Subwoofers and enclosures 101

[mxmissile]

OK, I said this would be coming, and I found some free time, so here are the the basics on boxes for anyone who wants to know.

I'm writing these threads to expound in detail on some of the more FAQ's around here, not to start debates. Please reply with real questions or information only - we all love to hear different opinions, but I'd like for this thread to be an information source for the beginers, not a debating ground for the old guard. Fellow tech freaks also note - I will not be addressing specific Q alignments and whatnot, this would make a long post a lot longer, and confuse the crap out of the newbies. Feel free to jump in with that stuff in any replies as you see fit. Now -

The job of a subwoofer is to produce the low frequency sound we love so that the smaller speakers in the system don't have to. This is accomplished by creating a pressure differential between the air on either side of the speaker cone. The different types of subwoofer enclosures affect the way that the air inside the enclosure reacts with the speaker and/or the surrounding air to accomplish the desired result, whether it be lower extension, higher SPL, etc. The enclosure also has a tremendous effect on a speakers power handling, because it will have a huge impact on the way that the cone moves. That said - There are many different types of enclosures (boxes), I am going to address the 4 most common - sealed, ported, bandpass, and freeair.

Sealed - Also called "acoustic suspension". The sealed box is the simplest and most popular type of enclosure. It works simply by containing the air behind the woofer to varying pressures, and keeping that wave from interacting with the outside air at all. ANY subwoofer will work in a sealed box of the correct size. It is a commonly stated that a sealed box is "tighter" or more musical than other types, this is NOT always the case. A sealed box can be "tuned" just like any other enclosure by varying the amount of air interacting with the speaker, making it tighter or boomier. Sealed boxes will generally play lower, and have higher power handling capabilities, but are are not as efficient - they need that extra power to play loud. With the way subs are designed now, sealed boxes will usually be quite a bit smaller than other box types. If you are building your own box, or will be putting a given woofer in a prefab enclosure, sealed is the easiest way to go. Just match the manufacturers recomended airspace with the box you'll be building or buying, sealed boxes are pretty forgiving if you're off a little bit.

Ported/Vented - Also called "bass reflex". The ported box is a more sophisticated version of the sealed enclosure. This type of enclosure also works by containing the rear wave, but adds a tuned port that interacts with it in a very specific manner at certain frequencies. The port has a very specific volume of air displacement in relation to the airspace inside the box. This air will provide a specific resistance and resonance when it reacts with the motion of the speaker cone. Technicaly, any woofer can work in a properly built ported box, but due to the drastic effects that this box has on cone motion, it is not recomended by some speaker manufacturers for some of their woofers (the Eclipse aluminum woofers are a good example - in the right ported box, they will POUND - but you greatly reduce their life expenctancy). Ported boxes have a reputation as being "boomy" - this is also not really accurate. Most high end home speaker companies use vented enclosures, they can be VERY accurate if they are designed correctly. A vented box will usually be more effecient, and power handling is exceptionaly high - but in a narrower bandwidth. System tuning is very important to get the most out of a ported box. Ported boxes tend to be larger than todays sealed enclosures, and unless the box is designed specificly for the woofer you're using, prefab ported enclosures are not usually a good idea. You can build your own if you have the right specs, but it's usually better left to the pro's.

Bandpass - Also called by their type, "single reflex" or "dual reflex", by the rolloff, "4th order" or "6th order", or various nicknames, "compression box" is the most common. Bandpass boxes work by controlling the pressure on both sides of the speaker cone using the principals outlined for either or both of the enclosure types listed above. They are INCREDIBLY efficient in the fequency range for which they are designed, gains of 6 to 8 Db vs. a sealed box for the same speaker are not uncommon. However, because of the passive forces that a bandpass box creates on the speaker, power handling is limited. If you're looking for the loudest possible bass with a limited amount of power, they are unbeatable. Sound quality in a bandpass box can be very good, but will always contain more distortion than a single chamber type enclosure. If you only get one thing from this post, let it be this - NEVER USE A PREFAB BANDPASS BOX UNLESS IT WAS BUILT BY THE SPEAKER MANUFACTURER SPECIFICLY FOR THE SUBWOOFER THAT WILL BE USED IN IT!!!! Read that 10 times, never forget it. Bandpass boxes are EXTREMELY picky, and are VERY hard on the speaker if not built right. If you want this type of box, it can be a lot of fun - having two 10's hit like 3 12's is a blast - but I highly recomend having it professionaly designed and built by an experienced installer. Note that bandpass boxes will be quite large, if weight and/or space are factors, this is not the right box. Bandpass is most effective when placed inside the listening area - i.e., in a hatchback or sport utility, etc., so all you Probe drivers are lovin' it. I personaly LOVE to put them in the trunk of a coupe or sedan with the port run to an opening cut in the rear deck, but if you don't want to cut a hole in your car, MX-6 owners should stick with another box type.

