DIY jobs aren’t for everyone. However, building a subwoofer enclosure is one of the best ways to add your personal touch to your car’s audio system. Whether it’s a basic sealed box or a precisely designed kerfed / ported box, making sure your sub is mounted in an appropriate box can make a world of difference.
Bracing is one of the most important things to consider when you’re building a subwoofer box. It’s nearly always recommended to prevent unwanted coloration from panel resonances.
So, when is bracing a subwoofer box necessary? How should you design bracing for your subwoofer box? And most importantly, How to properly brace a subwoofer box?
Why Subwoofer Box Bracing Matters
To make things clear, let’s start off by laying a little groundwork for our discussion. As you may already know, sound is defined as vibrations that travel through the air or another medium as an audible mechanical wave. To put it simply, sound is a mechanical vibration.
Sound reproduction is an attempt to store those vibrations in some fixed medium for later playback that will mimic, as closely as possible, the original waveform of the sound waves (such as spoken voice, singing, instrumental music, or sound effects…etc), with the term fidelity used to describe the degree of accuracy, precision, and how well the process of sound recording is done.
There are several factors that can negatively impact the fidelity in sound reproduction, but very few are less likely to cause problems as the final step, where the stored sonic event is converted back into mechanical vibrations by a loudspeaker.
In a bass system, the subwoofer’s main purpose is to vibrate and radiate the reconstituted sound. All the other components in the system, including the enclosure that provides physical support to the subwoofer should be completely inert and not undergo any mechanical vibration at all. In other words, the subwoofer box must be infinitely stiff to prevent extraneous vibration.
In the real world, this is almost impossible to achieve. However, you can attempt to cut down on any extraneous vibration and make it inaudible to the listener.
To achieve this, the enclosure has to be made of material that is strong enough to handle the pressure, dense enough that it doesn’t absorb sound waves, and is sturdy enough that it doesn’t vibrate excessively, in order to drive the natural frequencies of vibration out of the audible range, or at least out of the range where the cabinet vibration will unfavorably interact with the drivers.
To put it simply, you need to consider building a solid enclosure with bracing to prevent or at least reduce resonance and coloration of sound.
By bracing a subwoofer box, you’ll raise its resonant frequency to a point where the panel’s stiffness will keep it from moving, and hence, resonating.
Speaking of resonance, typical subwoofer box panels resonate in the hundreds of Hz or higher. If the panel can’t vibrate sufficiently to create audible coloration, its resonant frequency doesn’t matter that much. For example, if a panel’s natural resonance frequency is 600Hz, it will not resonate much from a subwoofer that’s operating from 40Hz to 80Hz.
This is not to say that the panel will not react to the internal pressure created by the driver. Quite honestly, if its natural resonant frequency is 600Hz, that means that’s the frequency it will primarily vibrate at in response to that pressure.P.S: It’s impossible to entirely get rid of all resonances. The trick is to get rid of as much as possible, in a controlled manner.
Furthermore, due to tremendous amounts internal pressure created by the driver at high output levels, strength is very important with subwoofer enclosures. This is especially true with today’s high excursion subs.
I mean, think about it: say you have two beefy subwoofers mounted in a sealed enclosure. As the subwoofers are moving into the box, they create a strong pressure on the walls. These walls will move away from the box to counteract this pressure. Likewise, if the subs are moving outwards the box, the walls will flex inwards.
In other words, the walls of the box will try to absorb the energy generated by the driver trying to convert its movement into sound.
That being the case, bracing is mostly used to add structural integrity to the enclosure by preventing the box walls from physically puffing out and sucking in at sub-resonant frequencies. It’s an effective way of making the panels stiffer, and stiffer walls will not flex or resonate much.
Generally speaking, thicker panels tend to outperform thinner braced panels. However, sometimes bracing can be even more efficient than building the enclosure walls with thicker material.
Speaking of which, thickness serves double duty on the baffle to support the subwoofer and to add rigidity. But for the rest of the box, one get away with smart use of bracing.
The reason we often recommend bracing is because it takes up less space, uses much less material, hence less weight. This results in a stronger and stiffer box than just adding extra layers will.
It should be noted that ported enclosures by nature have higher internal operating pressures than sealed enclosures. In other words, the amount of pressure acting on the enclosure walls at Fb (Port Tuning Frequency) is greater in a ported box than in a sealed one at the same frequency. Admittedly, you can’t have higher SPL outside the box without having higher SPL inside it.
This is the reason why bracing is more critical for ported boxes than it is for sealed ones.
When is Bracing a Subwoofer Box Necessary?
Well, the short answer is “always”. I mean, unless you are building an enclosure for a very small subwoofer where you can make the panels solid and stiff enough to not flex and resonate, you always need to consider bracing. This is especially true if you are a serious audiophile who’s into SPL competition.
How to Properly Brace a Subwoofer Box
As mentioned above, the deal with bracing is to increase the stiffness of the subwoofer box and to prevent unwanted coloration from panel resonances.
It goes without saying that you need to make the enclosure stiff enough that the motion of the panel due to the internal pressure fluctuations is kept to bare minimum. Not only that, but you also need to make the panel stiff enough to achieve a resonant frequency well above the low-pass crossover frequency of your subwoofer.
Generally speaking, the amount of bracing required for optimal results will depend to a great extent on the amount of energy you are dealing with, the shape of the box, and the panel size.
Keep in mind that for bracing to work, the braces must go through both sides of the box to prevent it from flexing not only inwards but outwards.
