Both of these curves should be centered on the X(mm) 0 line and have essentially the same shape on either side of this point. The dashed lines on the graphs are the mirrors of the actual test and make it easy to see if the curves match the ideal.
The Bl (X) (Graph 1) curve for the woofer almost perfect, with a shift of about 0.7mm. The Cms (X) (Graph 2) curve shows the same bias, in the same direction. This typically means the suspension is forcing the coil out of position. It's almost impossible to get these two curves perfectly centered and these are about as good as you can get.
The Bl (X) and Cms (X) curves can also be used to predict the maximum excursion for the speaker (Xmax). Klippel defines it as the point at which the compliance or Bl nonlinearities create 10 percent harmonic or intermodulation distortion. This happens when the Bl has dropped to 82 percent of its x=0 value (XBl) or Cms has dropped to 75 percent (Xc). For the MB Quart woofer, the XBl point is 2.7mm and the Xc point is 3.6mm. With an active crossover and subwoofer, these are good numbers.
Frequency Response
This is where the story starts to get really interesting...I started by measuring the speakers without crossover to see what the exotic materials were bringing to the table. Graph 3 shows the woofer and tweeter response on-axis. Not bad, but the tweeter has some non-linearities-certainly not the worst I have seen. If the crossover is done well and the off-axis response is good, this should be a very respectable system.
Graph 4 shows the system on-axis and individual responses together. The sensitivity of the tweeter is a little hotter, but the overall response is quite good. With a simple adjustment of the tweeter level, the system response is flat with a total deviation of about +/- 3 dB (Graph 5).
Graph 6 shows the three tweeter attenuation settings available by changing the + lead of the tweeter at the crossover. The setting includes a +3, 0 and -3dB.
I test component sets on-axis and both vertical and horizontal off-axis to see if the crossover design is flexible enough to work in the wide variety of car audio installation positions. Graph 7 shows the response 30 degrees off-axis with the tweeter mounted above the woofer. There is a small dip at 15kHz but the midband is perfect! Graph 8 shows the tweeter mounted next to the woofer (horizontal). This the harder of the two off-axis positions because of the additional time delay difference between components. Note that both tweeter + and - phase are shown. The - phase response (smoother red line) is the correct connection correcting the time delay. The off-axis testing shows that the QSC 216 has a well-designed and versatile crossover that is well suited to a wide range of car audio applications
I also test harmonic distortion at 1/2 rated RMS power to get some idea of where the limits are in the system. Graph 9 shows very low distortion levels in the throughout the important mid and upper band. The increasing distortion at the low frequencies is a result of the testing method.
Graph 10 shows a system impedance that is smooth and easy on any amplifier.
So what is my final opinion based on the tests?
On-Axis Response
8
Off-Axis Vertical
8.5
Off-Axis Horizontal Mount
9
Flexibility (above 3 averaged)
8.5
XBL
9
XC
9
Distortion
9
Parameters
8
I would say that MB Quart is back in the "high-end" car audio business. These are well priced, versatile and a good choice for anyone. My guess is that they sound as good as they look and test. -PT
For the subjective portion of this test report, read the December 2005 issue.
Graph 9 | 
Graph 10 | |