Subwoofers are probably the most misunderstood and hard to integrate components of a music or home theater system, but when done right, they have the potential to transform your speakers to whole new level. I’ve bought and built quite a few subwoofers over the years, and I’d like to share what I’ve learned about them with you.
The first few sections of this guide are about the basics like how to choose the right subwoofer(s) and how to make them sound good (they aren’t exactly plug and play). Later sections are about building your own subwoofers for a fraction of the price of the commercial units. Finally, for end game deep bass reproduction, you might want to read the section on the double bass array (DBA).
Please let me know if you have any questions or comments in the comment section. I will update this guide based on your input.
Table of Contents
First things first, what’s a subwoofer?
The image of a big black box housing a big speaker driver pumping out deep bass is probably the first thing that comes to mind. A subwoofer’s purpose is to complement the main speakers, many of which have a hard time reproducing the lower octaves of music and sound effects at satisfactory levels.
Types of subwoofers
For home use, the most popular types are sealed and ported (also known as vented). Sealed subwoofers have only the driver that produces bass while ported subs have one or more vents usually in the shape of a circle or rectangle that produce bass in addition to the driver.
Sealed subwoofers are compact. While large sealed subwoofers exist, a sealed subwoofer can be made only slightly larger than the driver dimensions (think of a 13" cube housing a 12" driver).
Ported subwoofers are quite a bit larger, but they can produce significantly more output (up to 10 db) within a certain frequency range as compared to sealed subwoofers. Some may suffer from port noise (aka chuffing) when you push their limits though.
What to look for in a subwoofer?
Box dimensions: It depends on where your priorities lie. If compact size is important to you then large ported subwoofers are probably out of the question. That doesn’t mean that all ported subwoofers are large – there are relatively compact ported subwoofers, but they have to sacrifice some deep bass output in order to fit in a compact cabinet.
Driver diameter: All other things being equal, as the driver diameter gets larger, the bass output produced by the driver increases. Subwoofer drivers typically come in nominal diameters of 8", 10", 12", 15" and 18" (20, 25, 30, 38, 46 cm respectively). The most popular diameters for home use are 10" and 12". A subwoofer can have more than one driver, but most have one.
A good rule of thumb is that as you go up one size you get about twice as much bass output. For example, a 12" driver has about double the output of a 10" driver assuming the drivers have similar technical parameters other than surface area, and the amplifier power used is the same. There are exceptions though. There exist monster 10" drivers that have more output than an average 12" driver, but those drivers can get expensive and they often require large amounts of amplifier power to reach their full potential.
Amplifier power: After driver size, the next important thing to consider is the available amplifier power. Most commercial home subwoofers have built-in amplifiers. The amplifier needs to supply enough power to the driver in order to reach satisfactory output levels. It’s a good idea to disregard any peak power ratings as they are often inflated and there is often no easy way to compare one brand’s peak ratings to another. What matters is RMS power (also known as continuous power) preferably given with a distortion rating (like 1% THD) to make true apples to apples comparisons.
There’s no need to sweat over small differences in amplifier power. For example a 25% difference may look big on paper when comparing a 400W RMS amplifier to a 500W RMS amplifier, but it only amounts to 1 db more output all other things being equal, which is so small that it’s hard to tell the difference in practice. For a meaningful output difference, an amplifier should be able to provide twice the power which is equivalent to 3 db of additional output.
Frequency response: Finally, take a look at the +/- 3db frequency response specification of the subwoofer. A subwoofer that can authoritatively go down to 25 hz or lower is ideal. However, frequency response by itself doesn’t mean much. These days a lot of sealed subwoofers use digital signal processing (DSP) or other means to boost lower frequency output at lower volumes. If the driver on a subwoofer doesn’t have enough surface area and the amplifier power is limited, it doesn’t matter if a subwoofer reaches down low frequencies because those low frequencies won’t be loud enough to hear or feel at higher volumes.
How many subwoofers?
How about zero?
It’s strange to suggest going without subwoofers on a guide about home subwoofers, but if you have terrible room acoustics that are hard to deal with, it might actually be a good idea to go without a subwoofer. Contrary to the common belief, boomy bass isn’t always caused by cheap subwoofers. In fact, more often than not, the main culprit is poor room acoustics – walls made of brick or concrete and little to no furnishings can cause plenty of nasty room resonance issues.
If you clearly hear an echo when you clap your hands inside a room, you will likely experience boomy bass when you put a subwoofer in that room regardless of the quality of the subwoofer. There are things you can do to fix some of the acoustical issues, but not all of them are practical. Don’t get discouraged though. Just be aware that in some cases it might be better to have no subwoofer than having one.
One subwoofer placed right
If you are going to have only one subwoofer like most people, getting the most even response out of your subwoofer is all about placement, and there are really only two optimal locations for a single subwoofer for that purpose.
