# Bass Trap Myth



## Dspencer

While I was planning my Lexus IS350 audio system upgrade, I came across several comments on different forums that stated their IS trunk was a "bass trap." I've heard this previously about other cars as well.

There is no such thing as a car trunk being a "bass trap." 

If you are not getting loud bass in the cabin, it's because you have cancellation from standing waves, not because the subwoofer is in the trunk.

This article is helpful in understanding how this works: http://www.teamrocs.com/technical/pages/aiming.htm 

My vehicle has a metal firewall behind the rear seats, and the luxury leather seats use thick foam padding. They are fixed and do not fold down - the vehicle has a small ski pass through door in the case something long (and narrow enough to fit through) needs put in the trunk for transportation - such as skis.

Yet, with the subwoofer I recently installed properly placed, the bass in the cabin is very loud - if I don't turn it down, too loud (loud enough to drown out front component speakers with 135 wrms per side).

Placement is key in vehicles, whether trunk, hatchback, or cargo area (van/suv).

If you think your vehicle is a bass trap, try a different subwoofer position. If you have it firing towards the front of the vehicle, just behind the rear seats, and it is not IB or sealed off behind the sub box, you are going to get cancellation, and the volume of bass in the cabin will be reduced.

You can test this by opening the trunk. The volume of bass in the cabin will increase. Close the trunk and bass volume in cabin again decreases. 

This is because with the trunk open, the bass traveling rearward is not being reflected back towards the cabin, but with it closed it is. 

The reflected bass is out of phase with the forward traveling bass from the woofer cone. Thus, cancellation. 

Move the subwoofer to the back of the trunk/hatch/cargo area. Even firing forward you will notice an increase in cabin bass loudness. Turn the subwoofer around firing towards the rear of the vehicle while close to the back of the trunk/hatch/cargo area, and you will notice a further increase in cabin loudness.

It may seem counter-intuitive to have the subwoofer facing the back of the car - you may be thinking, "I want more bass in the cabin, which is forward, so the subwoofer should fire toward the front of the vehicle."

Though that may be how you think of it, you are getting less bass response in the cabin that way. Turn the subwoofer around, and have it positioned close to the back of the trunk, hatch, or rear cargo door, and enjoy improved bass response in cabin. 

If any vehicle on the road could be a "bass trap," the Lexus IS250/350 would be (rear metal firewall, fixed rear seats, comes from the factory very well noise proofed (such a quiet ride!), and panels on the vehicle have tight tolerances - not much air gonna get through). Yet with the proper subwoofer placement in it, the cabin gain makes the bass too loud for music enjoyment - gotta turn the sub down so I can hear the music! lol

Thankfully, I learned this way back in late high school years when I was running those cheater amps and getting over 142db out of 2x 12" subs on soundstream amp. I took the time to learn how bass works to get the most out of my amp/sub combo. A friend back then had some rockford fosgate subs and amp installed by some store, and came over for a sound off. They placed his 2 12" subs rear facing, but pushed back all the way behind the seats, rather than close to the rear of his trunk. 

It was no context. Same track was played, same number of subs and rated power output on his RF amp vs my Soundstream, and the loudness in cabin was something like 10db higher in mine. He was so disappointed - he felt like he wasted his money on the RF equipment. I explained, it's not your equipment, they installed your subs in the WRONG PLACE!! He moved them, and sure enough, almost as loud as my system.

It's the same reason those in the know about home theater set up place the subwoofer in a corner of the room for wall loading - it applies a gain to the bass in the room.


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## SQLnovice

Great info. How close to the hatch should the sub be when facing the rear? I'm building a sub box and will take this into consideration. Tks in advance


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## Dspencer

SQLnovice said:


> Great info. How close to the hatch should the sub be when facing the rear? I'm building a sub box and will take this into consideration. Tks in advance


I've read that putting it too close to the back of trunk or a wall can cause issues with air pressure on the front woofer surface - too much air pressuring thus fighting the subwoofer, making it harder work on the subwoofer.

I personally haven't found this to be true in real life at any reasonable distance I've placed subwoofers.

I've placed mine so the woofer cone is 3 inches from the back of the trunk, 6 inches, and about 1 foot, and not much noticeable difference in the cabin at sub bass frequencies. The waves are long enough, that within 1 ft from the rear of the trunk seems sufficient to make sure the direct bass and reflected bass are close enough in phase to reinforce one another, rather than cancel out.

