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Old 10-18-2002, 09:07 PM   #1
AmIGrand
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Subwofers and Enclosures 101

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.
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Old 10-19-2002, 09:07 AM   #2
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First off, good job on all the sticky's AmIGrand.

I only wanted to throw in my $.02 about the enclosure materials. I prefer Marine Grade Plywood and Baltic Birch Plywood(13ply) over MDF for it's rigidity, void-free (a must for acoustics), and ease when building the box. I don't run into crushed corners or screw stripping with the plywoods.

And I have a little info on the tools. Most military bases have fully equiped wood shops. They usually let you use the equipment for something like $2/hour. Not to mention you can get Home Depot to cut your sheets up for a price, I'm not familiar with what they charge though. With good cuts on a table saw your box can be practically perfect. And if you are using a jigsaw to cut out the speaker hole, buy a rip fence for it. They are usually under $10 and allow you to make perfect holes with the right blade. An evenly cut hole of the correct size will make the seal between your woofer and enclosure a lot easier to maintain.
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Old 10-20-2002, 11:46 AM   #3
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The debate about ply vs MDF etc. has been here before, and I'd like to avoid going there again. They both work, each has advantages and disadvantages. Go with what works for you, and as long as the box is built to specs and sealed properly everything should be fine.

Good tips about the military bases and Home Depot. When I'm in a hurry and building the box at home, I'll draw out my cuts in advance, and have Home Depot cut the sheets when I buy the wood. First two cuts are free, and it's .25 or .50 per cut after that. I get home with ready to assemble pieces for an extra 2 or 3 bucks - and it's worth that much just to not hafta load and carry a full 4x8 sheet!
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Old 11-04-2002, 04:57 PM   #4
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a note about getting your wood cut at home depot, because of the type of saw they use ( i forget exactly what its called, but it sits upright and they move the saw) your pieces will NOT be perfectly square, infact they may be a good bit off. It will still work none the less, but please be aware of that. The real way to do it is to use a table saw, but I got my wood cut at home depot.
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Old 11-04-2002, 08:18 PM   #5
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If the saw is set up correctly, it cust straighter than most table saws.
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Old 11-05-2002, 03:23 AM   #6
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considering the design of the saw makes it impossible to make square, that isnt true. not only is the wood on large rollers, but the fact that the head of the saw pivots to cut either down or across, the angle isnt going to be setup 100% square with the rollers. I'm not saying its way off to where its noticable without a tsquare, but it definately isnt square all the time. also, my neighbor was a woodshop teacher for 20 years, id hope hed know his tools.
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Old 11-05-2002, 04:02 PM   #7
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I said straighter than MOST table saws.

Most of the saws that I've seen that are used day-in and day-out are a bit off. The gears that adjust the blades angle get worn, so it's a fraction off, the giude arm loosens over time, etc.... Plus, the person feeding the wood has to be vary carefull to keep it perfectly straight - that's a bitch with a 4x8 sheet of 3/4" MDF!! The wall saw has the wood on a (supposedly) level bottom roller, layed flat against a wall rack. The wood doesn't move, the saw does - down a metal guide. You are right in that it's seldom perfect, but in my experience that design tends to maintain it's settings better in the long run than a table saw under heavy use. Your teacher has had a different experience, nothing wrong with that!

Hey - they both work, right?
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Old 11-06-2002, 07:44 AM   #8
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Both saws work well for our applications. I bought a craftsman pro-series contractor setup to bolt onto my craftsman table saw. The damn thing cost about $10 less than the table saw, and it still doesn't cut perfect. I never trust the measurments from the guides. I always measure from both sides of the blade to the guide to ensure my cut is true. Staying within 1/8" is a good cut for what we are after, you can touch it up on your own from there pretty easily. It doesn't have to be perfect, it's all getting covered with carpet anyways.
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Old 11-11-2002, 12:25 AM   #9
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Quote:
Originally posted by AmIGrand
I said straighter than MOST table saws.