Going dual: Buy two get two free
If you have the funds and space for two subwoofers, dual subwoofers have several advantages over a single subwoofer assuming their placement in the room is right:
- Quadruple the output over a single subwoofer if they aren’t placed too far away from each other. You literally get two additional subwoofers worth of output for free thanks to a physical phenomenon called mutual coupling of sound waves.
- Little change in bass output on a horizontal plane. In other words, no matter where you sit on the sofa, the bass will sound about the same.
- Instead of a large subwoofer that is difficult to place (or hide), two smaller subwoofers can be used.
- Two subwoofers can function optimally in a lot more locations.
More than two subwoofers?
There is no upper limit to the number of subwoofers you can put to use, but for most people, having two subwoofers is the sweet spot. If you just need more bass output you can colocate subwoofers (a fancy word for placing multiple subwoofers in close proximity), and they will act like one large subwoofer.
You can also have more than two subwoofers distributed across the room, but they require careful measurements to function properly. If you are curious about non-colocated subwoofer setups with more than two subwoofers, head over to the double bass array section.
How to setup subwoofer(s)
It’s extremely unlikely to randomly place a subwoofer and expect it to sound good. Subwoofers require some effort to sound their best.
Where to place your subwoofer(s)
Try to avoid corner placement since it will produce uneven bass across the room. If possible, place your subwoofer along the front wall preferably right in the center. If the center front wall isn’t available, you can try placing your subwoofer along the center rear wall.
Having two subwoofers offer a lot of flexibility with placement. They can be placed in each corner of your front wall for a start. You can also move them closer toward the center of the wall as long as each subwoofer is roughly an equal distance away from the side walls. Placing the subwoofers at locations of 1/4 the front wall width is recommended to achieve the smoothest response. For example, if your front wall is 4 meters wide, try placing your subwoofers about 1 meter away from the side walls.
Where to sit
If possible, avoid sitting close to walls (especially the rear wall) where bass can get overwhelming in a bad way. Sitting in the middle of the room isn’t a good idea either where bass energy is at its weakest.
The Crossover Frequency
The crossover frequency is the frequency that your main speakers start to hand out bass reproduction to your subwoofer(s). 80 hz is usually a good overall crossover frequency, but if your speakers are particularly bass shy, you might want to go a little higher to 100 or 120 hz for a smoother transition. Virtually all AV receivers (AVRs) have built-in crossover capabilities. So, if you are using an AVR to set the crossover, it’s best to disable the crossover setting on your subwoofer(s) or set it to the maximum setting possible.
Once subwoofer placement is (near) optimal and you are hopefully not sitting too close to the rear wall, there is one more step left to enjoy smooth bass reproduction in your room, and that is applying room correction to fix the room resonances that can’t be solely fixed by subwoofer placement.
If you own a modern AV receiver, it’s a good idea to give your AVR’s automatic room correction capabilities a try. Many AVRs produced in the last decade support some form of automatic room correction, and they all work similarly although the results may vary depending on the quality of the room correction algorithm used (some are very basic) and the severity of the acoustic issues.
You place the so called calibration microphone that comes with the AVR in one or more listening positions, preferably attached to a tripod, and let the AVR to take measurements by playing test tones for each speaker including your subwoofer. After the measurements have been completed, corrections are applied at certain frequencies to reduce the audibility of resonances in your listening position. Although it’s called room correction, the effect of the corrections is usually limited to a small listening area where you have placed the calibration microphone.
For advanced users who have a good measurement microphone (you can probably get away with the AVR’s microphone although some caveats apply), there is a powerful free software application called REW (Room EQ Wizard) that can be used to measure your subwoofer’s response. Then you can apply corrections to it by using the parametric EQ capabilities available on your computer or an external device called DSP. More on that in the DIY section.
Granted, DIY (Do-It-Yourself) isn’t for everyone, but for those who are interested in building their own subwoofers and making their own acoustic measurements, the potential rewards can be quite high – you can achieve high quality bass reproduction for a fraction of the price of an equivalent commercial subwoofer setup and professional calibration, not to mention the personal satisfaction of building something yourself that would be put to good use years to come.
Building a subwoofer isn’t really hard. It can be done with little to no woodworking skills and minimal tools. Having had no prior experience, I built my first subwoofer with nothing more than a power drill, a jigsaw and a few wood clamps. After that, I’ve built many subwoofers over the years, and aside from the fact that latter ones are looking better, they’ve all worked fine.
A subwoofer consists of three components: one or more subwoofer drivers, an amplifier and a box. Sourcing the driver(s) and amplifier(s) is relatively easy. The only thing left is to build a box out of MDF (Medium Density Fiberboard) or good quality plywood. If you don’t have the tools to make the cuts, your local home improvement store will likely do the job for you for a small fee. You may even purchase ready-made subwoofer flat-packs online if they are available where you live. Once you have the pieces cut, they can simply be glued together with some wood glue and held together with a few clamps until the glue dries. More on box construction later.
