Firing subs into cabin vs. trunk
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BlownRiv Unregistered guest | i've got a sub in a sealed box (mounted in the trunk) facing forward - firing through the armrest hole and into the cabin of my car. the sub sounds really good, but when i pop the trunk lid, even a little bit, the bass will increase in volume level. i haven't metered it, but i'm guessing i may be getting a 6dB gain when the trunk is open. i've noticed this phenomena in previous installs, and i've also noticed that firing the sub toward the rear seems the give me the same kind of effect. my car is a large sedan with very well sealed doors and trunk. the car is virtually airtight. does anyone think they know what's going on here? i'm happy with the sound, i just want to know the physics behind this. why does this happen? is it wave cancellation? it's funny, when i want "megabass", i just pop my trunk (the lid stays down, but it cracks open enough to vent the trunk). but i'm half inclined to cut holes in the trunk floor and use actuators to operate doors that vent to the outside air, like mark fukuda used to do back in the day. thanks to anyone who can shed some light on the subject. |
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Silver Member Username: InsearchofbassPost Number: 881 Registered: Jun-04 | my personal opinion is that this is wave cancellation...i have noticed the same on other installs i have done my buddies have as well |
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Gold Member Username: GlasswolfNorthWest, Michigan USA Post Number: 4687 Registered: Dec-03 | not SWR or cancellation. this effect is, as you noted, due to the car being well sealed. in effect what's happening is your enclosure and car's cabin are acting to make the box function like a 4th order bandpass enclosure, with the cabin acting as a large sealed enclosure. when you open a window or pop the trunk, you're effectively changing the enclosure volume (of the cabin) and thus altering the response of the enclosure and the sub. the way to fix this is to vent the trunk to the outside in some manner, eliminating the sealed cabin you're experiencing. this is a common problem by the way. |
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BlownRiv Unregistered guest | thanks for the info. your explanation makes sense, but if my car interior is functioning as a bandpass filter as you say, why would it matter which way the sub is aimed? i've experimented in the past with box placement, and i've found that when the sub us firing toward the rear, it is always louder than when firing forward. what does the directional aiming of a subwoofer have to do with its response? i've built bandpass boxes in the past, and i wasn't aware that the orientation of the driver inside the box had an effect on the overall response of the system. it's still a bandpass, no matter which way the sub is pointing, right? also, i forgot to mention that in my case, opening the windows and sunroof does not boost the levels as does opening the trunk lid. i just don't get it - what is it about venting the trunk cavity that makes it different from venting up front? i feel like we're definately on to something here with the bandpass idea. i'm sure that's part of it, but there's got to be another factor involved that we're overlooking. |
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BlownRiv Unregistered guest | Jonathan, do you have any knowledge on this subject? |
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Silver Member Username: JohnnylemoineLaPlace, Louisiana United States Post Number: 198 Registered: Aug-04 | I got a slot ported box,2.5cu.ft., in an ext. cab chevy and when I role down my windows it hit 10 times as hard as when the windows are roled up. It did this even with my sealed box. |
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Silver Member Username: FishyTamarac, FL USA Post Number: 590 Registered: Sep-04 | Try placing your sub "near" the rear of the trunk firing backwards. Open the trunk and see if you gain as much. Sean posted a link in a thread to an article about cancellation which explained this very phenomenon. The article basically stated that the farther the sub is from a reflective surface the more the upper bass frequencies are lost due to out of phase reflections. Opening the trunk basicallly lets some of these bad guys "out". Article made sense to me anyways. Try it with your box firing a foot or two away and into the back of your trunk. If it don't help I'd go with Glass's bandpass theory.  -Fishy |
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Silver Member Username: FishyTamarac, FL USA Post Number: 591 Registered: Sep-04 | It would seem that the mere "cracking" of the trunk wouldn't reduce reflections that much so I dunno. hmmmm...... -Fishy |
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Bronze Member Username: DeathshadowPost Number: 51 Registered: Oct-04 | Same thing happens to me too, windows make no dif but if I pop the trunk and close it w/o locking it in place it is much louder... |
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Gold Member Username: Jonathan_fGA USA Post Number: 2404 Registered: May-04 | Read this, it'll explain your problem. It mainly has to do with acoustic phase. http://www.teamrocs.com/technical/pages/aiming.htm In your case, though, Glass is right about your car being very well sealed, you'll see a lot of benefit by running a port of some sort outside the vehicle. |
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BlownRiv Unregistered guest | thanks, jon. i had suspected wave cancellation. and that article made a lot of sense. thanks again! |
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BlownRiv Unregistered guest | was just thinking... instead of venting the trunk, couldn't i seal the trunk volume off from the cabin space by using a wall/baffle? if the sub fired into the cabin only, no sound could get into the trunk. this would prevent any cancellation, correct? |
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