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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > AUDIO/VIDEO + BLUETOOTH + Electronics/Alarm/Software > cutting slot in rear deck



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      06-14-2009, 12:04 PM   #1
xxxmuzk
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cutting slot in rear deck

hello everyone,

i have an 2008 e90 that i just added 2 jl audio 10w3v3 with and alpine pdx 600w amp. i notice that the bass is really muffled pretty well by the rear seat, and i don't have the ski hole pass through, but i have a carseat installed, so that wouldn't help anyway. so, i was looking at ways to "vent" bass into the trunk somehow, and i noticed there is like a slot in the metal right under the rear deck in between the 2 rear speakers. i felt my hand up in there and i felt the top of the rear deck. so, does anyone know if i cut a rectangular slot in the rear deck, would that make a significant difference in bass output coming from the trunk? has anyone cut the rear deck to allow bass to come out of the trunk? or is this more scientific than cutting a slot, similar to tuning a port when you have a ported sub box, and you would have to determine port length and width to "tune" it to a certain frequency? thanks.
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      06-15-2009, 08:21 AM   #2
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It sounds logic enough that a port would allow the bass to enter the cabin, as it is non-directional. If it was treble it would be a different story. I haven't encountered anybody that has done it, but I guess you could buy a replacement parcel shelf from the scrap yard and experiment with it! Good luck and let us know how it works out!
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      06-15-2009, 08:36 AM   #3
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I was thinking about doing this exact same thing. My idea was to poke a bunch of small holes in the material, then to put a plastic grill over it, like the grills over the rear speakers. If you do it, let us know how it works. The only other issue is whether heat or other road noise goes into the passenger compartment.
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      06-15-2009, 12:00 PM   #4
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ya, i would like to try it, but i'm too much of a chickensh*t to start cutting stuff up, so i was hoping that someone on this forum had already blazed the trail. but maybe i'll ask a few local shops i trust and see what they say, and i'll post my findings here. thx
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      06-15-2009, 08:43 PM   #5
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Quote:
Originally Posted by xxxmuzk View Post
hello everyone,

i have an 2008 e90 that i just added 2 jl audio 10w3v3 with and alpine pdx 600w amp. i notice that the bass is really muffled pretty well by the rear seat, and i don't have the ski hole pass through, but i have a carseat installed, so that wouldn't help anyway. so, i was looking at ways to "vent" bass into the trunk somehow, and i noticed there is like a slot in the metal right under the rear deck in between the 2 rear speakers. i felt my hand up in there and i felt the top of the rear deck. so, does anyone know if i cut a rectangular slot in the rear deck, would that make a significant difference in bass output coming from the trunk? has anyone cut the rear deck to allow bass to come out of the trunk? or is this more scientific than cutting a slot, similar to tuning a port when you have a ported sub box, and you would have to determine port length and width to "tune" it to a certain frequency? thanks.
In my E46 325i, I removed the rear deck speakers as I wanted a better front soundstage. That gave me 2- 6.5" holes. The sub sounded much better with the holes open so that some of the sub waves could hit the cabin. It wasn't perfect, but it helped a lot. Also, I had excellent 6.5" mid bass driver (Audiobahn), so I set the sub cutoff around 70-80hz.
The midbass drivers were good at handling low bass without having to go to reproduce below 80-90hz. I kept the door tweets for rear fill for when company was on board, powered by the stock amp.

I was using 1-12" Infinity Perfect sub in a box, driven by 425watts RMS from a Rockford 851X Power series amp. The front comps got 110watts RMS each. Good to very good overall, but not "very good" to "excellent" as the sub needed all that power to do what it did.
If I had a split rear seat it would have been MUCH better. In my previous 2000 Mits Eclipse setup I used Rockford "fanatic X" 6.5" comps and a 10" sub running about 325watts RMS and that system would "HIT" hard. It was a MUCH better overall system.
I don't know if Rockford still makes those comps.

Also, try moving the sub box around in your trunk and switch it facing forward and rearward, experiment to get the best sound you like. Don't forget to try the 180 degree out of phase setting too. If it's on your sub amp try the switch, if not, then simply switch the + and - at the sub connection.

Oh, and don't try to tune the sub placement with the trunk open, it won't sound right
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      06-16-2009, 01:36 AM   #6
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I wouldn't make holes to not diminish the value of your car. Bass is not destroyed. The volume is just reduced.
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      06-16-2009, 07:10 AM   #7
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The key here is correct placement of the box to take advantage of the car's transfer function. Another thing you should consider is amping the underseat woofers with mid bass frequencies and sending only the low frequencies to the trunk. The lower the frequency, the less directional the sound and the easier it will penetrate into the cabin.

The results will be 10x better than cutting an ugly slot in the rear deck or removing the rear speakers.
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      06-16-2009, 11:29 AM   #8
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thanks everyone for your input. i have a friend who has a similar setup to mine, where he added an amp and subbox to the factory bmw stereo, but the difference is he has the ski hole pass through in the back seat, and man, when he pumps the bass with the hole open, the bass in my opinion sounds better, like more powerful and not muffled like mine. but that's exactly the problem, i have a car seat, so i can't do that. if i did cut a slot in the rear deck, i would cover it with a speaker grille painted to match the top so it would look like a rear "center" speaker... hahah. but i'm still irked about the muffled sound... i thought of removing the back speakers so the air could pass through their locations but i like having sound back there...
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      06-17-2009, 09:33 AM   #9
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I took the rear speakers out on my car (no ski pass through either) to try to let more of the bass into the cabin. It doesn't help much IMO. I have just left them out as I only want front stage.
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      05-02-2011, 08:04 PM   #10
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Quote:
Originally Posted by xxxmuzk View Post
thanks everyone for your input. i have a friend who has a similar setup to mine, where he added an amp and subbox to the factory bmw stereo, but the difference is he has the ski hole pass through in the back seat, and man, when he pumps the bass with the hole open, the bass in my opinion sounds better, like more powerful and not muffled like mine. but that's exactly the problem, i have a car seat, so i can't do that. if i did cut a slot in the rear deck, i would cover it with a speaker grille painted to match the top so it would look like a rear "center" speaker... hahah. but i'm still irked about the muffled sound... i thought of removing the back speakers so the air could pass through their locations but i like having sound back there...
Did you ever go thru with it?
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      05-02-2011, 08:17 PM   #11
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Did you ever go thru with it?
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You should PM the person as I doubt he will see this.
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