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      09-22-2012, 07:07 PM   #1
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Drives: '08 335i
Join Date: Jul 2008
Location: JAX

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Getting the trunk sub to play nice with the underseats... (Scooby Doo Thread)

For those with trunk subs, you may want to try something very easy to improve the performance of the subwoofer and the midbass drivers, as well as improve the imaging of the subwoofer.

First, a little background:

The midbass drivers (underseats), as well as the trunk sub, play the same frequencies at the crossover point. So if you have them crossed over at 80Hz, they both play that tone. If the sub and midbass are the same distance from your head, then the sound waves will arrive "in phase", which is good because the sound becomes louder, and your brain cannot distinguish between the two sources, making the sound appear to come from one location.

Unfortunately it's not a perfect world, and in our cars the subwoofers are farther away from our heads than the midbass drivers. Therefore, the sound waves are often offset to each other, which creates a "phase shift". In the worst case, the shift is 180 degrees, which results in massive cancellation of the two waves. This drastically reduces the sound pressure level at the crossover frequency, and also enables our brain to locate the sub. When this happens, you can hear the sub play in the trunk, which is very annoying to say the least.

So what can we do to re-align the two waves (and make Scooby happy again)? - There are several ways to do this, but the easiest, cheapest, and fastest way takes literally seconds. All you need to do is to reverse the speaker wires going to your sub. So (+) goes to (-) and vice versa. What this does it flips the sub's sound wave and it brings it back in synch with the wave from the midbass drivers.

Try it, it may very well work! In case you cannot hear the difference between the two, there is an easy way to measure it:
  • Download an "SPL" measurement app for your smartphone (JL Audio Tools is a good one, and free). UPDATE: JL Audio Tools does not properly measure tones under 100Hz. I have tested the app SPLnFFT, and it works great at 60Hz and up.
  • Download a "test tone" from the internet, at the exact same frequency as your crossover setting.
  • Position the phone in the head position (you can just wedge it between the seat and the headrest, with the mic pointing forward)
  • Disconnect your subwoofer (so only the midbass drivers are playing) and play the tone through your MP3 player (you are sitting in the passenger seat). Increase the HU volume until you get around 70 - 80dB (it doesn't have to be exact). Record the max. dB reading.
  • Now do the opposite and disconnect the midbass (so only the sub is playing). Don't change the HU volume. Adjust the sub amp gain until you get the same reading as before.
  • Reconnect the midbass, remeasure, and record the max. dB reading
  • Now reverse the sub's speaker wires and remeasure

Whichever connection gives you the higher reading is the one with better phase alignment! In my car, it was clear:
  • SPL with sub in phase: 72dB
  • SPL with sub out of phase: 87dB
Mobridge DA2 > JBL MS-8 > ARC XDi 600/4 > JBL Gti 408 Mids & Vifa OT19 Tweeters; PG Ti2 1600.5 > Morel HCW-10 & IDMax 15

Last edited by kaigoss69; 07-28-2014 at 02:59 PM.