Secrets to box building

[djjdnap 78]

So does anyone wish to share thier secrets?

Only thing i've heard is use iron rod instead of wood for bracing.

[mrray13 30]

good, quailty glue...

screws/brads no closer then two inches..no further then about 4 inches

round all port edges

and take ur time.

if ur chasing tenths on a meter (or are just anal) resin the inside of the box, round or 45* all corners, especially in the port, allthread makes a good brace, just make sure u seal it..make it ugly. ugly is louder..lol

wheeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

[djjdnap 78]

i've seen the inside of 1 box.

I had a roounded interior, kinda like a skate board half pipe.

[djjdnap 78]

lmao

Sealed box

This is the simplest type of box. The back wave can't interfere with the front wave because it's trapped in the box. Typically, sealed box systems are the least efficient designs since half of the radiation from the driver (i.e. the back wave) is wasted. Despite this, sealed designs are quite popular because: 1) they're easier to build than other enclosure types, and 2) are better behaved below their tuning frequency. The greatest disadvantage of sealed enclosures has little to do with efficiency, but is that their lower -3 dB frequency (F3) is always higher than the driver's free air resonance frequency (Fs). Variants of the sealed box include the infinite baffle and the acoustic suspension. An infinite baffle is where the volume of the box is so large that the driver acts (approximately) as if it were suspended in free air, An acoustic suspension design uses a very small box so the air inside can be used as a pneumatic spring to support the driver. Typically, sealed boxes are stuffed with acoustic material to increase their apparent size by changing the enclosure's characteristics from adiabatic to isothermal. All sealed box designs ultimately roll off at 12 dB/octave.

Ported/EBS box

Also known as a vented box, a ported box is a non-sealed box with a port (a tube between the box and the outside air) which is tuned to extend LF response. The port usually uses a constant cross-sectional area (excluding flares on the ends). Both the port length and diameter may be varied independently to achieve the desired tuning. Ported boxes are typically 3 dB more efficient than a sealed box below the sealed box's F3 point, since the back wave is used to augment the driver's output. The characteristics of this augmentation are determined by the port's tuning. Response rolls off at a 24 dB/octave slope.

Ported enclosures are typically lined with acoustic treatment to break up standing waves, and may contain a low density fill, but should never be densely stuffed. In either case, the filler should be held securely in place to prevent it from being blown out the port.

A popular variation of the ported box is the Extended Bass Shelf (EBS) design. This is simply a ported system in a larger than optimal box, tuned to a lower than optimal resonance frequency. The frequency response is distinguished by a slightly depressed shelf at the bottom end. Although F3 is actually higher, usable in-room bass response is extended, while providing better phase and delay performance. For more information on EBS alignments, see The Subwoofer DIY Pages.

Bandpass

A bandpass design is a special type of ported box. It contains two or more chambers, any of which may be sealed, ported to the outside, or mutually ported with another chamber. The driver is mounted on a baffle between two of the internal chambers, so the sound can only get to the outside through a port. As with ported enclosures, bandpass enclosures are lined but not stuffed. As their name implies, bandpass designs exhibit both low- and high-frequency roll offs of 12 dB/octave in the case of 4th order systems. In all but the best implementations, this leads to boomy, one-note bass - impressive for casual listening, but not particularly accurate. As a result, bandpass enclosures are mostly used to provide the perception of lots of bass from a small box. Good bandpass designs are possible, but they remain subject to Hoffman's Iron Law.

Tapered Quarter Wave Tube (TQWT), also known as a Voigt pipe

The TQWT is a hybrid technology, related to both TL's and horns. Although more obscure than TL's, TQWT's have traditionally been backed by more solid theory. This is due to the fact that they were the work of one highly respected audio researcher, Paul Voigt, in the early 30's. Both straight and folded versions are known. Also like TL's, TQWT's benefit greatly from a good stuffing strategy. Although arguably as arcane as TL's, the TQWT's principle use has been as a low cost, easy to build alternative to horns. TQWT's are often used with full-range drivers, such as those listed in Section 3. Having said all that, if you want something different, yet lack the woodworking skills to build a good folded horn or TL, a TQWT might be just the project you're looking for.