At 3 inches from cone to rear of trunk structure, I've not noticed any reduced sub loudness or clarity, as if the pressure created by sound wave reflection is fighting the subwoofer motor force. It may be happening but my ears can't tell the difference.

This is probably because even if the woofer cone is only 3 inches from the back of the trunk, there is plenty of space above and to the sides for air pressure to escape quickly, so it does not build up at the cone's surface.

Obviously, don't put it so close that if the subwoofer is performing at maximum cone excursion, the cone will hit the back of the trunk - but most subwoofers are not going to have more than 1" maximum one way excursion. So, anything more than an inch away will not come into contact with the cone at max excursion.

Though I haven't tested it scientifically, I leave at least 3 inches from cone to back of trunk to let the subwoofer "breath." It's more of a precaution, as I have played subs within 1 inch and it still seemed to perform just fine - again, no doubt because the space above and to the sides of the enclosure are open for air pressure to escape.


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## Dspencer

There is a downside to rear firing subs close to back of the trunk - trunk rattle.

Though this position gives the maximum in cabin sub bass response, it also means the rear trunk panels and trunk lid are taking a real pounding! 

Make sure to properly/thoroughly dampen the trunk panels, especially rear of trunk and trunk lid. Some vehicles may need something done about the rear tail lights and housing as well, with powerful sub bass firing so close. 

Thankfully, with proper panel deadening my Lexus doesn't have trunk rattles even with the sub close to trunk rear, and the tail light assembly is so tightly built and installed they don't rattle either (and I did nothing to them). This probably won't be true on many vehicles, though. Not all are built like a Lexus.

I hate trunk rattle so much, that if I had a vehicle that even deadening panels didn't help get rid of it, I'd go with an IB set up (sub firing into cabin behind seats, and sealed off mounting wall and rear deck), OR sub facing rear but pushed back to rear seats with a sealed false wall between the sub enclosure and rest of trunk. 

Those designs cut down drastically on trunk rattles AND on sub bass cancellation in the cabin. But, they are more complicated and harder to properly implement - and WILL require vehicle alteration.

I was unwilling to do any major vehicle alterations for my upgrade, as I may want to sell the Lexus later, and Lexus buyers will expect it to be in stock condition. So, i deadened the hell out of trunk panels and did the sub location rear firing close to back of trunk. You can now pound the trunk lid with fist from the outside, and no rattles whatsoever - and at full sub volume I can see it flexing like a SOB, but no rattle.  OH! and don't forget about the license plate - I used weather stripping behind it so when it's flapping with the bass no metal on metal - quiet as a church mouse...


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## SQLnovice

Thanks, I'll take these into consideration.


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## syc0path

Dspencer said:


> I leave at least 3 inches from cone to back of trunk to let the subwoofer "breath."


The rule of thumb that I've heard is 1/2 the diameter of the sub -- so the cone of a 12" sub should be about 6" away from the trunk lid. I haven't tested this scientifically either, but it probably doesn't make an audible difference. Maybe it would make a difference at an SPL competition if u'r trying to squeeze out an extra .1dB.

The other issues w/ mounting the subs close to the trunk lid are more practical. It reduces usable space in the trunk, it's harder to get items in and out, and it may block access to the spare tire. It could even affect the car's handling if the subs and enclosure are heavy enough.

For these reasons (as well as the fact that the box is ezr to securely mount in place), my sub box is positioned between the strut towers, against the back of the rear seat, w/ the subs firing toward the trunk lid.


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## Infinity

This is why downfiring and loading panels are effective. You don't see much of either in car audio though.


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## Dspencer

syc0path said:


> The rule of thumb that I've heard is 1/2 the diameter of the sub -- so the cone of a 12" sub should be about 6" away from the trunk lid. I haven't tested this scientifically either, but it probably doesn't make an audible difference. Maybe it would make a difference at an SPL competition if u'r trying to squeeze out an extra .1dB.
> 
> The other issues w/ mounting the subs close to the trunk lid are more practical. It reduces usable space in the trunk, it's harder to get items in and out, and it may block access to the spare tire. It could even affect the car's handling if the subs and enclosure are heavy enough.
> 
> For these reasons (as well as the fact that the box is ezr to securely mount in place), my sub box is positioned between the strut towers, against the back of the rear seat, w/ the subs firing toward the trunk lid.


