Earlier in the week, I went over the characteristics of an “SQ” subwoofer driver. When it comes to a subwoofer, however, the driver itself is only half of the way it will sound. The best SQ driver out there will still sound like garbage in a poorly designed and built subwoofer enclosure (also known as a sub box). The purpose of this article is to briefly outline the various types of subwoofer boxes one can expect to run into for mobile audio, highlight some design principles of SQ sub boxes, share some of my own techniques to give you an idea of what goes into a well built subwoofer box, and ultimately give you an understanding of how crucial the design of a given subwoofer box is in how the subwoofer will sound. It should give you some insight into subwoofer box design.

 

What is the purpose of a subwoofer box?

Before we begin talking about subwoofer box design, we need to first understand the purpose of a subwoofer box. Why do we even mount a subwoofer to an enclosure? There are three primary reasons for this. The first is to separate the back wave of the subwoofer from the front wave. As you may have noticed, frequencies become decreasingly directional the lower they get. Inversely, frequencies become increasingly directional the higher they get. From about ~120hz down, it is difficult if not impossible to identify the location of a subwoofer in a given room. These frequencies are large, and they wrap around objects. When we play a note through a subwoofer, the front of the cone is playing that note. However, the rear of the cone is also playing the same note, out of phase. Since these low frequencies wrap around objects so easily, they will cancel them out. If you take your subwoofer out of its box and attempt to play a bass note through it, you may notice that you don’t hear any bass at all. This is the reason for that.

The second reason we put a subwoofer inside an enclosure is to limit the excursion of that subwoofer. While some subwoofers are designed to work in free-air environments, most subwoofers will need to be inside an enclosure in order to prevent form bottoming out, or exceeding their mechanical excursion limitation. The air inside a box acts like a spring; compressing and decompressing as the cone of the subwoofer oscillates. The larger the box, the less influence the volume of air has on the subwoofer’s excursion.

The third reason is, in the case of vented and bandpass alignments, to provide an augmented output for the subwoofer over what you would find in a sealedbox. Augmented output by definition refers to output provided by the box in addition to what the subwoofer’s cone produces on its own. While there are many forms of augmented output (including passive radiators and various horns), we will be briefly discussing vented and bandpass alignments as they are the most commonly found.

 

Types of Subwoofer Boxes

To start, we have three primary types of subwoofer boxes used for mobile audio purposes as we referred to in the previous post. We have sealed, vented, and bandpass designs.

Sealed subwoofer enclosures are the simplest of all of the designs, and quite arguably the highest in sound quality. In most instances, they also require the least amount of space, least amount of weight, and least amount of complexity. They are my enclosure of choice for SQ-based installs. We will focus on this type of enclosure for most of this article. These boxes exhibit the lowest overall group delay, and in many cases, the most linear frequency response.

Vented subwoofer enclosures are slightly more complicated as they introduce a port, or a vent. The vent’s volume is calculated based on the net volume of the enclosure to create a tuning frequency. At a given range around a tuning frequency, the enclosure itself limits the excursion of the subwoofer, and the port instead creates additional output. This easily provides a 60% increase in output near that tuning frequency. However, as with any augmented output, it introduces a potential problem called group delay. More on that later.

Bandpass subwoofer enclosures are often the largest of the three, and include two chambers as well as the possibility of an additional port, depending on the type that is designed. These typically have an excellently low group delay in their passband, and provide even more augmented output than a vented box. However, they are only effective in producing frequencies in that passband, and will have a sharp roll-off in frequencies higher than and lower than the given passband. For example, a bandpass box may be tuned to produce very high output at 40-65hz, but will fall flat above 65hz. They are great for SPL boxes, but are often poor choices for SQ boxes due to the limited frequency response they exhibit.

 

So, what makes a great SQ subwoofer enclosure?

A great SQ subwoofer enclosure allow the subwoofer to have as linear of an in-car frequency response as possible, while reducing group delay to inaudible levels, and providing an adequate amount of structural rigidity to prevent any box flexing or vibration.

Group Delay

We’ll begin with group delay. With sealed boxes this is not an issue as the subwoofer’s cone is producing all of the frequencies. However, this is a different story with a vented box. Since the subwoofer cone itself is not producing the vast majority of the output there is a delay between when the cone itself applies a force to the airspace inside the box, and when the port produces that note. As a rule of thumb, the product of the group delay at a given frequency, and the frequency itself, should not exceed 400 in order to keep from being audible. Let me provide an example:

The above is a group delay plot for a box I designed for a Cerwin-Vega 15D subwoofer. A high pass filter was applied, but for the purposes of this article, we will ignore it. Instead, pay attention to the lighter blue, thinner group delay line. At 30hz, we have 16ms (milliseconds) of group delay, for a total of 480. At 40hz, we have ~6.5 ms of group delay, for a total of 260. Based on my rough calculations, group delay begins to become a real issue at frequencies below ~32hz. I wouldn’t consider this to be too great of an issue, as any bass in that region is synthetic anyway, and most group delay at that low of a frequency cannot be distinguished with the human ear. However, let’s look at another.

