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oh boy, where do i start.
first, lets talk about cancellation. when a sub moves in/out of the box, it produces sound waves. the number of waves emmited from the speaker per second is the frequency. most subs will play frequecies of importance of around 160hz and down dependant on the slope and the cut-off point for the low pass on the crossover (this is assuming a 12db/octave at 80hz for those who care). in other words, at 40hz, the sub is transducing one wave function 40 times per second. thats a lot of waves!
now lets take about interference. when a driver is producing 40 (relativly large) waves every second, where do they go? if you are standing in an anechoic (non-echoing) chamber with the speaker elivated 6 feet off the ground, and you are standing a few feet back from the driver (facing it directly), you will hear the waves amplitude peaks/dips all in order, just as you should. your eardrum will be altered by positive pressure at the peak of amplitude (when the speaker moves "outward"), and then a negative pressure when the wave dips to its negative peak (when the speaker moves "inward"). in a car, you arent afforded this...you have surfaces that both muffle and reflect the waves as they leave the speaker. that can send the waves boucing all around the cabin of the vehical. how they interact with each other from that point is called interference. see, these waves can lay on top of each other and align in two different extreems, with an infinate amount of variation between the two.
deconstrictive interference: remember how i said there is a cycle: compression followed by rarefaction (as the sub moves in and out to creat one whole wave)? it is possible that one waves rarefaction cycle can overlap another waves compression cycle. this causes BOTH waves to nullify each other. its like introducing compressed air into a vaccum. no wave=no sound=no pressure (negative OR positive).
constructive interference: it is also possible that two waves can align themselves in a way that both positive peaks line up, as do both negative dips. this form of wave combination results in a stronger wave.
why the hell i just said all this: when youve got a box in the trunk, facing backwards, the waves are reflecting from a relativly close surface, and are all in such close proximity that the overall effect is an increase in constructive interference. this is called "slot loading" or "corner loading" in home audio jargon. when a box is sitting in the backseat, the waves are bouncing/modifying all over the place (off the windshield, off the rear glass, the floor, the seat, the headliner, you, etc..). because it is so random, the likelyhood of deconstructive interference is huge. if you want to put a box in the backseat, it is EXTREMELY important that you block off the front face of the box from the trunk (with a wall, as you suggested). this does 2 things: (a)it eleminates all possible waves from bouncing around the back of the car(b)read my note below. ive already explain why the traditional box-in-the-trunk method works well.
*note (b): by building a wall in the backseat, the main advantage is a smaller area to compress. imagine it liek this: the pressure of blowing up a baloon will be greater than exhaling inside a room. pressure is a whole other devil though, so ill leave it alone for now.  :shhh: i do cocaine :/shhh:
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