I've tested it out, and there is no audible difference in sound even with a 12" subwoofer 3" from the year of the trunk - thankfully too, as I've had no other choice before but to put it that close. 

There is enough open airspace above and to the sides in such a scenario to allow the air pressure to dissipate quickly, so there is no forced air back onto the woofer surface. 

In fact, you can get away with less than 3", such as in the case with many home theater downward firing subs. Those home theater setups often only have like 1.5-2" clearance from the floor, and sound great.


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## BuickGN

For what it's worth I've had the same experience with it facing forward near the seat with the trunk sealed off vs open. When I first faced it forward there was a huge loss in bass, especially sub bass. After sealing it similar to IB style it sounded a lot better, lots more bass but not as bloated as it sounded facing backwards.


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## T3mpest

This is NOT true in my experience. I had a 99 cadillac eldorado with a thick back seat as well as a layer of what appeared to be ensolite behind the rear seat. Long story short I cut out behind where my arm rest comes down and removed that leather. I also pulled out the ensolite that was behind that from there. Just doing that was a drastic audible change in my bass. It was louder to the ear and more noticable than that, their was much more air movement. You could feel your clothes moving more and things of that nature. I had 2 15's in that car, I don't have termlab to verify, but it was easily louder and everyone who got in my car and heard it after that noticed.. "wtf did you do it's way louder now"!

That and luxury cars are known for putting up bleh scores on the termlab from the SPL crowd.


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## amalmer71

T3mpest said:


> This is NOT true in my experience. I had a 99 cadillac eldorado with a thick back seat as well as a layer of what appeared to be ensolite behind the rear seat. Long story short I cut out behind where my arm rest comes down and removed that leather. I also pulled out the ensolite that was behind that from there. Just doing that was a drastic audible change in my bass. It was louder to the ear and more noticable than that, their was much more air movement. You could feel your clothes moving more and things of that nature. I had 2 15's in that car, I don't have termlab to verify, but it was easily louder and everyone who got in my car and heard it after that noticed.. "wtf did you do it's way louder now"!
> 
> That and luxury cars are known for putting up bleh scores on the termlab from the SPL crowd.


Thank you. The preposterous thinking that bass can't get trapped in a trunk was starting to get to me. 

Let's use a little logic.

Stand behind the car with the trunk closed and the bass cranked, then open the trunk lid. Which way sounds louder while standing outside the car? With the trunk open, right? Why? Because the bass is trapped inside the car with the trunk lid down.

Same thing happens with a lot of cars, including mine. I have a '12 Civic with a fold down rear seat. There is a huge difference in the amount of bass that enters the car with the rear seat up and the rear seat down. Why? Because with the solid back seat up, the sound waves have a wall they have to get through. 

AAMOF, they technically don't make it through. They end up vibrating the rear seat, which then creates sound waves in the cabin. I.E., the energy is transferred from the air to the back seat then back into the air again.

The energy passing through the seat into the cabin is called sound transmission. The downfall is, not all of the energy makes it through. Some of the energy is trapped in the trunk. The energy that doesn't make it through and stays in the trunk is called sound reflection.

There's no way around this except for me to lower the rear seat and let the sound straight into the cabin.


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## therapture

Having the two stock 8" holes open in my rear deck made an amazing difference in sub output and quality. The rear seat, even with the ski pass open, blocks far too much sound energy.


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## Rs roms

amalmer71 said:


> Thank you. The preposterous thinking that bass can't get trapped in a trunk was starting to get to me.
> 
> Let's use a little logic.
> 
> *Stand behind the car with the trunk closed and the bass cranked, then open the trunk lid. Which way sounds louder while standing outside the car? With the trunk open, right?* Why? Because the bass is trapped inside the car with the trunk lid down.
> 
> Same thing happens with a lot of cars, including mine. I have a '12 Civic with a fold down rear seat. There is a huge difference in the amount of bass that enters the car with the rear seat up and the rear seat down. Why? Because with the solid back seat up, the sound waves have a wall they have to get through.
> 
> AAMOF, they technically don't make it through. They end up vibrating the rear seat, which then creates sound waves in the cabin. I.E., the energy is transferred from the air to the back seat then back into the air again.
> 
> The energy passing through the seat into the cabin is called sound transmission. The downfall is, not all of the energy makes it through. Some of the energy is trapped in the trunk. The energy that doesn't make it through and stays in the trunk is called sound reflection.
> 
> There's no way around this except for me to lower the rear seat and let the sound straight into the cabin.