The above is a group delay plot for an Alpin SWR-1521D subwoofer in a large vented box. As before, we will focus on the light, thin blue line. Here, we find a group delay of 15ms at 40hz for a total of 600. This begins to creep dangerously into a region where group delay becomes quite audible. It is not as bad as some of the SPL boxes I’ve seen as the tuning frequency is still very low, but it is too high for SQ use. To create a comparison point, here is that same subwoofer in the same net volume in a sealed box:

As we can see here, group delay has been reduced to practically nonexistent levels.

One must be careful when designing a box to check the group delay. In SPL-based pre-fabricated boxes that I’ve modeled for people in the past, I’ve found group delay in excess of 50ms at 45hz. This kind of box would sound horribly uncontrolled, muddy, and boomy. There would be no detail to the bass notes being produced and would instead sound like a loud rumble.

Frequency Response

This is a big one that’s also sorely misunderstood among many self-proclaimed experts. Frequency response refers to the amplitude of all frequencies that one would expect to achieve out of a given loudspeaker. The frequency response of a subwoofer is determined by a variety of values including the airspace inside the subwoofer box, the tuning frequency of a port (if applicable), and any filters you may have applied on your amplifier. In all respects, a sound quality oriented subwoofer should have as close to a perfectly flat frequency response as possible. In otherwords, if we are playing a 40hz note at 100db, we should expect to also play a 80hz tone at 100db. If we have large variations in frequency response between certain frequency ranges, this will change the overall sound of the subwoofer and will almost always result in an undesirable sound that may come off as “boomy.” While designing a subwoofer box, many will model a vented alignment using BassBox Pro, WinISD, or a variety of other enclosure modeling software. After a few quick steps, they will end up with a raw frequency response that looks something like this:

Here, we’re paying attention to the green/black line. On the surface, we would be led to believe that this is a relatively flat frequency response that will sound relatively good. Unfortunately, this is not the case with car audio. When you place a subwoofer inside a car, you apply two concepts called boundary loading and room pressurization gain.

Boundary loading refers to the nature of low frequency sound waves wrapping around objects. These frequencies will wrap around the box, and reflect off of the surface above, below,  to the sides, and behind the box. If a subwoofer is placed in the corner of a room, for example, that corner improves the boundary loading of the subwoofer and acts as a horn; amplifying its output. Room pressurization gain refers to the pressure created by the subwoofer in a given volume of air. The smaller the volume, the more pressure is created by that subwoofer. The combination of these two concepts is also referred in many car audio circles as cabin gain. Without the ability to model or simulate cabin gain, our simulated raw frequency response is useless. Fortunately, there are tools out there that allow us to do this. Here is a rough simulation of the cabin gain of a typical midsize sedan where the subwoofer is placed directly against one side of the vehicle’s trunk:

Now, let’s see what the frequency response of the subwoofer we modeled earlier looks like with that cabin gain applied:

Here, we find that the subwoofer we previously modeled as flat in its raw frequency response (green line), is actually not at all flat. Theoretically, a 6db increase represents a doubling in output, although when it comes to frequency response, the human ear typically perceives a 10db increase as a doubling in output. Here, we see that this subwoofer in this box will produce a 35hz tone twice as loud as it will a 95hz tone. This box would not be suitable for sound quality use, and highlights the problem exhibited by almost all of the pre-fabricated vented subwoofer boxes I have come across.

The challenge when designing a subwoofer enclosure is to achieve as flat of a frequency response as possible in the range you expect it to perform while keeping the box size reasonable and group delay manageable. While some subs thrive in vented alignments, I prefer to stick to sealed boxes as they will typically exhibit a very flat in-cab frequency response. Our focus with regard to frequency response will be 30-100hz, as our door midbass drivers are expected to pick up the frequency response from that point forward, and very little synthetic material will play below 30hz. 20hz represents the lower limit of human hearing. 40hz is about the lowest note one will expect to play with natural instruments. For reference, 41hz is the lowest note on a bass guitar.