Horn

Horns for bass systems are almost always folded to conserve space. Both the geometry and subsequent design can be tricky and will require the greatest woodworking skills of any enclosure type. Horns are characterized by their flare rate - i.e. the rate at which the cross-sectional area increases between the driver and the mouth. Lacking finite element analysis tools, the best way to design a horn is using a tractrix (a specific geometric shape) profile. There are several software packages (none free that I know of, though) which can be used to design tractrix horns (some also do hyperbolic and/or exponential profiles as well). The mouth area and length of the horn determine the LF cutoff of the horn. Raise the cutoff frequency of the horn, and the enclosure size drops dramatically. Much of the traditional literature describes horns which are front-loaded, i.e. the driver is in a box (which can be sealed or ported, BTW - there are some good articles in recent issues of Speaker Builder magazine discussing horns with drivers in ported sub-chambers) which, in turn, drives the horn. Much contemporary interest in horns involves the use of a direct radiating, high-efficiency, full- or wide-range driver, with horn loading of the driver's back wave. A horn acts as an acoustic transformer, greatly increasing the efficiency of the system. Horns exhibit a LF cutoff, based on the circumference of the mouth. Below this cutoff, output drops precipitously - typically 24 dB/octave or greater. Rear horn

Transmission line (TL)

Think of a TL as a pipe containing a driver in one end and with the other end open. Traditionally, a TL had no tuning frequency (Fb) in the conventional sense as with a sealed or ported box. "Tuning" a TL was a "simple " matter of making the line length 1/4 wavelength of where the driver begins to roll off, so that the rear wave can reinforce the front wave. The line was tuned to either the driver's F3 (-3 dB) or F10 (-10 dB), depending on whose design methodology you used. Similarly, unlike a port, which acts as a Helmholtz resonator, the parameters of a TL were independent, the cross-sectional area being determined by the driver's Sd, and the length being determined by the driver's F3 or F10, as previously noted. Some TL's (notably the Focal Daline series) were hybrids, utilizing a small enclosure which vents to the outside via a more conventional TL. Even in a Daline, though, the cross sectional area of the TL where it joins the box was much larger than a typical port, usually 1-1.5 times the SD of the driver. TL's are usually stuffed, often with stuffing materials of varying density. Sound complicated? It was, which was why many people avoided TL's as involving as much luck and voodoo as science.

Well, that was then, this is now

[Nikuk 1]

General Box building:

Measure three times, cut once.

Gorilla glue will stain your hands for a couple of days, but it'll destroy your mdf if You try to dismantle the box. The glue joint literally has more tensile strength then the wood.

Glue like a clean surface, take 5 seconds to wipe off the saw dust.

Be sure to set any adj. saw blades to 0 or 90 degrees unless otherwise needed. You'd be amazed how much a 10* list will fudge up your corner.

Pick up two or four of these: http://img.photobucket.com/albums/v492/Njc...uip/90brace.jpg 90* or corner braces. Will save You 10 - 30 minutes per box.

When pre-drilling pilot holes into the edge of a board, use a slight countersink. this will make for easier finishing of the box (carpet, bondo, paint) and in theory gives the screw more holding power.

When cutting the mounting hole for a speaker, be SURE to use the proper cutout size, an extra 1/8" may not sound like much, but it's too much.

If You don't like unsightley wires hanging out of Your port, drill (2) holes into a hidden surface of the enclosure, JUST BARELY larger then your speaker leads. If You are having trouble feeding the wire through, spray it down with some WD-40 or similar. Once the wire is long enough (will reach ~12" out through the baffle), silicone the holes.

When mounting a speaker, be sure to screw @ a level angle. 90* from the surface. If You are off by 30-45* You could very well skip through the edge of the wood & end up splitting the wood -or- even damaging the speaker possibly.

If You do a proper job cutting Your mdf and using the gorilla glue, You will not need and silicone or caulk on the interior seams. The same goes for foam tape around the speaker cutout.

Port length, less is more. The shorter that Your port is, the less 'intrusion' upon the acoustic energy in the box. This will help with port noise, spl, transient, and cone control.

When building an enclosure for more then one speaker, try to place the port or any openings as central as possible... I have seen woofers over exert simply because they were mounted over the port, vs others that were not.

Slot ported woofer enclosures with the port on one end and two (or more) woofers side by side: Depending on the equipment and power levels, You will tend to see "servo-ing" of the woofers - the woofer closet to the port excurting more then the further one. If this becomes a reoccuring problem, try a loading board. A vertical board (1/2 the width of the woofer) directly behind the woofer closeest to the port. There are many variations of a loading board, this is simple a rule of thumb to start with.

When working with smaller baffles & mulitple drivers - dont be afraid to get the baskets touching each other. BUT, if You do, be sure to rotate the woofers in a fashion that the screws are not within 3-5" of each other. Otherwise that area could suffer some weakening.

When builing slot ported boxes with 'labyrinth' ports, be sure to account for the full length of the port. By that, I mean keep in mind that each 180* bend will have a length of 2x the width of the port.

The 9:1 port area ratio... Try to stay under it. The more surface area or perimeter of the port area, the more likely You are to suffer from Laminar Flow - the air on the outer edges of the port opening slowing down and the air in the center of the port area speeding up. This is why round ports are best - minimal perimeter.