I have exactly the opposite experience, when i open the trunk lid, the bass fades away, outside and inside the car


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## Mitsushi

I don't know about bass traps in a car but here in my studio, i'm mixing songs a lot easier since i got 2 bass traps. It really helps when you have a square room to eliminate standing waves. It tights up the bass a lot!


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## Weigel21

Rs roms said:


> I have exactly the opposite experience, when i open the trunk lid, the bass fades away, outside and inside the car


Ditto, as there's now no surface for the subwoofer(s) to properly load off of.


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## OGJordan

While there are MULTIPLE incorrect statements in the first post, I'll just point out one. The ABSOLUTE WORST place to have a home audio subwoofer is in a corner. Home Theater In A Box guys may place them there so they have more "boom" but if you want accurate bass it is terrible.


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## gstokes

Infinity said:


> This is why downfiring and loading panels are effective. You don't see much of either in car audio though.


Not all subs are compatible with down firing and gravity does put increased stress on the suspension..


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## gstokes

OGJordan said:


> While there are MULTIPLE incorrect statements in the first post, I'll just point out one. The ABSOLUTE WORST place to have a home audio subwoofer is in a corner. Home Theater In A Box guys may place them there so they have more "boom" but if you want accurate bass it is terrible.


A large carpet remnant hanging on the wall extending in both directions from the corner minimizes reflections and keeps all the "booms" in phase


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## gstokes

Dspencer said:


> I've read that putting it too close to the back of trunk or a wall can cause issues with air pressure on the front woofer surface - too much air pressuring thus fighting the subwoofer, making it harder work on the subwoofer.
> 
> I personally haven't found this to be true in real life at any reasonable distance I've placed subwoofers.
> 
> I've placed mine so the woofer cone is 3 inches from the back of the trunk, 6 inches, and about 1 foot, and not much noticeable difference in the cabin at sub bass frequencies. The waves are long enough, that within 1 ft from the rear of the trunk seems sufficient to make sure the direct bass and reflected bass are close enough in phase to reinforce one another, rather than cancel out.
> 
> At 3 inches from cone to rear of trunk structure, I've not noticed any reduced sub loudness or clarity, as if the pressure created by sound wave reflection is fighting the subwoofer motor force. It may be happening but my ears can't tell the difference.
> 
> This is probably because even if the woofer cone is only 3 inches from the back of the trunk, there is plenty of space above and to the sides for air pressure to escape quickly, so it does not build up at the cone's surface.
> 
> Obviously, don't put it so close that if the subwoofer is performing at maximum cone excursion, the cone will hit the back of the trunk - but most subwoofers are not going to have more than 1" maximum one way excursion. So, anything more than an inch away will not come into contact with the cone at max excursion.
> 
> Though I haven't tested it scientifically, I leave at least 3 inches from cone to back of trunk to let the subwoofer "breath." It's more of a precaution, as I have played subs within 1 inch and it still seemed to perform just fine - again, no doubt because the space above and to the sides of the enclosure are open for air pressure to escape.


the average subwoofer enclosure is 36" wide and 15" tall so even with the enclosure 1" from a flat wall there is still 66 sq inches of air space for the sub to breathe (36+15+15=66), that's why you don't notice a difference..


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## subterFUSE

I had a 94 Acura Legend Coupe with 3 x 10W6 sealed, down firing in the trunk. Seats did not fold but there was a ski pass thru.

That car had the worst bass cancellation I ever experienced. With the trunk lid open, the low end extension was phenomenal. The bass would vibrate your body to the core. It was awesome.

Close the trunk and instantly the bass would die. It was like the subs were turned off. I was still a teenager at the time and didn't know much about cancellation so we never really tried anything to correct it.


Sent from my iPhone using Tapatalk


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## subterFUSE

Rs roms said:


> I have exactly the opposite experience, when i open the trunk lid, the bass fades away, outside and inside the car



That just means you have a peak in the response at the LP instead of a null. So the peak goes away when the trunk is open. It's the same phenomenon just observed from a different location.


Sent from my iPhone using Tapatalk


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## gstokes

Okay, so i tested the theory and it works exceptionally well..

My van is full size Ford E150 with bed in very back that is 4ft deep and extends to both sides, my subwoofer cabinet is 3.2 cu ft with 1.5lbs of polyfill/side and it was facing forward just under the front edge of the bed and sounded okay, i had plenty of bass..

