# Mounting sub under trunk floor



## Bash12 (Oct 26, 2016)

I'm looking at mounting my sub under the trunk floor, however I carry a lot of equipment in the boot most of the time and also have a rubber mat to protect the carpet. Just wondering in how this would affect the sound coming from the sub and whether it's feasible or whether I should mount the sub in a different position?


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## rockinridgeline (Feb 2, 2009)

the output from your subwoofer has to get into the vehicle somehow. Having a rubber mat over it is not going to work.


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## Bash12 (Oct 26, 2016)

Thanks for confirming my suspicions, just wanted to double check as I spoke to a shop who said it should be OK! Just to confirm, the sub would be mounted in the spare wheel well, then a couple inch air gap, trunk carpet with rubber protector on top of that.


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## bugsplat (Nov 7, 2014)

You may run into vibration issues if you hard mount it to a structure. That bass is going to transfer through your frame and shake lord knows what. Dash vibrations suck to find and fix. I've never had success with hard mounts if thats your intent. I put a couple layers of carpet or rubber between the box and car frame. Id also not seal the sub up under the floor. Its going to restricted the pressure waves a lot.


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## Bash12 (Oct 26, 2016)

So basically I'll be a lot better off building a custom Fibreglass box that'll fit in the side of the boot and fitting a decent Grille to stop anything damaging the sub?


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## bugsplat (Nov 7, 2014)

Bash12 said:


> So basically I'll be a lot better off building a custom Fibreglass box that'll fit in the side of the boot and fitting a decent Grille to stop anything damaging the sub?


I would. Some guys like to replace their spare tire with a sub box. Not my taste but not going to knock it. If you do secure a fiberglass box to your car use rubber to help keep down the mechanical vibrations. Rubber grommets like these will help a lot.


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## Lanson (Jan 9, 2007)

Bash12 said:


> I'm looking at mounting my sub under the trunk floor, however I carry a lot of equipment in the boot most of the time and also have a rubber mat to protect the carpet. Just wondering in how this would affect the sound coming from the sub and whether it's feasible or whether I should mount the sub in a different position?



You were getting misinformation from some folks here. Bass will pass right through rubber or carpet or whatever you have. 

Allow me to give an example, this is from my Focus ST build









I built the grille height to match the false floor height exactly. Over this, I run the factory floor carpet









And there is no way any of that will affect or stop the sub's output. 



HOWEVER, there are times when the sub can affect other things around it. Example, my Durango build, I have a hinged top cover and the subs directly underneath rattle that pretty hard, so I have to flip the hinged panel open so the bass vibration isn't audible in the cabin. I'll be rebuilding that with a downfire setup and no hinge, in the future. 

Point is, your plan, provided you let the sub breathe a little bit like I have shown, will not be a problem.

edit: I feel like I should back up my claims. Here's a build I did with a monster SI sub,

















That panel in the middle has the sub underneath, and under that carpet is this flushed-in heavy steel grille. The sub is flushed into the box underneath that, and a 3/4" gap exists. 



















If there were ever a sub that would shake the car apart (minus the heavy deadening work we did), this would be it.



I do mostly false-floor builds so I can safely tell you, as long as you give the sub a medium that bass can travel right through (carpet, light rubber, whatever), and the originating wave doesn't excite and shake something and rattle it to pieces.


further edit: Bugsplat was mentioning isolating the sub box. I agree with this. But this is a separate issue with false floors. I use deadening and foam to isolate the box from the vehicle to keep the bass from exciting damn near everything. But as far as getting the bass to propagate through the cabin, you'll have no issues


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## rockinridgeline (Feb 2, 2009)

Sorry fourthmeal, but I think you missed something really important. He said that he would be covering the subwoofer with a rubber floormat. Your examples are showing subs firing through carpet covered grilles. Of course, your examples will work perfectly. But there is a big difference if you cover your carpeted panels with a sheet of rubber. Whatever your experience is, the plain physics of it say that the response in doing so will be very difficult to predict. It is irresponsible to point a guy in the direction of doing something that costs money and time that might not work out well.


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## SHAGGS (Apr 24, 2011)

Forthmeal is correct. The rubber floormat will not impede the low frequency sound waves.

Simplicity in Sound has done more false floor/spare-well builds, than I can shake a stick at, and a lot of them retain the factory/aftermarket rubber mat.
This applies to the OEM carpet mats with rubber backs, as well as the WeatherTech, full plastic style.