Most of the saws that I've seen that are used day-in and day-out are a bit off. The gears that adjust the blades angle get worn, so it's a fraction off, the giude arm loosens over time, etc.... Plus, the person feeding the wood has to be vary carefull to keep it perfectly straight - that's a bitch with a 4x8 sheet of 3/4" MDF!! The wall saw has the wood on a (supposedly) level bottom roller, layed flat against a wall rack. The wood doesn't move, the saw does - down a metal guide. You are right in that it's seldom perfect, but in my experience that design tends to maintain it's settings better in the long run than a table saw under heavy use. Your teacher has had a different experience, nothing wrong with that!

Hey - they both work, right?
This is true. Unfortunately, I found out just how off my wood was today when i started assembling... although i just trimmed it down to be straight and all is good in Brian land.
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Old 12-09-2002, 03:07 AM   #10
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i just wanna put in my 2 cents here. i went to ye ole home depot, and had them not cut my dimesions but just cut the huge fiberboard into 3 equal pieces so i could get it into my car. to my dismay that focker cut the last piece off by like....... 5 inches man, it was horrible. anyhow i am just saying if you want it cut right do it yourself. i dont trust those guys with the saws. i mean, i got a friend who works at home depot and i DAMN sure wouldnt let him direct me in anything that required or involved tools. ok im out
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Old 02-19-2003, 06:24 PM   #11
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yeah home depot sux, i dont thing i will have them cut my wood anymore, the last box i built they were way off on some pieces, like 1/2 inch on one side, i knew i shouldnt have left the guy alone . sure the wood stays in place and the rollers are level, but have you seen the saw dust? the dust gets on the rollers along with other things and the MDF no longer sits on the rollers level, so it ends up being off, personally i think the way to do it is have them cut your wood within an inch or so of your dimensions then take it home and cut it to exact length on a table saw, it's much easier to get a strait cut on a smaller piece of wood, and if you can help it try and get a factory edge on every piece to square up on when you get home to the table saw, i have built many boxes and i usually cut everything myself and it comes out pretty good... but i am a perfectionist and i have to recut a piece if it's even 1/16-1/8th off, and it kinda ****es me off, so i think the next time i am going to have one custom built by a professional so i dont have to deal with it, it will probably take a long time to try and figure out how i want to fit this new audiobahn 18" sub anyways, i'm just gonna hand it to em and say "build it!"
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Old 05-07-2004, 04:08 PM   #12
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How do you find the volume of a wedge? I have to build a box on the back hatch of my transam, and there are angles involved. I have a rectangular part, and then i can devide it into a triangle on the back side of the box? How do i get the volume for that, so i can add it to the rest of the box?
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Old 05-07-2004, 05:16 PM   #13
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I actually covered that in the original post in this thread. You average out the difference and use that as your width. For instance, (assuming that the rest of the box is basicly square) lets say the box is 10" wide at the top, and 15" wide at the bottom. Add the two together and divide by 2, you get 12.5. Use 12.5 as the measurement for that particular figure, and you have the correct figure.
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Old 05-07-2004, 05:25 PM   #14
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Height x Width x ((Depth1 + Depth2)/2) = XXXXX

XXXXX/1728 = Equals internal volume.

If you include the wood thickness in the equation your volume number will be off.
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Old 06-23-2004, 01:22 AM   #15
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Am I the first person to realize that in the title, subwoofers is spelled with one "o"?
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Old 06-23-2004, 09:42 AM   #16
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Oops.
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Old 06-23-2004, 10:33 AM   #17
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I'm going to throw my $0.02 about making your own sub cabinets...

If you are working with a big sheet (4x4' or 4x8') it can be difficult for the home builder to get a good cut on a table saw, unless you have one with a really large table or a bunch of feed rollers to support the full sheet. I found that using a hand-held circular saw can be a lot easier.

