Originally Posted by
mattkim1337
I'll contribute, for the people that will actually benefit from and attempt to apply this information. Two seat tuning is a balance of compromise.
1) Delay should be used to align groups of speakers that are on the same side. Example: Left midbass 50in, Left midrange 40in, Left Tweeter 30in. You still need to align them with each other to arrive at the same time. This is most easily done by entering the distances to a point in between the headrests. However, you do not use delays to align the left and right side. Doing so will create a biased listening experience in only one seat and ruin the opposite seat. Aligning the sub to midbass using delay is still necessary as well.
2) All pass filters are used to compensate for the destructive interference caused by the path length differences of speakers between the left and right. Example: Path length difference of left to right side is 20 inches. There will be a frequency where half the wavelength will be 20 inches. Half a wavelength = 180 degrees out of phase = Complete cancellation. That is the frequency where you apply a 2nd order all pass filter to just one side. What you've effective done is correct the phase cancellation in both seats at the same time, since the path length difference is 20 inches from both seats. This has a similar effect as time alignment (not quite as exact) Which side you apply it to, and what the Q should be, I can't answer accurately at the moment.
3) The mic should be placed at the drivers seat, then moved to the passenger seat. The two measurements should be then averaged. Huge difference compared to placing the mic in the point between the headrests. Matching the left and right side's frequency response from both seats is impossible, as one change will skew the opposite listening position. The main objective of this step is overall level matching, problem area cutting, and global tonality adjustment. You and your passenger aren't going to listen with your heads smashed together in the middle of the car while you're driving. You want to mic to pick up the unique acoustical behavior at each headrest (reflections off the side window, etc). Luckily, the center channel will have almost identical response from either seat because the reflections are identical.
4) You can use the Helix Virtual Channels to remove Mono (Center) information from the left and right virtual outputs. If you don't attenuate it, you will likely hear the mono information coming from the speaker closest to you because it's too loud (improper imaging). You want to center channel to dominate the center info and pull it to the center of the car. All you have to do is sum the Left channel with Center flipped in polarity (Maybe 80% left and -20% center), with real center FX turned on). This will remove some center information from the side channels.