And +1 on the rubber grommets, for decoupling the enclosure from the mounting points.


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## diy.phil (May 23, 2011)

Bash12, please help to identify what vehicle this is.... is it a sedan, hatchback/coupe, stationwagon or SUV? Yeah that makes a difference. A sedan is already difficult with the subwoofer pointing into the trunk space as it's confined. Placing it under the floor will probably make it worse. However if it's hatchback, wagon or SUV, just go for it as fourthmeal and shaggs mentioned... it'll be just fine as we're sitting inside the same air space as the subwoofer.


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## Lanson (Jan 9, 2007)

rockinridgeline said:


> Sorry fourthmeal, but I think you missed something really important. He said that he would be covering the subwoofer with a rubber floormat. Your examples are showing subs firing through carpet covered grilles. Of course, your examples will work perfectly. But there is a big difference if you cover your carpeted panels with a sheet of rubber. Whatever your experience is, the plain physics of it say that the response in doing so will be very difficult to predict. It is irresponsible to point a guy in the direction of doing something that costs money and time that might not work out well.


Have faith that I know what I'm talking about here. The rubber floor mat will not affect the bass much, because it is not dense enough and thick enough to impede that bass energy. A rubber mat will do a great job blocking midrange, road noise, and higher frequencies. But unless that mat is LEAD-lined and/or very, very thick (like in inches) you've nothing to worry about. Just don't "suffocate" the sub, it needs some space above it to breathe.

Another advantage of false floor builds, they usually have less audible cancellation nodes, compared to rear-firing and usually front firing setups have certain frequencies that null-out usually, and are very hard to fix. This is due to the sub-frequencies reflecting off, say, the trunk back end and another part of the wave making it slightly before that one because it was omni-directional from the sub itself. Creates comb-filtering, highly annoying. A false floor sub is usually right back in the very rear of the cabin, and much like placing a home theater subwoofer in the corner to have the largest room gain with the fewest nulls, the false floor setup has the most to gain, typically.

With a false floor sub, I just try to have the front stage take over early (like 80hz cross or lower) because too high and the back end will resonate with that higher frequency energy. You start getting into frequencies that excite the panel(s) above the sub and it can get noisy. You can break out the measurement mic and do sweeps to understand what parts of the car resonate at what frequency, and then add deadening/blocking materials to try and reduce that somewhat. 

I really prefer to build in false floor setups when I have the opportunity. I've done it many times and each one works out great. You can even cut in steel or aluminum bar stock slots with a table saw, if you need a "Home Depot" solution. The big bar grille I showed in my first pic has been my favorite way to do it though. Just have to match the height of the surrounding floor exactly, so there's no excessive weight on the grille itself. The rest is easy.


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## Bash12 (Oct 26, 2016)

Thanks for all the advice and help on this one. Just to confirm it's a hatchback it'll be going into. 

I've decided to give it a go and have begun the build this afternoon. I'm going to go with some Mdf to line the base of the wheel well then make a Fibreglass box/casing for the sub to sit in. I'll make sure the false floor then sits so that the sub has room to breathe. How much gap should I ideally leave between the cone of the sub and the false floor?


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## Lanson (Jan 9, 2007)

Bash12 said:


> Thanks for all the advice and help on this one. Just to confirm it's a hatchback it'll be going into.
> 
> I've decided to give it a go and have begun the build this afternoon. I'm going to go with some Mdf to line the base of the wheel well then make a Fibreglass box/casing for the sub to sit in. I'll make sure the false floor then sits so that the sub has room to breathe. How much gap should I ideally leave between the cone of the sub and the false floor?


I would say at least 1" from the highest point of the sub, but more would be better if you have it. Again, consider the grille I showed you and how you can level it out with the rest of the false floor, those grilles are cheap (usually called subwoofer bar grilles) and they give excellent clearance for subwoofer excursion. 

If you don't need a ton of airspace, you can do this without fiberglass, btw.

Example:



















7.5 degrees is the answer to how that works. Put your saw at 7.5 degrees per cut, and you can make this real quick.


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## Jayhovah (Jul 3, 2013)

Resurrecting this thread just in time for Christmas.

The general consensus in this thread is that there will be little effect from leaving the OEM carpet over the sub... what about a port?

I am planning an 8" ported enclosure to fit in a dead spot in my hatch floor (which is currently occupied by a styrofoam plug put in at the factory). Both the sub and the port will fire upwards into the OEM carpet mat. Any issues here?


