Building A DIY Sealed Subwoofer (2023)

Hereat Audioholics, we have a great affinity for bass. Unfortunately, gettingcopious amounts of deep bass usually requires a fair amount of cash. However,if you’re comfortable with a bit of woodworking, there is a cheaperalternative: DIY.

Herewe’ll look at how to design and build a DIY sealed subwoofer. Sealed subwoofershave a few important advantages for the DIYer. First and foremost, they’re relativelyeasy to build compared to a vented box or a more exotic alignment like a tappedhorn or transmission line. The sealed alignment is also relatively tolerant ofminor changes in box volume and driver-to-driver manufacturing variations. Lastbut not least, they need less protection than ported subs, which can quicklyoverload below their tuning point. On the down side, without a port to augmentsystem output, you need a meatier driver to achieve solid deep bass.

Thereare three basic components needed to build a sealed subwoofer: the driver, theenclosure, and an amplifier. Building a successful subwoofer requires somedesign work in terms of matching the driver to the enclosure, and specifying anamplifier that delivers enough output for your needs without the risk ofsmoking a voice coil or bottoming out the driver. Back in the good old days,this kind of work was no small feat. Today with modeling software like WinISD,real world data from sites like data-bass, and a little guidance, it’s a lotless complicated. So where do we start?

The Driver

Thefirst step in building any subwoofer is selecting the driver, as this willdetermine how large an enclosure you need as well as what you need in terms ofamplification. You can find a dizzying array of raw drivers from companies likeParts Express and Madisound, as well as from smaller shops like iST and StereoIntegrity. Driver selection is all about what you’re looking to achieve. If youwant enough deep bass to rattle grandpa’s dentures you’ll need a lot ofdisplacement, which is a fancy way of saying a large driver with a lot of conearea (aka Sd) and linear excursion (otherwise known as Xmax). If you’re livingin an apartment where high SPL is a surefire way to get an eviction notice, a10” - 12” driver might be more appropriate.

Funfact: an 18” driver has a bit more than double the cone area of a comparable12” model, meaning it is capable of an additional 6dB+ of output when all elseis equal. As an added bonus, sensitivity will tend to be higher, while distortiontends to be lower.

The Dayton Ultimax 18” Subwoofer driver

Ofcourse, there’s a lot more to the driver than just size. Basic frequencyresponse is one important consideration that can make or break the soundquality of your subwoofer. Thiele/Small parameters like Fs (resonantfrequency), Qts (total Q of the driver at Fs), and Vas (equivalent compliancevolume) will also determine how a driver will perform in any given box. Sensitivityand impedance are also critical considerations, since these will help determineyour amplification requirements.

Notethat some drivers utilize dual voice coils, which allow the end user to changethe impedance of the driver as seen by an amplifier. For example, if a driverhas a pair of 2 ohm coils, wiring them in series gives you an impedance of 4ohms, while wiring the coils in parallel nets you a 1 ohm load. We recommendavoiding wiring for a lower impedance (<3 ohms), since this will place greatstrain on a partnering amplifier.

Crunching the Numbers

Atthis point you may have your eyes on a couple of drivers that look like they’dbe suitable for the task. Now it’s time for the rubber to meet the road, andsimulate the performance of our driver with modelling software. Regardless ofwhich particular program you use, you enter the T/S parameters from thedriver’s spec sheet into the software, and let it work its magic. Now it’s timefor some actual design work.

Simulated data for Driver X into Qtc’s of 0.5, 0.707, and1.0

Aboveis a sample driver modelled into three different box sizes, yielding a Qtc of0.5 (~7.75 cubic feet), 0.707 (2.7 cubic feet), and 1.0 (1.15 cubic feet).Looking at these graphs, some things stick out. Starting with a Qtc of 1.0, thefirst item of note is that the frequency response develops a pronounced hump,which can be further exacerbated if the woofer you selected also has a nativepeaky response. The smaller box also has a much lower sensitivity in the deepbass, a function of Hoffman’s Iron Law, which means more EQ and power toachieve a flat in-room response. What’s not seen here is that a Qtc of 1.0 willhave some ringing associated with its humped frequency response profile. Distortionwill also be higher in the deep bass relative to a larger enclosure. All takentogether, one can see the problems associated with building a box that’s toosmall.

Onthe other end of the spectrum we have a Qtc of 0.5. The first thing that shouldcatch your attention here is the enclosure size; 7.75 cubic feet worth ofworking volume for the driver is absolutely enormous by most standards. On theother hand, there are some dividends associated with going big. With a Qtc of0.5, the box is said to be critically damped, meaning the driver won’t ring onand on. Hoffman also dictates that a large box will equal better low-endsensitivity, which is reflected in the response graphs. Beyond size, however,there’s an additional downside to keep in mind. As a consequence of being moreefficient down low, driver excursion must go up for a given power inputrelative to a higher Qtc box. To put it another way, a large box does less toprotect the driver, such that high powered transients at very low frequencieshave greater potential to do damage.

