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Discussion Starter · #1 ·
none has looked at this in the car audio area, that place is the abyss. I think some of you might find this an interesting question and maybe I'll get a couple good answers up here in DIY...hope it doesn't bother anyone! hey, thats what mods are for...


Im just playin around still waiting for money. no sawdust, no pics, no vids here...


so I build a van with a few very large drivers walled off in a very large vented box. Say 40-60 cubic feet with a huge shared 14" round port. Responce in open air is obviousely different than in-house which is different than in-car. But see, here's the thing. I know a car flexes. I know it has leaks. I know the SPL guys break out layers upon layers of deadener, cans and cans of expanding foam, concrete... welders... steel tube and sheetmetal...MDF, fiberglass, resin.... it goes on and on. 2x4's..layers of 2x4's and MDF.


Lets say I get a van, gut it out, fill every single space I can with foam. No airleaks anywhere. Hit everything with a few layers of resin. It's not as good as it could be, but, it's good. What if I only build a wall... no enclosure behind it. Behind it is sealed. It's pretty solid. I know you hate it. But take a sec and get over it. So it is an "IB" wall even though it is NOT an "IB" wall. I have a port in it anyway...


I now have 50-80 cubic feet behind the wall, it is pretty lossless...not perfect. Well dude... the front 1/4 of the van is only 1/4 the airspace. The port goes between both. The drivers between both. It's more like a bandpass box with NO ports to the outside... just a single port connecting the 2 chambers together. Which volume of air is being "tuned"?


reason I was thinking about it..yet again... was with that method correct airspace is hard to determine to figure port length for tuning so I was thinking of measuring the impedance curve to find the saddle... well, both sides of the cabin will affect tuning!


like this!

 

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You are correct that both sides will affect the tuning. The joys of car audio. In a vented box the tuning is at the minimum point between peaks on the impedance curve. You'll likely see 3 peaks and 2 minimums. One thing I think may work is if you could get the correct volume and tuning so that the smaller chamber ends up being tuned to a harmonic of the larger chamber. That could likely get you some extra reinforcement.


John
 

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Discussion Starter · #3 ·
so basically build it all first, leave the port at it's longest length, test imp curve... which dip would be tuning..both? In that case, if I want a low tuning, go with the lower of the two? If too low, cut the port down, re-test, repeat?
 

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The tuning of the rear chamber versus front chamber has little to no effect if you're sitting in the front chamber. Response there will be dominated by cabin gain and boundary effects.


The tuning of the front and rear chamber does have an effect if your listening position is outside the vehicle. In most cases the result is a very high Q system, and that's why passing cars will have a boomy 'one-note' sound that seldom reflects what's heard inside the car.
 

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Discussion Starter · #5 ·
I thought that was Doppler Effect and because no cabin gain, the low lows are rolled off and the highs just go and go
 

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Discussion Starter · #6 ·
I guess the bigger question is which chamber(s) will affect excursion due to tuning. Will there be 2 tunings, and will excursion will be lessened at both of them?
 
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