take this off axis into consideration when designing their equipment.

• Small Room Effect: The vehicle is in essence a very small room. Because of this, the nodes and room gain (lets call it car gain) are much higher in the frequency band than at home. This also means that the car gain is very different depending on vehicles. A Lincoln Navigator has a much different gain than a Honda Civic.

• Bass Response: There are many factors to clean bass in the car. In the home, room resonance is typically very low, whereas it's much higher in a car. Due to the the air volume restrictions of a car, that air volume plays a heavier roll in response and loading. Remember that when in the car, you are actually sitting inside a speaker enclosure.

• Imaging: This may be the hardest thing in Car audio. Where do you sit in relation to the front stage when listening to music? How about 5.1? The answer is equidistant from the speakers. This is where technology comes to play.

• Road Noise: No matter what make, or how expensive the model, you always have to consider road noise. The C5 exhibits 93 dB at 65mph (87 dB at idle), with the top up and the windows up. There are ways to combat road noise, and we will definitely have to address it.

The C5 Corvette

Head-Unit:

The head-unit in the C5 is a Delco CD player with changer control. It has large buttons and plain styling. The Bose Speaker System here is very odd, as there is a proprietary connection from the head-unit to each speaker. At each speaker there is a small amplifier. This makes upgrades of the rest of the system almost impossible without replacing the head-unit, which for our cause is just fine.

The head-unit supports CD, CD-R, and has changer control for the factory changer. Note that the changer doesn't fully support CD-R. One feature that is nice is the SCV (Speed Compensated Volume). SCV raises the volume as you drive faster, so that the music appears to have a constant level. Very handy for convertibles.

Amplifiers:

Due to the proprietary connection, there is really no way to bench test the small Bose amps. There is a single amp in each door powering the 8" mid-bass. The 3" driver is powered from the deck. The Bose amp appears to be a class D design. We were able to get about 18 watts out of one of the amps, measured by voltmeter. However, 18 watts probably isn't the max, as the measurement was taken during the "best THD at 500hz" test.

Speakers:

Each door houses an 8" mid-bass and 3" full range (!) driver. The 8" driver uses an inverted magnet structure to reduce depth. The lack of a tweeter is very apparent in the system, as the 5 kHz range is very undefined. The 8" driver goes into heavy distortion on any signal lower than 40 Hz at any meaningful volume. Overall, the system severely lacks punch. With no tweeter, and running at 15-18 watts per channel, the sound isn't crisp, and it lacks detail. The mid-bass is muddy, and vocals are harsh.

There is a low SPL limit to the factory system. The frequency response was taken at 100 dB. Trying to push past that SPL generated large amounts of ringing in the THD tests. When you subtract the road noise level from 100 dB you get a good idea of your system's dynamic range. Now, you can increase that by going louder, or by making the car quieter. Since I value my hearing, we are taking the second approach.

Next Page - The Bench Tests

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