2 - The Bench Tests
About the Measurements: A calibrated microphone was set up at the driver's head level in the driver's seat, pointing towards the dashboard. During each test, the volume was adjusted to 100 dB, then the measurements were allowed to stabilize. Since we are evaluating a stock system, there was no reason to test on-axis response. Due to the proprietary Bose speaker system, it wasn't possible to bench test the electronics separately from the vehicle.
The system scored 2.18% THD at 100 dB, and the best we could achieve (at low volume) was 0.45% THD. Consider that this "best" THD was achieved at a volume lower than the road noise level. Not so good. Now consider that this is the premium sound option, so was it worth it? Maybe it would be wise to forget the premium sound system when you order your car, and put a custom system in later.
As you can see from the graphs, there was significant ringing at 100 dB. Odd and even harmonics were very pronounced. It wasn't very nice to listen to. We are striving for a much lower THD. Again, the lack of any real power to the speakers is a major suspect in these high distortion readings. Audiophile speakers are often under 0.2% THD. The lack of tweeters was telling, as the system couldn't pass the 100 dB test for 5 kHz at the listening position. The most we could get without THD going crazy was 98 dB.
With an input of 1 kHz and 1.5 kHz, the IMD was 1.57%. IMD tests sound bad, but the ringing this system produced gave me a headache. The stock systems just aren't made for quality playback. With better electronics and speakers, I'm sure we can make this number go way, way down.
RTA 0 Hz - 200 Hz
RTA 200 Hz - 20 kHz
Frequency response was measured with pink noise at 100 dB, averaging the results over a one minute period. It is a simple RTA rather than quasi-anechoic response, because the main sitting position is not movable, and speaker placement is pretty much unchangeable as well.
This is where things got really interesting. You can clearly see a frequency dip under 45 Hz. 8" drivers (especially underpowered) aren't cracked up for sub-bass. It's down 30 dB at 25 Hz and down 20 dB at 30 Hz. There is a significant dip right at 400 Hz (-10 dB). At around 1 kHz, the system is losing some information, and also at 1.5 kHz. Overall, the frequency response was very poor, and really shaky from 200 Hz to 2 kHz. This system could sure use some attention.