How important is a box?
 

I recently had a customer come in and ask me this question. A subwoofer box or enclosure (which I prefer saying especially if it is custom made!) is one of the most important things to invest in and consider when it comes to adding bass to your car.

What are the different kinds of enclosures?
There are many different types of subwoofer enclosures but there are 9 that are most used:

1) Sealed
A sealed enclosure uses a calculated volume of air as a spring" to help dampen and control woofer cone movement. Sealed enclosures can have a relatively smooth roll-off and flat response.

Pro's: Small boxes, higher power handling, excellent dampening and transient response, easy to design and build, can be forgiving of design and construction errors. Sometimes subwoofers can be inverted in this enclosure but provide no benefit other than to show off the magnet of the subwoofer.

Cons: Efficiency vs. output

Ported (my personal favorite)
Ported enclosures use a tuned port or vent to increase output at lower frequencies. Air inside the vent acts like a piston or motor that moves in the phase with the woofer reinforcing frequencies.

Pros: Increased efficiency, increased low frequency output, relatively low distortion.

Cons: Larger enclosures less tolerant of design/construction errors, no woofer control below tuning frequency.

Aperiodic
Aperiodic enclosures utilize an external membrane to dampen the woofer and flatten out the impedance curve of the enclosure.

Pro’s: Extremely small enclosures, very flat response, excellent low extension.

Cons: Low efficiency, usually needs larger subwoofers to perform, EXTREMELY difficult to design and tune properly

Isobaric ("push-pull")
An enclosure that uses two woofers mounted face to face, one wired reverse polarity. This alignment effectively cuts the vas in half. The general rule of thumb for an isobaric enclosure is twice the woofers, twice the power, and half the box size3.

Pros: Very small enclosures, increased power handling, driver non-linearities canceled out (lower distortion).

Cons: Very low efficiency, due to the small size of the recommended enclosures for our woofers an isobaric alignment is not practical

Acoustic Coupled
Two woofers sharing a given volume.

Pro’s: Can be easier to build than a divided enclosure.

Cons: Output can be affected each driver relieving slightly different information, enclosure is more likely t flex without the additional bracing of the divider (usually need additional bracing).

Free air or infinite baffle
A woofer mounted to a surface that isolates the front wave from the back. (A woofer in an enclosure which has a larger vas than the woofers vas).

Pros: Very low system resonance, minimal space used in the vehicle.

Cons: Limited efficiency/output, no control of the woofer below its fs, usually very difficult to seal front wave from back

Single reflex band pass
Sealed enclosures that play into a tune enclosure that acts an s a low pass filter... This increases efficiency within the “pass band” or usable frequency range of the enclosure.

Pros: Can be made very efficient within the pass-band”, can be designed for increased gain in a band of frequencies, increased power handling within the band.

Cons: Distortion hard to hear so, it’s very easy to destroy woofers, increased gain designs sacrifice low end response, relatively large enclosures very difficult to design and build, very sensitive to design/construction errors.

Dual Reflex band pass
A ported enclosure in which the woofer plays into a tune-ported enclosure that acts a low pass filter. This creates a highly efficient system with two resonance frequencies.

Pros: Can be even more efficient than a single reflex band pass; excellent dampening at resonance, increased power handling at resonance

Cons: EXTREMELY difficult to design/construct, not forgiving of designs/construction errors like a ported enclosure it looses woofer control below f3, steep low frequency roll-off (24db) very large enclosures.

Labyrinth or transmission line
A form of Transmission Line, this enclosure uses a port that is a fraction of a wave length to couple with the woofer.

Pros: Can be built to maximize out put at a particular frequency (SPL purposes)

Cons: Difficult to build, giant enclosures, easy to blow woofers, not practical unless used for SPL competition, experimental.

As I have stated before, ported is by far my favorite enclosure to work with. For the most part, it plays lower than most sealed, plays nice and loud and doesn't require ridiculous amounts of power. A ported enclosure can be custom built for as little as $300 but if designed and built properly for the specific subwoofer and car, it can bring years of bass enjoyment!

You mean I CAN'T just leave my sub driver just sitting on the back seat??

Sadly no. However, it is perfectly acceptable to just cut a hole in the cardboard box that the sub came in and seal it with DAP*!





*Just kidding, please don't do this. If you do however, please record it on video and post it here for all to laugh at. :)

Which brings us to the other important factor: whatever you use for a baffle or enclosure, it must be SOLID and STIFF. If the box flexes with the changing pressure inside, you're losing efficiency there too.

My old GLC had those crappy cardboard door panels and my 6.5" Alpine door speakers sounded okay, but had no bottom end at all. I used the cardboard as a template for some solid 1/4" MDF panels, and suddenly I had some serious (for 6.5" full-range three-ways) bass coming out of my doors.

Same goes for mounting a free-air sub in the rear deck of your car - sound (air pressure) can still "leak" through the cutouts in the metal deck, the cardboard, and the seat backs.

Yes you can. Just don't expect it to be there 5 minutes after you park your car.

I am a huge believer in double baffles, angled bracing, routered port edges and popping wires directly through drilled holes as opposed to them plastic terminal cups.

nice write-up. you should do one for electrical upgrades

Mmmm, cups

:lawl:

Nice write up.
Things I learned after building a couple of boxes:
If space is an issue a flared aeroport is great option, the overall outside dimension of the enclosure will be smaller verses a slot port but aero's are pricey.

MDF vs Birch, I have been using Birch plywood over MDF due to the substantial weight savings, birch cuts cleaner on the table saw and overall easier to work with. Downside is in some places its almost double the price and birch requires lots of bracing.

Fi BTL N2 15", 6 cubes net 33hz port tune, 8" flared aeroport on 3.5K watts.
http://farm9.staticflickr.com/8387/8...26bb48e1_c.jpg

Fi SP4 15", 4.5 cubes net 30hz port tune, 8" sonotube.
http://farm3.staticflickr.com/2810/8...35108997_c.jpg

The lack of woofer control below tuning frequency in vented enclosures is irrelevant with infrasonic filters (more commonly, but incorrectly, known as subsonic filters), which have been commonplace on amplifiers for the better part of a decade.

Really, outside of a slight degradation in sound quality (which, for a subwoofer, is almost imperceptible, anyways) and some potential overheating issues around the tuning frequency of the enclosure (if the driver's design relies heavily on airflow for cooling the voice coils and you tend to play music with long, droning bass near the tuning frequency), there are no real downsides to a vented enclosure outside of the space demands. They're also relatively simple to design and construct.

But, yes, the enclosure is very important. The reality is that the enclosure is more important than the driver itself.