Freeair - Also called "infinite baffle". The freeair method works on the simplest possible concept - the air on either side of the speaker has little or nothing to do with the way that the speaker produces the sound in this application. As long as the wave on either side is seperated from the other side to avoid cancelation, it's all up to the speaker to do the dirty work. Because the sub will derive no support from backpressure, power handling and efficiency are VERY limited in this type of system. The majority of subs out there would not last long in this type of mount, if you want freeair bass, you should get a woofer designed for it. Sound quality in an infinite baffle system is EXCELLENT with the right sub - on par with or better than the best sealed boxes and similar to a perfect ported enclosure - but you're not going to get real loud. A lot of the options available for freeair setups are disappearing, and aperodic damping and whatnot is pretty technical stuff anyway. I will let it suffice to say that freeair is great for taking up very little space, and the sound quality is awesome. If you want to build your own freeair setup, it'll be more work than you might expect, but most DIY's could manage with a little patience. The "wall" which will be your mounting point must be VERY solid, pretty much airtight front to back, and you'll probably need a lot of Dynamat or related material to keep things quiet. Amp choice and system tuning will also be very important here.

I'll finish with some notes on box building, those questions also come up a lot....

If you ain't got access to the right tools and materials, don't start till you do. A table or wall saw, a drill and/or screwgun, and woodglue are necessary. Hammer & nails can be used, I prefer to pre-drill & screw things together, and you will need to screw down the sub. A jigsaw or sabresaw is commonly used to make the hole for the speaker, a router will make your life easier. You may want to use a silicone type sealant to make sure things are airtight, but if your cuts are straight the woodglue should hold things tight.

Use 3/4" or thicker MDF or HDF. That's Medium or High Density Fiberboard. There are lots of things you can use, but heavier & more dense is better. Plywood & particle board should not be considered. Flex and resonance are the enemy here. Any surface that'll be more than 18 or so inches long and more than 8 or so wide should be braced internaly - a piece of wood 2" thick running across the large surfaces every 18" or so would do nicely. If you want the box to be REALLY airtight and solid, put a sqaure block of wood along all your right-angle joints as you build the box. This will help ensure proper angles and help seal off the corners from pressure.

To figure airspace, find the volume of your box, Length x Width x Height, and divide by 1728 - that's the number of cubic inches in a cubic foot. If you're metric, do the conversion - just because I think America should switch to metrics doesn't mean I know 'em yet. Remember to find the volume of all the crap that's going in the box, and subtract it from the total space - i.e., a 12" by 2" brace made of 1" thick wood will subtract 24 cubic inches from your volume.

To convert a round port to a square vent for building ease, square the radius of the recomended port and multiply by pi, the length remains the same. So a recomended 4" port 10" long becomes a sqaure or rectanglar or whatever you want vent with 12.57 inches of surface area, still 10" long. The radius of a 4" circle is 2", squared, times pi - 2 x 2 x 3.1417 = 12.5668. Take the port surface area and multiply it by the length to get the port volume, don't forget to subtract that from your box volume!

That's all that fits here, which is good, 'cause that's all I feel like writing now. There will be questions, feel free to post them, but I hope this puts ya'll on the right track.

Next up - series/paralell wiring and impedence (ohms)!

[scubasteve]

Holy Cow Batman .... you make my job easy ... Im thankfull for you as a member ... sticky for this one .... , are there any more, if so I make a colaboration ....

[mxmissile]

Hehe - thanx for the props....