Structural Rigidity

This is one aspect that makes the custom subwoofer boxes I build for people unique. To begin, MDF is the material of choice as it is a dead material that does not buzz or resonate, and is relatively rigid. It is also inxpensive, coming in at $25-$35 for a 3/4″ 8ft x 4ft sheet, and can be purchased in 1/2″ and 3/4″ thicknesses. 1″ thick MDF can be found, but it is uncommon.  The disadvantages of using MDF is that effective dust extraction and a dust mask should be used, as the dust particles blown into the air are tiny and will not sit well with your lungs long-term. In addition, it dulls saw blades and router bits at an accelerated rate when compared to real wood.

In building a subwoofer box, one needs to understand that the primary strength of the box is in the joints, which are glued together using a reliable wood glue like Titebond II. Any other extra strength glue such as Gorilla Glue is absolutely unnecessary for subwoofer box building and provides no benefit. Two ends of a subwoofer box glued together with a simple clamped butt-join with Titebond II will never be cleanly separated. The joint will be stronger than the MDF it is holding together, making screws unnecessary. Furthermore, a healthy bead of glue will create an air-tight seal that will produce a box with zero leaks, eliminating the need for caulk. My method of choice for building subwoofer boxes is simply glue and lots of clamps.

Wait about 30 minutes for the clamps to dry before removing them, and continue on to the rest of the box.

I prefer not to use screws to tighten boxes together due to the inherent risk of the material splitting, and the added expense of the screws. I do not believe that screws provide any benefit except for allowing one to build a box more quickly than they would with clamps. This is a benefit I am willing to forgo to build some of the best subwoofer boxes money can buy.

Once you have the box built, the next aspect of structural rigidity is the use of internal bracing. Internal bracing is designed to add rigidity to the MDF in flat areas where it can flex or vibrate. My preference for subwoofer box bracing is 4-piece routed arc bracing.

 

This method allows me to use the largest braces I can, increasing total box rigidity. Each wing of these 4-piece braces adds rigidity to three panels on the box. Together, they make the box strong enough to be used as a car jackstand.

The last part of structural rigidity is the use of extra-thick baffles. For all but the strongest subwoofers, I use either two 3/4″ thick sheets, or one 1/2″ thick sheet. This extra thickness serves three purposes. The first is to strengthen the plane on which the subwoofers actually mount to. This receives the force of the subwoofer directly and needs to be the strongest in the box. The second purpose is to provide a flush mount for the subwoofer to better protect it from anything one might have in the trunk. The third purpose is for aesthetics. A flush mounted sub looks incredibly good.

 

It is worth noting that the above requires precision routing work. A circle jig on a good router is absolutely necessary for achieving these results, but the effort is well worth it.

 

Acoustic Stuffing/Wall Treatment

One last aspect of an SQ subwoofer that is important is the use of acoustic stuffing. Many will attempt to write this off as insignificant, but I have seen a notable difference in both mobile audio and home audio subwoofers. My personal preference is mineral wool, as it’s easy to use and isn’t as irritating to the skin as rigid fiberglass board. I use 2.5″ thick material, and I line the entire back wall of the subwoofer box with it, attaching it with 3M adhesive spray. The purpose of the acoustic stuffing is to absorb some of the backwaves coming off the subwoofer. A subwoofer will produce harmonic frequencies much higher than the range in which it is designed to play. What happens here is that those higher frequency harmonics will “bounce around” inside the back of the speaker due to the cone playing backwaves, and will eventually find themselves hitting the cone again with a delay. While that is a simplified explanation and doesn’t do full justice to what is actually going on, it should give you a general idea of what I’m referring to. This is considered distortion, and compromises the transparency of the subwoofer. The acoustic stuffing aids in absorbing these harmonic frequencies, which are far higher in frequency than the subwoofer’s filtered range. An additional benefit to acoustic stuffing is that it helps flatten the frequency response by simulating a lower Q, typically reducing peaks by 1-2db.

 

Appearance

Lastly, of course, is your cover. As everyone perfectly well knows, a good looking box always sounds better than a bad looking box. Your mind will often fool you into thinking something that looks good also sounds good, so a professional carpeting job is generally required in a subwoofer box. I purchase latex-backed cabinet fabric from Parts-Express.com, but have also used a variety of other fabrics from a local fabric store, even black shag!

 

In closing…

There you have it. I hope that the above has served to outline the characteristics you will find in a well-built and well-designed subwoofer box and give you some good knowledge on general subwoofer box design. While there are more technical and more advanced topics to discuss, this article should at least have given you a decent understanding.

As you may have picked up through this article, I design and build high-end SQ subwoofer boxes as a side job. Each box is custom made and designed to allow a subwoofer to sound as tight, accurate, clean, and flat as it possibly can. If you are interested in having me build you one, drop me a comment and I will get in touch with you.

If you have any questions or comments, feel free to also leave a comment.

– XTR