How to choose a driver
Among DIY subwoofer builders, the driver of choice is often a car subwoofer driver for a number of reasons:
- They are widely available and many are reasonably priced.
- They have the necessary moderate to high excursion capabilities to reproduce special sound effects like explosions at high volumes, a desired feature for home theaters.
- Many are designed to work in relatively compact enclosures, which is always a plus.
- Finally, many of them are tolerant of abuse since car subwoofer manufacturers know it all too well that car subwoofers are pushed beyond their limits quite often.
Most manufacturers provide the Thiele/Small (T/S) parameters for their subwoofers. The T/S parameters can be used to compare drivers objectively, and they are also essential for determining the right box size. For home theater use, the most important parameters are:
- Maximum linear excursion (Xmax), the maximum distance a subwoofer cone can travel in one way without gross distortion. Xmax is important because as you go down one octave such as from 40 hz to 20 hz, the subwoofer cone needs to travel 4 times the distance to have the same level of output.
- Usable surface area of the driver (Sd) is the area of the driver excluding part of the surround and outer frame. The more the surface area is, the easier it is to produce bass.
- Resonant frequency (Fs) is the natural resonant frequency of the cone. This is the frequency where the driver is the most efficient (outside of a box).
You will want Xmax as high as possible and Fs as low as possible. Sd doesn’t change significantly across the same diameter drivers, but it’s very important because when you multiply Xmax with Sd, you get what is called Vd (volume of displacement).
You’ve probably heard the expression “there’s no replacement for displacement” used for car engines. Similar to car engines, the larger the Vd of a driver is, the larger the deep bass output will be all other things being equal. For example, if the Sd of two drivers are similar (they both have the same diameter), the one with twice the Xmax will have twice the deep bass output again all other things being equal. The natural conclusion to this is that a large diameter driver with little Xmax and a small diameter driver with plenty of Xmax can produce about the same amount of deep bass although you will probably need to feed the small driver a lot more power to get there.
There are other T/S parameters such as QTS and Vas which are important in determining the optimum box size for a subwoofer. If you require a very compact box size without sacrificing too much deep bass output, make sure that these two parameters are as low as possible.
The easiest subwoofer box to build is a sealed box with no ports. They can be made fairly compact and are tolerant of mistakes. There are free software applications such as WinISD that can help you simulate the behavior of subwoofer enclosures after you enter your driver’s T/S parameters. They also let you see whether you exceed the excursion limits of a driver when you feed too much power to it, but for most drivers, unless the manufacturer recommends otherwise, you can simply use the following approximate sealed box volumes:
- 10" driver: 14 liters (0.5 cubic foot)
- 12" driver: 28 liters (1 cubic foot)
- 15" driver: 56 liters (2 cubic foot)
If you are going to put two drivers in the same box (such as a dual opposed subwoofer build), you need to multiply the required volume by 2. Please keep in mind that when calculating the volume of a box, you need to multiply its internal width, height and depth dimensions.
While car subwoofer drivers are popular among DIY subwoofer builders, when it comes to amplifiers, inexpensive multi-kilowatt professional audio (pro audio) amplifiers are clearly preferred. Pro audio amplifiers may not have great signal to noise specifications, but they excel at providing lots and lots of power which is precisely what is needed for power hungry subwoofers. Some of them even have built-in DSP capabilities which make it easy to dial in crossovers and fix response anomalies caused by room acoustics.
There is one major downside to using pro audio amplifiers though – virtually all of them have built-in fans and some of those fans can get quite noisy. It’s hard to keep an amplifier providing kilowatts of power cool without a fan. Fortunately, there are models with relatively quiet fans that only start spinning when driven hard. For those amplifiers with loud fans, putting them in another room or replacing the existing fans with quiet ones are the options to consider, but the latter would likely void your warranty.
Another potential downside is that unlike commercial subwoofers that use built-in plate amplifiers, pro audio amplifiers are often stand-alone units. Plate amplifiers do exist for DIY subwoofers, but they aren’t very cost effective. On the upside, stand-alone amplifiers are potentially more reliable because they have better cooling.
Building a box
Building a basic sealed subwoofer box isn’t hard. It requires six sides cut out of 18 mm (~3/4") MDF or good quality plywood (preferably made out of void-free hard woods such as Baltic birch), a cutout made for the driver, a brace or two to make the box stronger, some wood glue and a few clamps to hold things together until the glue dries. Using silicon caulk to seal all internal joints will make the box air tight.
While it’s not strictly necessary, you can put some polyester fiber stuffing inside the box to make the internal volume appear slightly larger to the driver(s) so that they can play a little deeper. For large boxes, stuffing also eliminates most resonances caused by sound waves bouncing around inside the box.