Thankfully,subwoofer builders have a happy medium to choose from with a Qtc of 0.707. Thisalignment is typically referred to as maximally flat, since it provides themost extended response before the system begins to roll off. The enclosurevolume of 2.7 cubic feet is by no means ultra-compact, but it’s not somethingyou’d mistake for a refrigerator either. It’s not as efficient in the deep bassas a larger box, again blame Hoffman, but the payoff here is that the driver isat far less risk of over-excursion as well. Finally, while the design isn’t“critically damped”, ringing is nonetheless reasonably well controlled relativeto a high Qtc box.

Nowit’s time for a reality check. Simulations are a great starting point inunderstanding how Thiele/Small parameters and box size will affect performance.However, in the real world, nothing ever performs quite like the simulationssuggest they should. Some of this can be explained by variations inmanufacturing, or manufacturers fudging their specifications. Some of it comesdown to the reality that as you push a driver to its limits, non-linearbehavior creeps in to throw off the math. This is where real world data comesin handy. We recommend anyone considering dabbling in DIY to visit data-bass, which is run by bassaholic extraordinaireJosh Ricci. The site, as the name might suggest, is a database containingdetailed measurements of a wide range of subwoofers, including many raw driversin test enclosures. Raw drivers are also tested to see how closely they conformto their rated specifications.

The Enclosure

Withyour driver selected and knowing the internal volume needed to achieve yourdesired Qtc, the next step is actually building the enclosure. As with anyother loudspeaker, your subwoofer enclosure will ideally be an inert structurethat doesn’t add or subtract from the output of your woofer. This tends to meana reasonably thick cabinet (>0.75” of MDF or high quality, void free BalticBirch works well) and a reasonable amount of interlocking bracing and stuffing.Don’t forget that internal bracing will also subtract from the volume thedriver sees when designing your box, while stuffing will help raise theapparent volume the driver sees. We also recommend a double thick front baffle,as this will improve cabinet rigidity as well as allow you to recess the flangeof the driver for a cleaner look. If you’re using an external amp, you willneed either a pair of high quality binding posts that accept heavy gauge wire ora Speakon connector as well. Finally, you’ll also need additional wire for theinternal connection(s) to the driver, as well as spikes or rubber feet.

Pro Tip:It’s always easier to build a box a bit larger than you need and pad it down asnecessary, than to find out the box you built is too small and have to startfrom scratch.

Forthose who only have a passing interest in woodworking, flat packs sold by PartsExpress and others are also a viable option. A flat pack is to subwoofers whatIkea is to furniture. You get a bunch of pre-cut pieces to build an enclosure,which you assemble at home. Needless to say, this can save a bunch of time andeffort, but they may not fit your exact requirements, i.e. building a tall,shallow subwoofer versus a basic cube.

A flat pack cabinet offered at Parts Express made for theDayton Ultimax 18” subwoofer driver

More Power!

Onthe face of it, picking an amplifier seems like a simple task; more power isbetter than less, right? However, this is an area where great care must betaken, as too much power can quite literally break your subwoofer, either byburning up the driver’s voice coil or by beating the driver to death viaover-excursion. At the same time, an amplifier needs to be powerful enough todeliver the output you’re looking to achieve into the impedance load that yourdriver presents. Getting this balancing act right is the key to maximizingperformance without putting your investment at risk. This is also an area wherecommercial subwoofers have an important advantage; through the use ofcustomized DSP limiters, they can keep a woofer out of trouble while still havinga boatload of power on tap.

Achievingthe right balance means understanding the limitations of the subwoofer you’rebuilding. Some of this is relatively straightforward. Via the excursion graphsseen in the modelling section above, you can get an idea of how much power isrequired to reach the driver’s rated Xmax (maximum linear excursion) and Xmech(maximum physical excursion of the driver before damage occurs). Needless tosay, if you have enough power on tap to exceed the driver’s rated Xmech, greatcare must be taken with playback. Also as mentioned above, over-driving awoofer tends to be a lot easier with a low Qtc alignment, versus a Qtc of 0.707or higher. An important implication of this is that due to thedisplacement-limited power handling of a low Qtc enclosure, upper bass outputis also limited relative to a higher Qtc box.

Pro Tip:If you’re looking to power a sub or six with thousands of watts ofamplification, dedicated circuits with adequate amperage are a requiredaccessory.

Theother side of the equation is the driver’s thermal handling ability, wherethere’s a little more gray area. Yes, most drivers have a power handlingrating, but most raw drivers that we’ve seen tested can tolerate a great dealmore than rated power for short periods of time. Given the dynamic nature ofreal world content, this is something you can take advantage of. To put it inreal terms, pumping 4kW through a driver rated to take 1kW for some fraction ofa second isn’t liable to cause any harm. On the other hand, 4kW worth of sinewaves for a couple minutes is a great way to end up with a melted voice coil. Wecan’t stress this this enough: with great power comes great responsibility.