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## Lanson (Jan 9, 2007)

Jayhovah said:


> Resurrecting this thread just in time for Christmas.
> 
> The general consensus in this thread is that there will be little effect from leaving the OEM carpet over the sub... what about a port?
> 
> I am planning an 8" ported enclosure to fit in a dead spot in my hatch floor (which is currently occupied by a styrofoam plug put in at the factory). Both the sub and the port will fire upwards into the OEM carpet mat. Any issues here?


Thinking yes, port efficiency will be slashed, because restrictive media will be affecting the port. I was able to add grille cloth for a port, and that worked









But at higher volumes, you could hear that grille cloth moving around a bit. Of course at those volumes, the output from the sub and port was loud as hell and the grille cloth noise was something you had to get close to hear. Tuned to an extremely low frequency (yes that's the same AE sub as I had in my Focus...this sub gets around.)

For your situation, what if you cut in a sort of grille? Like maybe a flush metal drain type grille? Could be interesting. 

lol wtf not


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## Jayhovah (Jul 3, 2013)

fourthmeal said:


> Thinking yes, port efficiency will be slashed, because restrictive media will be affecting the port. I was able to add grille cloth for a port, and that worked
> *-snip-*
> 
> But at higher volumes, you could hear that grille cloth moving around a bit. Of course at those volumes, the output from the sub and port was loud as hell and the grille cloth noise was something you had to get close to hear. Tuned to an extremely low frequency (yes that's the same AE sub as I had in my Focus...this sub gets around.)
> ...


Thanks for the reply! That makes sense since the port will need to breath a bunch more.

I am going for a fully stealth/OEM looking install so I would like to avoid installing a toilet directly into my hatch floor lol =)

I am thinking, if I can space the grill/carpet 1" (or maybe more) off of the port and allow free airflow out the sides of the face of the enclosure, maybe that might help? Might be difficult to visualize, but the grill would just be there to support the carpet to make sure nothing is in direct contact with the driver and port opening. Make sense?


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## Lanson (Jan 9, 2007)

Jayhovah said:


> Thanks for the reply! That makes sense since the port will need to breath a bunch more.
> 
> I am going for a fully stealth/OEM looking install so I would like to avoid installing a toilet directly into my hatch floor lol =)
> 
> I am thinking, if I can space the grill/carpet 1" (or maybe more) off of the port and allow free airflow out the sides of the face of the enclosure, maybe that might help? Might be difficult to visualize, but the grill would just be there to support the carpet to make sure nothing is in direct contact with the driver and port opening. Make sense?


Darn near everything effects port tune, like boundaries materials. So it will be a game of exactly what. IMO, I'd go sealed and take advantage of the cabin gain in the vehicle. But it depends on the sub I suppose.


edit: A passive radiator would probably not "care" about these things, but ports tend to.


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## Jayhovah (Jul 3, 2013)

fourthmeal said:


> Darn near everything effects port tune, like boundaries materials. So it will be a game of exactly what. IMO, I'd go sealed and take advantage of the cabin gain in the vehicle. But it depends on the sub I suppose.
> 
> 
> edit: A passive radiator would probably not "care" about these things, but ports tend to.


Thanks! I was hoping to go ported just because I have never done it, and to shave a little weight off the deal...but it sounds like I should just add another sub and call it a day.... bummer I will need to sell the model I have and get two new ones to have the proper voice coil configuration... but weight shouldn't be too bad. I am planning to run 8" Alpine Type R's and they weight less than 5#/ea

edit: While I am here - any recommendations for small volume 8" subs that provide a good bang/buck?


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## Lanson (Jan 9, 2007)

Jayhovah said:


> Thanks! I was hoping to go ported just because I have never done it, and to shave a little weight off the deal...but it sounds like I should just add another sub and call it a day.... bummer I will need to sell the model I have and get two new ones to have the proper voice coil configuration... but weight shouldn't be too bad. I am planning to run 8" Alpine Type R's and they weight less than 5#/ea
> 
> edit: While I am here - any recommendations for small volume 8" subs that provide a good bang/buck?


PR wouldn't need another sub. Just a PR. It would be fun to model.


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## Jayhovah (Jul 3, 2013)

fourthmeal said:


> PR wouldn't need another sub. Just a PR. It would be fun to model.


lighter than a second sub too. Brownie points for you!! I will have to do some research since I have no idea how to set something like this up. Thanks so much for your input!


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