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This may not exactly be the correct place to get this answered, but i'd rather try and get info from professional audio engineers than from random petrol-heads - please tell me if i am off-topic.

I am thinking about building a custom enclosure to put a pair of sub-woofer speakers & an amp in the boot of my car; I have seen people who just buy thick plywood & mount the speakers without much care about anything else, i have also seen professional car audio techs who put a lot of thought into their enclosures to the extent of - what i believe i heard anyway - creating ports from the internal 'chambers' that have a width relative to the wavelength of the sounds it will be producing, is this sort of stuff necessary? I don't want to go over-kill but i do what to take the correct amount of care to not ruin the sound.

The speakers will mainly be used for showing off at car events looking flashy & unique, so I guess i don't need to worry too much about sound quality vs how they look, but in honesty i tend to be a bit of a perfectionist and want to understand exactly how something works before i just go and do it willy-nilly.

Are there any pitfalls to building an enclosure to house a subwoofer that should be carefully planned around? Are there any commonly made mistakes that can be avoided?

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Certainly you can optimize (or at least increase) the performance (efficiency, flatness, coverage, etc. etc.) of a speaker system by using the standard tools. You can use the Thiele-Small parameters of the driver(s) to calculate the optimum enclosure volume and port area/length. There are whole university courses and books published on the subject, and likely online calculator web pages for those of us who are more practical.

Only you can make the trade-off decision how much time and effort you want to put into this exercise vs. what it is actually worth. And doing this in a car is even more challenging given the odd size and shape constraints of the space you have to work with.

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The worst mistake you can make is to put an 18" woofer in a compact car. All you hear is wind blowing. You hear the bass 3 meters away from the car.

The hardest bass I have heard was either 4 10" woofers in a compression design firing into the cab of a small truck. They were driven by a Fosgate 1000 watt amp. I could hardly catch my breath.

Close to this in super-bass was an old style Cadillac with 2 12" woofers in a box in the trunk with the speakers pointed down 45 degrees. This created a gap so they could 'breath'.

In addition to Richard Crowley's answer remember that woofers of a given width need at least that much space in cubic feet x 2 to 'breath' properly.

An exception would be Cerwin Vega's whose specs require more room because the cones move further in and out. Being modest for a moment, a couple of 8" woofers in a tight but just-enough-room space can produce good-tight bass, without blowing your hair out of place.

We used to have contest to see if a recent install would blow out a match or lighter from the bass at full volume-and how far away it would do that.

Bottom line is to do was us installers had to do. Measure the space available, stay within the limits I gave and read Richard Crowley's links.
Don't let money or fast talkers put in the wrong sized speaker or a poor cabinet design.

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