As far as SPL numbers - 45* reflection boards in the corners DONT ALWAYS help, sometimes they may hurt. Sometimes You may need 30* boards, You may need 70* boards, experimentation helps. I've found the method that helps most often is to: cut a cardboard tube in half (from a 1" dia. mailing tube to a 8" diam. concrete tube); tack it into the corner; fill the gap created with pretty hot resin or epoxy; once the pour is cured, pull the cardboard off and sand the pour smooth as possible.

Simple rubber stips on the bottom of the box will go along way toward keeping the box from sliding around in the vehicle.

when double-walling a box or baffle, try to interlock the ends of the panels with the adjacent panels. makes for a stronger corner.

when making a box that has the port on the far end of the baffle, try not to put the woofer on the far opposite. generally dead middle to 2/3rd's positioning will yield a higher score then a woofer then is butted up against the far wall.

When making a box with variable tuning, I find it much easier to make a port 'plate' rather then a port plug, or swappable port. To bring a 10"x8" port down to 8"x8", simply make a plate that is 2"x8" that can screw over -or- be flushed into the port opening to reduce area. This will change Your tuning more easily and quikly... and it's generally easier to "make pretty" then a fully swappable port assembly.

IMO - nothing more then 15 sq in of port per cu. ft. is necessary for music or daily. No less then 7 sq in though.

Use at least 1 5/8" screws for 3/4" mdf.

When painting a box with out bond'ing it, be sure to sand down the showing cut edges well, then spray with automotive "high build" primer.

All thread > all for bracing. Use a double baffle & BIG washers where it'll be bolted though. This will help structural integrity as well as allowing You to "sink" the nut flush - therefore hiding it.

Inverting drivers raises cooling efficiency, but the only by a negligible amount.

I can't think of anything else at the moment... it's early still.

Hope this helps.

-Nick

[finebar4 0]

Some real good tips in there, thank's Nick. Where did you get those corner braces at?

[ANeonRider 3]

As for screws, I use 1.75" (when using 0.75" MDF) coarse threaded decking screws.

Sealing... I use the MDF dust I make, mix it up with a ton of carpenter's glue, and spread it on the seams, makes for a very very solid seam.

[Nikuk 1]

Home Depot: $1.89 or so...

[jntar 15]

Some good tips there Nick

[CBFryman 2]

djjdnap, good post of information but a lot of the stuff is misinformed when speaking of an enclsoure truely devoted to the 2 lowest octaves and infrabass.

Mainly, you do NOT want to dampen the interior of a true subwoofer enclsore because there are NO standing waves. the waves of bass frequincies are FAR too long to exist inside the enclosure and bunch up around bountries.

Same goes for the "sealed/acoustic suspension" enclosures. a true subwoofer will always be acoustic suspension because usualy the waves cannot exist.

Also, TL is the evolved more technical version of the 1/4 wavelength enclosure.

it didnt differentiate between 6th and 4th order bandpass.

Ported is more technically named Bass Reflux since it isnt technically a port.

isobaric is generally usless, you get no extra useable displacement so a single driver properly powered in a good enclosure can yeild similar results to an isobaric set up.

and horn roll off is generally NOT 24dB/octave... if this where true really bass horns with extension down to 20Hz would have to be even more massive than they already are

but it did have some good information.

[djjdnap 78]

^^ yea, this site was based on home audio, i believe, thus the horns

[CBFryman 2]

when speaking of Home audio most of that is true when spekaing of midbass drivers in ported and large sealed enclosures.

just an FYI for anyone who doesnt already know.

Suffing in an acoustic suspension enclosure for a subwoofer is NOT to dampen standing waves but to lower enclsoure Q resulting in lower extension with a smaller enclosure when compared to an enclosure of the same design.

[trex-btnhfan 0]

For bracing, use threaded steel rods. Make sure you have bolts with locking washers on all 4 sides. Insides and outsides.

[djjdnap 78]

Placement, Brace types

This is what a properly braced ported enclosure for a single driver would look like. Note in particular the liberal use of wood in the construction of the baffle itself (where the subwoofer actually mounts--here on the right side of the box just below the slot-port). This provides an extremely solid, vibration free anchor for the subwoofer to mount.

For enclosures that house multiple drivers, just follow the same general procedure which is to basically just go nuts with the bracing. You can never have too much bracing (unless of course you were to brace your box to the point that it would be reduced to a solid block of wood in which case it wouldn't be a box--it would be ballast).

It doesn't have to be pretty, just effective. Just make sure that you compensate for the braces' displacement by over-voluming the enclosure by just the amount necessary to make up for what the braces displace. Do not simply add a fixed percentage of fudge factor, particularly when dealing with ported or bandpass enclousres as this practice can result in serious mis-tunings (read: it'll make your sub sound like crap). Sealed systems are a bit less finicky and a percentage-based or rough guestimate over-volume will be less of an issue, but we still don't recommend the practice. Don't be lazy...better to do a little math now than to regret the hours you spent desiging and building the box later!.