Turned the cabinet around and slid it all the way back facing the back doors which are nice and flat and provide the optimal reflection, the cabinet is 1 to 2" from the doors and OMFG did it make the most incredible difference, it feels and sounds twice as loud and shakes my entire seat and me with it..

Myth Confirmed: The subwoofer cabinet sounds best when facing backwards and up against the wall..

To add to this conversation, I'm convinced that a flat wall like the back doors of a van are the ultimate setup, in a car you got all these curves and stuff in the trunk to reflect sound in 714 different directions..

Because of the fantastic results I obtained this gets my vote for legendary thread


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## gstokes

If you do turn the box around backwards and place it up against a wall or in my case rear doors prepare to do a boatload of damping, the bass being reflected off the doors is enough to flex the outer body panels, actually had to move the enclosure forward to lessen the effect because reducing the bass is not an option..


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## YukonXL04

This holds true in just about every vehicle I have ever had. Question though, can you recreate this by placing mdf in front of the box? Does it have to go floor to ceiling like a wall? Or can you place it just in front of the sub and ports? I have a yukon xl, and if I have my box against the 2nd row facing the rear I loose a bunch of output and frequency response... if I slide it to the rear doors then it's awesome... but I was wondering if I stuck a piece of mdf the width of the cargo area, to the top of the box if it would recreate the effect and allow me to put the box against the back seat again. Don't want to lose 4ft of cargo area by having the box against the rear doors lol


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## cajunner

YukonXL04 said:


> This holds true in just about every vehicle I have ever had. Question though, can you recreate this by placing mdf in front of the box? Does it have to go floor to ceiling like a wall? Or can you place it just in front of the sub and ports? I have a yukon xl, and if I have my box against the 2nd row facing the rear I loose a bunch of output and frequency response... if I slide it to the rear doors then it's awesome... but I was wondering if I stuck a piece of mdf the width of the cargo area, to the top of the box if it would recreate the effect and allow me to put the box against the back seat again. Don't want to lose 4ft of cargo area by having the box against the rear doors lol


not really?

the effect of having bass "in phase" by loading the box at the rear of the vehicle, is not approximated by a panel that way.

but don't fret.

make a bandpass box with a long vent, that replaces the floor to the vehicle.

make the exit at the rear of the vehicle, and the subs fitting inside the box vent to the rear via slot porting.

a platform appx. 3" high across the whole bed is a lot of airspace, so you wouldn't need anything more than the size of the subs for volume displacement, and you have all the deck space back save for the appx. 13" rectangle at the seatback where the 3 12" woofers are enclosed...


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## gstokes

YukonXL04 said:


> .. but I was wondering if I stuck a piece of mdf the width of the cargo area, to the top of the box if it would recreate the effect and allow me to put the box against the back seat again.


It needs to be strong, like maybe 1" minimum and i wouldn't use MDF, plywood or appearance board, something very stiff..

I would build the wall about 6" taller than the enclosure and put a 90 degree ledge at the top facing forward with a 45 in the corner to redirect the sound forward before it hits the roof..

Kinda like the corners inside a sub enclosure where they use 45's to smooth out the corners and reduce turbulence..


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## jdsoldger

I have tried every spot I can in my 2011 Honda Fit (except front seat) and I still have a nasty cancellation at 70ish hz...

Any thoughts on that? I tried everything from putting it in the back seat to jammed up against the hatch, both facing forward and backward....


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## cajunner

jdsoldger said:


> I have tried every spot I can in my 2011 Honda Fit (except front seat) and I still have a nasty cancellation at 70ish hz...
> 
> Any thoughts on that? I tried everything from putting it in the back seat to jammed up against the hatch, both facing forward and backward....


that be a listenin' position problem?

as a suggestion, not sure if it's going to help at all, or make it worse...

the tuning of body panels using dampener, especially the roof, could alter that null you've got by redistributing some of the resonant energy like a bass trap, which is also a possible plan, if you don't mind customizing the headliner's parts.


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## FelipeGoncalves

I have my 2 fosgates firing the cabin, right behind the back seat. I was experiencing a bass cancellation with the trunk closed. The problem is that cant place the box in other location. I was thinking about sound dampening all the trunk, in attempt to absorb the back wave of the box, to avoid cancellation with the front wave Will work?


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