You'll need some 4x4" lumber (the kind used for fenceposts) long enough to support your sheet of board. Lay them on the floor, and lay the board on top. Use a long straight edge (I used a 6' aluminum spirit level) and some clamps to set your fence, allowing for the distance between the saw blade and the edge of the base of the saw, then make your cuts keeping the saw against your fence. Set the cutting depth on the saw to about 1/4" more than the thickness of your board so you won't cut the 4x4" posts too much if you cross them with the saw.

One more tip... when cutting from a large sheet of board, always use the same edge as a reference to make sure the panels you cut are square. Sometimes a 4x8' sheet of board will not be completely square, so if you measure from different sides you can end up with out of square panels. Been there, done that, had to start over again.

When you have all the panels cut and have done a dry run to make sure everything fits where it does, and you have no unwanted gaps. If you are going to screw the panels together, one of these countersink drill bits will make life a lot easier - it drills the screw hole and countersinks it so it's flush with the surface in one go. For sealing the joints, something like liquid nails is better than silicone. Some silicone sealants contain acetic acid which evaporates off when it cures. This can damage copper, so it's not a good thing to have around speakers, unless you leave the cabinet empty for 48 hours or so to let the silicone cure properly.

If the speaker you are using does not have a cabinet recommedation, you can use a program called WinISD. You enter the speakers Thiele-Small parameters (available from the speaker maker, often included with the speakers) and the program will calculate the optimim sealed, ported or bandpass enclosures, and it will plot the projected frequency response for each speaker/cabinet combination. The only issue I have it is doesn't show the vehicle transfer function, which affects low frequency response so you only see the speaker/cabinet response, not the in-car response. It's also interesting just how undersized most store bought cabinets are.
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Old 08-08-2005, 05:34 PM   #18
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i need a good box for 3 10" JBL Power series its there new 2005 line subs and i have 2 infinity amps that push 760 watts
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Old 09-27-2005, 10:38 PM   #19
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one thing that is not really covered here is cabin gain. as mentioned by daxx, programs like WinISD do not account for cabin gain/transfer function. in general, below a certain frequency, you will experience a 12db/octave boost in frequency response when compared to a free air response curve. this is generally in the 40-50 hz range, but this varies depending mostly upon the length of the vehicle. in general, the shorter your vehicle is length wise, the higher the frequency the cabin gain will kick in. this is, in part, why crx's are such powerful spl machines. now for those users of WinISD, i have a few different filters you can use to simulate your possible cabin gain:

internal car length = l [m]
Linkwitz transform
f0 = c / 2l
fp =15
Q0 =0.707
Qp =0.707

Eg
internal car length = 1.9m
Linkwitz transform
f0 =90
fp =15
Q0 =0.707
Qp =0.707

internal car length = 2.2m
Linkwitz transform
f0 =80
fp =15
Q0 =0.707
Qp =0.707

internal car length = 2.5m
Linkwitz transform
f0 =70
fp =15
Q0 =0.707
Qp =0.707

internal car length = 2.9m
Linkwitz transform
f0 =60
fp =15
Q0 =0.707
Qp =0.707

internal car length = 3.5m
Linkwitz transform
f0 =50
fp =15
Q0 =0.707
Qp =0.707
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Old 09-28-2005, 08:09 AM   #20
MIAaron
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Join Date: Aug 2002
Location: Shelby Township, MI
Posts: 584
Vehicle: 2000 GT Coupe
MIAaron Gettin' there
It's more complicated than a standard 12db curve, as there is usually a bump in the 40-60 range. I usually design my car enclosures for a high Q response. The early rolloff of the high Q box blends well with the cabin gain , and then I can usually kill the res freq with one band of eq. Using a low Q box almost always forces me to waste another band of EQ on my low end.

There is a thread here by quinton where he shows how to get a rough idea of the transfer function of the vehicle. Sub location and orientation will determine a lot of this to, so realize you will need to take that into account when getting your measurements. And yes, the rat shack $40 analog db meter is well worth it. You'll need it when you get into home theater anyways...
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