Drivers can be mounted by using regular wood screws (pre-drilling pilot holes are highly recommended to avoid splitting the wood). Heavy-duty drivers weighing over 10 kg (~22 pounds) may require thicker baffles (mounting surfaces) and stronger screws.
Dual opposed subwoofers:
Subwoofer boxes housing a single driver tend to “walk around” when pushed hard. To fully eliminate that behavior, you can use two drivers placed on the opposite sides of the box. Any movement made by one driver will be immediately cancelled out by the opposing driver. You can even balance a coin on top no matter how much the drivers are moving! Dual opposed subwoofers also have the added benefit of more even bass output across the room regardless of where the drivers face.
When using two drivers, you can connect them in series (doubles the resistance) or in parallel (halves the resistance). For example, two 4 ohm drivers connected in series would appear as a 8 ohm load to the amplifier. When connected in parallel, they would appear as 2 ohm load. Subwoofer drivers often come with instructions on how to connect multiple drivers together. Having a multimeter helps to verify the final resistance. Just keep in mind that some amplifiers may not support low resistances especially when they are set to bridge mode.
Making it look good:
Building a box isn’t hard, but making it presentable may take some effort. There’s of course nothing preventing you to leave the box as is. It won’t affect the performance one bit.
If you have a router, you can apply a roundover or chamfer to the edges for a nicer look. A router also comes handy when making perfect circular cuts for the drivers. If you are going to flush mount the drivers, a router may even become a necessity.
There are basically two options for finishing. You can paint the box or apply a thin veneer made of vinyl or wood. Whatever path you use choose, the surface needs to be sanded smooth first. This can be done by hand, but if you are going to build more than one box, you might want to buy or borrow a random orbital sander which can speed up the process significantly.
If the box is made of MDF and you’ve decided to paint it, the edges must be sealed first – a mixture of half water and half wood glue can be used for that purpose. Otherwise, the edges will soak paint, and the surface finish will be uneven. You can use a foam brush to apply paint if you don’t mind some brush marks, but spraying is clearly superior. If you don’t have a paint gun, spray cans can also be used. After the paint dries, it’s a good idea to apply several coats of clear coat to protect the paint, which also gives a nice satin or gloss sheen.
Measuring and correcting output
The frequency response of subwoofers in-room has nothing to with their natural outdoors response. Resonances (also called standing waves) occur inside the room, and the resonant frequencies are determined by room dimensions. The smooth open-air response of a subwoofer is replaced with peaks and valleys (also called nulls). Peaks cause boomy bass while nulls do the opposite: weak bass. To make matters worse, the magnitude of peaks and nulls change from one location to another in the room – any corrections made by using the automatic room correction features of an AV receiver or by using DIY parametric EQ will be only applicable to a small area where the measurements are made.
A powerful free software application called REW (Room EQ Wizard) is a popular choice to make measurements and to automatically calculate the corrections necessary to reduce the magnitude of peaks. Nulls are better left alone because trying to correct them can easily overload your subwoofer’s amplifier. REW also has a quite accurate Room Sim feature that can be used to simulate the frequency response of your subwoofers in different parts of your room without having to move the subwoofers around and make hundreds of measurements.
End Game: Double Bass Array
If you have a rectangular room with all parallel surfaces (no sloped ceilings) and have access to at least two subwoofers, there is a special configuration called double bass array (DBA) to eliminate virtually all room resonances up to a certain frequency.
For two subwoofers:
- Center a subwoofer (half way from the side wall and floor) against the front wall and another subwoofer against the rear wall.
For four subwoofers
- Raise the front two subwoofers half way from the floor, and place them at locations of 1/4 the front wall width. Repeat for the rear wall.
Now, you need to delay the signal going to the rear subwoofers by a certain amount, which can be calculated by simply dividing the length of the room by the speed of sound (about 0.343 meters/milisecond). For example, for a 5 meter long room, the delay would be 5/0.343 = about 14.5 miliseconds. You will also need to reverse the polarity of the rear subwoofers, which can be done by flicking the polarity switch on powered subwoofers or switching the positive and negative wires for passive subwoofers powered by an external amplifier.
Here is what happens: The sound wave generated by the front subwoofers are cancelled by the rear subwoofers at the exact moment they arrive at the rear wall due to the delayed rear signal and reversed polarity. It’s as if there is no rear wall and hence no resonances between the front and rear walls. The resonances between the remaining walls aren’t excited either thanks to the strategic placement of the subwoofers in the first place.
The great thing about DBA is that the bass output across the room will mostly be free of resonances independent of where you sit. You should still avoid sitting too close to the walls though.
Keep in mind that delaying the signal to the rear subwoofers may require using an external DSP device or an amplifier with built-in signal delay features. It also helps to have the ability to make measurements to fine tune the setup. For more information, you can take a look at the DBA Wikipedia page.