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View Full Version : cancellation? trunk acting as bandpass?



Eddie Bloom
09-08-2013, 06:18 AM
i'll keep it simple...

96 impala ss, with a single JBL GTI 12 ported off of about 300 watts. sounds great, hits low, almost perfect. pop the trunk, +6-12DB's of gain...

everyone says its some cancellation issue, well no $%&*. anyone have an idea how to fix it?

yes, there is a port thru on the rear deck lid, yes we have played with phasing and eq. the sub is pushed towards the rear deck and side firing, changing positions made little to no difference.
the difference in output is dramatic. easily 12 DB of gain with trunk open. so please don't tell me what's wrong, we know what's wrong, tell me how to fix it.

someone suggested to me that the trunk itself is acting like a bandpass enclosure but still even if that were the case then it should give me the boost with trunk closed, not open as that would mimmic an enclosure that wasn't sealed properly or had leaks...

thanks!!

maylar
09-08-2013, 09:06 AM
Trunk is acting as a vented enclosure when it's open.

NASTY08IMPALA
09-08-2013, 10:17 AM
i'll keep it simple...

96 impala ss, with a single JBL GTI 12 ported off of about 300 watts. sounds great, hits low, almost perfect. pop the trunk, +6-12DB's of gain...

everyone says its some cancellation issue, well no $%&*. anyone have an idea how to fix it?

yes, there is a port thru on the rear deck lid, yes we have played with phasing and eq. the sub is pushed towards the rear deck and side firing, changing positions made little to no difference.
the difference in output is dramatic. easily 12 DB of gain with trunk open. so please don't tell me what's wrong, we know what's wrong, tell me how to fix it.

someone suggested to me that the trunk itself is acting like a bandpass enclosure but still even if that were the case then it should give me the boost with trunk closed, not open as that would mimmic an enclosure that wasn't sealed properly or had leaks...

thanks!!

I have always loaded off the trunk ..meaning subs and port back with great results ...i recently joined the craze of trying side port ..resulted in about half the output ..very disappointed ....so i turned the box sideways with the port facing back and boom problem solved

gckless
09-08-2013, 10:26 AM
Is the only opening into your cabin through the rear deck? Or do you have ski pass or the seats fold down? I found in my last car, which only had a ski pass, that the ski pass made the entire trunk behave like a 6th order, and acted as a bandpass. That was with the box facing back. Didn't have much output below about 33Hz IIRC. Turned the box around and pushed it as close to the seats as I could and lost output, but gained those lower notes. Same thing might be happening here.

phemps
09-08-2013, 10:27 AM
Incident wave and reflected wave are causing cancelation cause you have waves entering back into the trunk and returning to the cabin out of phase. When you open the trunk the out of phase wave thats entering back into the trunk is being released and your cancelation disappears. Having the sub and port load close to the trunk in thjs case would keep the waves close and stop cancelations

T3mpest
09-08-2013, 11:06 AM
One thing to realize is that the trunk doesn't have walls thick enough to act as a true bandpass at any bass frequency. It can absorb higher upper frequencies, 100hz an higher, using the rear seats to absorb along with small holes out of the trunk, but that's the extent of the bandpass effect. If your louder with the trunk open it is a cancellation issue. Changing the subwoofer orientation will fix it. Try firing the sub directly into the trunk lid, I would pay good money that says that you won't be louder with the trunk open then. Although you will have some rattles. Other option is to keep facing the woofer into the car, but fully seal it off so that the wave can't travel behind the box to make it to the trunk lid.

Beatin'
09-08-2013, 12:09 PM
Why the FFFF would you fire the port through the rear deck when the sub is playing inside of the trunk? I had to read multiple time to ensure you weren't talking about a 4th/6th order.

maylar
09-10-2013, 04:24 PM
It's not about cancellation, it's about air flow. Easy to prove - close the trunk slowly while someone listens inside. If it's reflected waves coming back to cancel, then you should hear the drop in volume as the trunk lid comes down. You won't. Not until the trunk is nearly fully closed and sealed. Or cover the trunk lid in some sound absorbing material like CCF. That should diffuse the reflected waves and at least change the difference open/closed. It won't.

Side Show
09-11-2013, 12:27 AM
take out rear seats