I figure some time spent writing these will save a lot of time answering questions later, and reduce the clutter of repetitive question threads.

Just tryin' to help....

[NorthATheBorder]

Ok, when it comes down to making a box what is the best shape? I have to make a box soon for my 2 10s, and I am going to go sealed. I was thinking that rather than making a rectangular box, I should make one with a sloped face. I figured it would sound better than a regular rectangular box. Because wouldn't the sound waves in a traditional style box reflect off the back wall of the box, and cancel itself out somewhat?
I may be way off, enlighten me...

Oh, and btw, great info.

[mxmissile]

The shape of the box doesn't make a whole lotta difference. If you get into it with a way technical audiophile (like me!!), then a really crazy discussion on standing waves inside the enclosure can develope - but realisticly, in a small box for moderately powered woofers intended to go in the trunk of a car, I wouldn't worry about it too much. Make the box the best shape to fit your needs and trunk, fill it about 2/3 to 3/4 with pillow batting or other fiberfill, and be happy.

[NorthATheBorder]

fill it about 2/3 to 3/4 with pillow batting or other fiberfill, and be happy.

What, put pillow batting of a fiberfill in the trunk? Are you talking sound deadening? Can you please elaborate.

[mxmissile]

Fill the box 2/3 to 3/4 full of fiberfill. It helps "deaden" the air inside the enclosure. The sub thinks it has more air than it does, and it helps absorb standing waves. Open up any quality home speaker or prefab preloaded sub box, and it'll be full of the stuff. Not necessary, but an easy step to take to improve the sound of a subwoofer enclosure.

[NorthATheBorder]

So where could I buy this stuff, a hardware store?
And you just dump it in the box? Wouldn't that get inside the speaker?

Sorry about all the questions, Im not much of a stereo buff...

[mxmissile]

Don't apologize for asking questions, that's why we're here, man!

The fiberfill is most easily found in a crafts store or fabric store, though I imagine a hardware store may have something along those lines....

The fill should come in a large "sheet". Apply a little glue to the back of the box, and press the first layer in pretty good. Than just fold the material in untill the enclosure is about 2/3 full. It's not an exact science, so don'y worry too much. Even if the box is pretty full of the stuff, it won't hurt the speaker.

[NorthATheBorder]

Great, thanks for the help.

Karma for you....

[JBHELL]

From my knowledge, you dont have to worry about the shape of the box as the wave lengths for bass frequencies are quite large. The dimensions of the box in a car is alot smaller than the size of the wave lenght and therefore there is almost no chance of getting 180 degree wave length bouncing back to create any cancellation. All you should really worry about is the volume of the box and secondly the port(s) where if it is a vented box.

ok if im looking for volume of a sealed box and i want a sloped face how do i find out the volume when the width will be diffrent from top to bottom?

[NorthATheBorder]

Originally posted by LovnDaMX6
ok if im looking for volume of a sealed box and i want a sloped face how do i find out the volume when the width will be diffrent from top to bottom?

Ok, thats not hard. Say your box dimensions are supposed to be 10" deep x 15" wide x 15 " high. The second two dimensions would stay the same. The only difference would be the depth. All you have to do is take for instance 2". You add this to the bottom depth, the the depth of the botom peice of the box is now 12". you then sybtract 2" from the top depth, this leaves you with 8" deep for the top piece. Its that simple. If you want more of a slope, use 3" instaed of two, its that easy...

[mxmissile]

I think he wanted to calculate volume, not figure the slope. Or maybe I didn't fully uinderstand the directions you were giving.

Anyway, what you do to calculate volume, and what NorthA The Border might have been getting at, is simple. If their is only one slope, and the box is otherwise square, take the difference between the short & long sides of the slope and average them.

Example - a rectangular box 10x10x10 would have a volume of 1000. but if the back is sloped so that its 10x10 with the top 9 and the bottom 11, you'd just average it out - in this case back to 10x10x10. Another example for good measure (I kill me.... ) : if the slope gives you a top measurement of 8 and a bottom of 11, you'd have an average of 9.5 - use that as your depth, multiply length and height normally.

[NorthATheBorder]

Yeah, thats exactly what I was trying to explain. I think you explained it better though, mine was to wordy...