While there are other methods of bracing involving extremely complex shapes, the following is probably the easiest to implement as it requires no pesky angle cuts and is also quite effective.

Brace Placement

When deciding where to place your braces inside the enclosure, it is a good idea to make sure you use asymmetrical placement so that the span between any brace and the nearest wall (or the next brace brace) is unequal to subsequent spans between braces and walls (and you thought this was going to be easy). This placement technique will minimize the possibility of standing waves forming in the wood panels that make the walls of the enclosure which would cause nasty losses and potentially bothersome colorations.

If you are using actual partitions as bracing in your particular design (when using multiple drivers, this is adviseable), this rule does not apply--it is only for braces used within the individual chambers.

Basic Brace Types

Using this scheme, there are two types of braces that are commonly used. To keep things really interesting, we'll call these two types of braces types "A", "B", and "C".

Type "A"

A type "A" brace is simply a solid piece of wood that does not span the entire width of the box (it contacts the enclosure walls on three of it's four edges). It gets a little more fun as we go along so hang in there.

Type "B"

- or -

B-type braces involve a little more work, but they are a bit more effective since all four edges of a B brace are in contact with the enclosure walls. A type "B" brace is made by simply cutting holes into a piece of wood that would otherwise be considered a partition--it touches all four walls. To maintain the brace's rigidity while keeping displacement to a minimum (allowing air to flow freely through the brace), cut circular or rectangular holes and make sure that the distance between each hole's edge and the edge of the brace is no thinner than one (1) inch.

The bracing used in the photograph at the top of this page are "Type B" braces with rectangular openings. The shape of the opening in the wood is of little consequence for the most part, but it should be noted that using circular holes does offer a slight strength advantage.

Type "C"

http://www.jlaudio.com/tutorials/bracing/images/braceC.gif[/imgA type "C" brace takes a little more skill and/or patience to make (due to the angled cut), but are very much a worthwhile investment of your time. The braces are nothing more than triangular corner braces that help assure you of a very solid connection between adjacent walls of your enclosure. This not only reduces cabinet losses but also makes your enclosure much less susceptible to being turned into a rhombus when subjected to the "Jumbo Test".

Make them as big or small as you like, but always keep in mind that the more braces you use, the more air they'll displace!

[Steven Kephart 0]

One thing I like to do when building enclosures is when the edges of the MDF are going to be exposed, cut that piece 1/4" large. Then when you build the enclosure, you should have 1/8" of wood sticking out in those areas. Now use a flush trim bit on a router to cut the excess off and you are left with professional looking edges.

Steven Kephart

Adire Audio

[mrray13 30]

X2 with the flush bit!! then use a 3/8" roundover bit (even with carpeting) for that ultra profressional look.

as too the bracing...there is such as thing as too much. for spl purposes anyway. u should just brace enough to stop any flexing/resonance.

wheeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

[Airmack 0]

I find it easier to joint one side on a jointer then cut the other to the exact dimensions on a table saw. But I am going to have to try that router bit technique by leaving excess material to trim around the edges. I like to use a through dado if I have two seperate chambers for two subwoofers. I usually have rabbits on the side peices for a better joint. My next box will be using tongue-and-groove joints on the sides. It will be for my 12" RL-s. Forstner bits also aide in the process of screwing it all together.

Has anyone every tried using finger joints or dovetails on a sub box?

As for wood braces... Could other woods work such as ash? I was thinking about making a frame within the box out of ash.

Edit: Does anyone have pictures of bracing techniques or bracing that they did? Also, any pics or "C" braces?

Edited November 4, 2005 by Airmack

[Supa_c 1]

Good luck with the tounge and groove.....

[///M5 2,833]

Has anyone every tried using finger joints or dovetails on a sub box?

Errr, probably not with MDF. Not that it wouldn't hold up with a lot of glue but my yearly limit of profanities would probably fly in the process of trying. As for your bracing question, stiffer is better as it will flex less. Ash is not as strong as a threaded steel rod, but if it placed well it is fine. I needed to compensate a little volume on my last box and just used 2x2 for bracing. If you do use wood and not a rod, make sure to attach it really well as it needs to brace in both compression(the wood itself) and tension(your screws and glue). If you are wondering on locations of where, look at your box and imagine where it would flex the easiest and put it there. IMO you shouldn't ever "center" your bracing though as the two pieces of box you just separated will now share the same acoustic bending mode. Just move it off by a couple of inches and you should be fine.

[azbass 0]

i sink the 2 end sides of the box in a little. it helps with getting it carpeted.

[Amish Marauder 0]

Gorilla glue shouldn't be used when building enclosures. It expands when it dries. A good glue like Elmers pro bond should be used. Fantastic stuff by the way.