Here are some basic bracing guidelines to keep in mind when you’re building a subwoofer box:
- Strive to turn long spans into multiple shorter spans.
- Keep in mind that the more panels a brace joins together, the better.
- Brace across to opposite panels when possible, otherwise brace to adjoining panel.
- Transfer load from weak areas to stronger ones.
- Minimize bracing in corners where it is un-needed.
- Avoid bracing where it’s unneeded such as in the corners and near the edges.
- Make sure your bracing takes up the least amount of internal volume as possible.
- Keep disruptive bracing to bare minimum in high airflow areas.
There are many different ways to properly brace a subwoofer enclosure. The most effective way to do it is by bracing from one side across to the other. What this does is that it “shorts out” the internal pressure effect because the forces are equal and opposite on each end.
Additionally, by dividing each panel into small and well-supported sub-sections, you significantly increase the resonant frequency of the panels much higher.
Here are a few different ways to brace a subwoofer box:
1. Matrix Bracing
Matrix-like structure is one of the most advanced, and most complicated type of bracing. It consists of two-axis grid (intersecting sheets) that form an an ultra-solid brace, coupling all the cabinet panels with each other.
If you opt for this type of bracing, it would be much easier for you if you assemble the bracing first and then build the rest of the box around it, than if you build the box then try to fit the bracing.
2. Shelf Bracing
A shelf brace is just like it sounds: It links the panels together on three sides, and has at least one side that is open.
Generally speaking, one shelf bracing is usually more than enough. However, you can also add more shelves to further increase the cabinet rigidity and therefore its resonance frequency. Just make sure you place them so they don’t come into interference with the driver back emission.
Since shelf bracing don’t extend all the way across the box, they can be quite solid. However, they still have room for improvement. For instance, you can skeletonize them to make them more acoustically transparent and to save some weight.
Here is a skeletonized version:
3. Corner Bracing
Just like its name suggests, corner bracing is used when bracing across to an opposing panel isn’t possible
4. Window Style Bracing
There’s a wide range of window style bracing types. Each of which has its own pros and cons.
Window Bracing 1
This is one of the most common way people use to do window braces. It’s easy and it looks good, but unfortunately it doesn’t do anything as there’s no material linking the middle of the panels together. All this does is that it takes up space.
Window Bracing 2
This is another mediocre way to do window braces. While this has the braces where you need them (the middle of the panels), there isn’t enough surface area for glue to stick to. But it’s definitely better than the previous window type bracing.
Window Bracing 3
This window bracing style is much better than the previous ones, in that it has braces where you need them to be, and has lots of surface area for glue to stick to. It’s not the best window bracing style though.
The main downside to this type of window bracing is that it has lots of material in the corners where it doesn’t serve any purpose.
Window Bracing 4
Here is an improved version of the last window bracing type. This is the best way to do window braces, bar none.
This has all the right stuff in all the right places, but without unnecessary material in the corners of the box where it does not serve any purpose, making it more acoustically transparent.
Its main disadvantage is that it’s more of a hassle to make.
5. Wooden Dowels Bracing
Another common way to brace a subwoofer box is to use wooden dowels. However this is not super efficient especially when you’re dealing with larger panels.
The biggest advantage of dowels is that they are cheap and really easy to use. The downside, however, is that they only brace along one dimension.
This is not to mention the fact the wooden dowels are thin, which makes them prone to flexing at the woods resonance making them almost useless.
When you are using dowels, drilling mounting holes through the panels, and then driving the dowels (with glue) into the holes, is really necessary. Because otherwise, they’ll come off really easy if you only glue the end of the dowels to the panels.
Due to such small surface areas on both ends for glue ups, some people opt for using screws to do the bracing, which is not aesthetically pleasant.
6. U Brace
“U-brace” is another common way to brace an enclosure. However, it’s the least effective bracing methods, as it does not directly connect opposing panels where they flex the most, in their middles.
Does Bracing Affect SPL
Bracing isn’t likely to have an impact on SPL, except in the most extreme situations. The main purpose of bracing is to make the box more solid and reduce the requirement of thick walls. It will only impact SPL to a significant amount if the walls are FAR too thin.
Some people believe that internal subwoofer box bracing can restrict the air flow created by the driver and the port, thus, reducing SPL.
The reason why this is not true is because air flow does not happen inside a subwoofer box. Because sound waves are pressure waves, which means they consist of particles being condensed, these air particles will jostle each other and pass on vibration like a Chinese whisper. To put it another way, the waves will move along, but there is no flow.
That said, bass won’t travel as a wave inside a subwoofer box. This is because its wavelength is so much bigger than the box that it is not possible for reflections to occur. This pressurises the box. A 40 Hz wave is 8.6m long, which is way much bigger than a subwoofer box. And An 80 Hz tone is 4.3m long, which is again way bigger than the box.
All in all, bracing won’t have a serious impact on SPL. Aerodynamic factors can, however, affect your subwoofer box.
I mean, if you’re building a vented subwoofer box, you need to ensure clearance around the vent on both ends (internal and external). Otherwise the tuning may be affected in ways you can’t predict resulting in a less than optimal response. You may even end up with serious turbulence reaching audible levels.
This is reason why it’s a pain in the butt to properly design the bracing where you have to accommodate a vent. This gets worse when you’re dealing with a small subwoofer box, although the requirement for bracing then is way less and you can just focus more building a thick box instead of bracing.