A 700W plate amplifier from SpeakerPower

Inaddition to raw power, one other aspect for judging an amplifier’s performanceis its effective bandwidth. So what’s the deal here? As it turns out, someamplifier manufacturers include a fixed high pass filter around ~20Hz. In manycases, this can be beneficial since the high pass filter acts to removeultra-low frequencies that most systems are ill-equipped to produce, whilereducing the load on the amplifier at the same time. Win-win right? In the caseof a subwoofer, this high pass filter will have the effect of neuteringperformance in the infrasonic range. Depending on your performance goals, thismay not be a big deal. On the other hand, if you’re looking to achieve in-roomextension down into the single digits, an amplifier with a fixed high passfilter is no bueno. In addition, poorlyimplemented filters can lead to ringing and phase anomalies, which can have anegative effect on sound quality.

Movingbeyond the capability of the amplifier, there is one other question toconsider: do you want a plate amplifier or an external amp? While plateamplifiers have the benefit of allowing an all-in-one-box solution, there are alot of reasons to consider an external amplifier. First and foremost, you canbuy a lot of power for cheap in the form of a pro-style external amp. If you’rebuilding multiple subwoofers, wiring is a lot easier as you don’t need a power outletadjacent to each subwoofer. Last but not least, it’s much easier to replace anexternal amplifier, as a new/different plate amplifier may require differentcutout dimensions in your enclosure.

Accessorizing

Asmentioned, a DIY subwoofer primarily consists of a driver, enclosure, andamplifier. Of course, there are a few additional ingredients that are worthconsidering as well. Measurement suites like Dayton’s Omnimic or XTZ Room Analyzer coupled with asophisticated EQ system like miniDSP will enable you toshape the response of your subwoofer. This is especially important when it’stime to actually use your subwoofer in your room. Without any sort of EQ, eventhe best subwoofer will have ugly peaks in the response that are a recipe forboomy bass. EQ will also let you shape the low end of your system; depending onthe amount of room gain your space exhibits, you may be able to achieve flatresponse well below 20Hz with little more than a low shelf filter. However, oneshould be careful in boosting system response, as this requires more amplifierpower and excursion from the driver. Remember, a 6dB boost equals 4 times thepower and 2 times the driver excursion, making it easy to run out of headroomin a hurry.

XTZ’s Room Analyzer II measurement system

Recommendations & Resources

Sonow the question becomes a little simpler: what the heck do you buy?Fortunately, there are quite a few solid drivers in all sizes out there from awide range of brands. Excellent performance can be achieved with woofers fromthe Dayton Reference HO and Ultimax lines, products from Stereo Integrity and iST among other popularbrands. An ideal driver will have a lot of linear throw (xmax), power handling,low distortion, and a reasonably flat frequency response up to at least 200Hz.In practical terms, this tends to mean a driver with a fairly large,multi-layer voice coil as well as faraday/shorting rings to improve linearity.A reasonably low Qts is desirable as well (>0.5), as system Q (Qtc as discussedabove) cannot dip below the driver’s Qts.

Whenit comes to power, again there is no shortage of high quality power amps outthere, particularly from pro-manufacturers like Crown, Crest, QSC, et al. Forplate amps, we give a nod to SpeakerPower, who is the OEM supplier for ReactionAudio, Power Sound Audio, and JTR among others. Also as mentioned before, aflat pack from Parts Express can also make enclosure construction a far lesslaborious task. For wiring, we recommend no less than 12AWG cable, though thelower the better for high powered applications.

How Much Does it Cost?

The flat pack/Ultimax 18" bundle is currently priced at $400 on Parts Express. If you use external amplification instead of the plate amp shown above, a Behringer iNuke NU3000 amp which can be bridged for a whopping 3kwatt peak output into a 4 ohm load runs $345 retail, but street price is less ($230 on Sweetwater). If you figure in a few bucks for things like paint, you'd be at around $7-800 for a very good 18" sealed sub. Cutting your own wood might be a little cheaper, but not by enough to really matter (especially if you make mistakes). You would be hard pressed to find a commercially available sub with this level of performance at this price.

Conclusion

Areyou a serious bassaholic on a tight budget? A DIY subwoofer might just be thefix you’re after. For not a lot of cash it’s possible to build a sub that cancompete with the very best subwoofers on the market. Beyond that, you will havethe pride in knowing that you built an integral part of your home theater, andthe knowledge that you have a unique piece of equipment that Joe Sixpack can’tjust pick up at the local big box store. However, realizing the valueproposition of DIY requires great care, both in terms of getting the designright, as well as the implementation. Don’t forget the age old tip: measuretwice, cut once. Are you a DIY subwoofer enthusiast? Make sure to share on ourforums, including pictures of your build.