5 DIY subs

If you are planning to build a DIY subwoofer, here are 5 options that you might consider. They are based on some of the best value products available for budget conscious enthusiasts, and their popularity is based on their performance and value.

The drivers are Exodus Audio Shiva X2 and Tempest X2 which are 12” and 15” drivers respectively. The amps are O Audio subwoofer plate amps. The drivers have been chosen for their value for money, and suitability for music and home theatre applications. The amplifiers have been chosen for their flexibility. There are many different plate amps available, but most lack the sensible controls that allow for proper filtering as needed for a subwoofer. It’s common for many plate amps to have poorly chosen filtering which results in them being unsuitable for serious use without modifications.

The fifth sub uses a Behringer power amplifier EP4000 which provides very high output, but requires a separate sub crossover which would typically mean a small DIY electronics project. This might be more than many are prepared for, but for those who want to get the most output possible, it’s worth the effort.

Overview

1. Shiva X2 - 45L sealed – O Audio 300w
2. Shiva X2 - 80L vented – O Audio 300w
3. Tempest X2 – 150L sealed - O Audio 500w
4. Tempest X2 – 300L vented - O Audio 500w
5. Tempest X2 – 300L vented – Behringer Europower EP4000


Drivers:
Shiva X2 Tempest X2
Exodus Audio Website >

Amps: O Audio 300 & 500w plate amps
O audio Website >
Behringer Europower EP4000



These subs range from 109 – 119 db predicted maximum output at 1m half space. If built as designed and correctly configured, the driver will never exceed xmax regardless of the material used and the vented versions will never chuff. The only exception is #5 where the extreme power capacity of the Behringer amp means there is enough power to drive the sub beyond safe excursion limits. It’s necessary to carefully determine the limits by trial and error where you intend to get close to the limits then make sure you don’t even set the gain above a safe level.

Sub #1 is the budget compact choice that you’d best hide from your partner. It lacks EQ and extension and relies on room gain, which you can never count on. Sub #2 is a little more impressive but still retains a reasonable size. Being a vented high excursion sub, it presents more of a construction challenge – read on for advice on how to beat commercial vent options. Sub #3 with a more powerful amp and bigger driver achieves slightly more output in a sealed box. It’s simpler to build than a vented box and has just the right balance of features for most people. If you are unsure which to build, choose this one. Sub #4 is twice the size and is all about how much output and extension we can squeeze out of that driver with the O Audio amp. It not only plays deeper than the sealed version, it also achieves about the same output of two of sub #4 with only a slight increase in cost. Sub #5 doubles the amp cost and gives us up to about 50% more output again, but if you want to get really serious about bass and get a second sub, you already have the amp.

All of these subs will perform very well for music and movies if integrated properly. The main differences to consider are the construction challenges, size, depth and output. If you are using EQ for the bass with a unit like Behringer FBD or Ultracurve then the extension is not so important as having the desired headroom.

Common Settings

The O Audio plate amps are modeled with a high pass filter (HPF) which serves to prevent excessive cone excursion. Some music and DVD material includes very low bass, which can bottom drivers and cause excessive vent velocity leading to compression and unavoidable vent chuffing. This ultra low frequency content adds nothing useful and it makes sense to filter it out. Each sub has a 4th order Linkwitz Riley low pass crossover set at 80 Hz. This will integrate best with sealed main speakers set to small which should mean they will receive a 2nd order high pass filter at 80 Hz. The result will be a symmetrical 4th order summed crossover.

Shiva X2 subs

The sealed version is simple and compact – only 45L is needed for adequate damping. To keep it low cost the lower powered amp is chosen, but this doesn’t allow for EQ. One might choose the 500w amp for this reason, otherwise we are counting on room gain and unless you have measured your room, this isn’t a good idea. You may not get the room gain others on every audio forum will tell you to expect – many who tell you such things have never measured a real room. The vented version is almost twice the size and has impressive performance at the cost of added complexity.

For this sub to work well we need a 120mm diameter PVC vent 970mm long! That means either a tall sub, or at least one bend, with care needed to ensure it is as smooth as possible. The vent should be mounted to two sheets of 18mm MDF with a flare at each end. The simple way is to use a roundover bit on the router – the biggest you can find which is typically around 32mm in radius. The cost is around AU $75 for such a bit but it can prove useful for speaker baffles as well to reduce baffle edge diffraction. The vent velocity will peak at just under 19 m/s.

A typical design would include a 100mm vent with a 12mm radius on the roundover, and many would run this without a rumble filter. The result is a vent velocity of 35 m/s, almost double what we have achieved! The vent being smaller will itself only handle 20 m/s and the flare would need to be 54mm diameter to avoid adding extra turbulence. By using the rumble filter and a larger vent, we cut down the size of the problem dramatically. As a result we only need a small flare that is easily achieved. A little more effort and care means we’ve achieved much greater performance with a vent that won’t chuff when the sub is pushed hard. It will also suffer much less port compression. High velocity in a vent causes the vent to gradually “lock up” and restrict the output. This shows up in many commercial subs when measured as severe compression.

If all this sounds like too much hassle, then you might spend a little more on a bigger driver that can achieve the output without a vent.

Tempest X2 subs

The extra output achieved with the bigger driver and amp isn’t much to get excited about, especially where the box is quite a bit bigger and the driver is actually working harder. The vented Shiva only uses 14mm of excursion, while the sealed Tempest uses all of it’s excursion for a similar output level. However, many feel that a sealed sub provides a tighter response. Whether this characteristic is due to the frequency response or is a trait related to sealed vs vented designs is a matter for debate.

For those that want to reach THX levels, we need a bigger vented box. A THX sub should achieve 115 dB output, allowing for a level 10 db higher than the mains, which should be capable of 105 dB. Achieving this level in your listening position isn’t as easy to predict as you might think.

The Tempest is a driver designed for a big box. The larger box allows extra efficiency down low. If we step the size up to 300L, we can now exceed 115 dB with an F3 below 20 Hz. The excursion is now down to a lower level at 19mm. If we want to go one step further, we can get up to 119 dB with the more powerful pro amp. We need more than twice the power to do it, and we now use up all the excursion of the driver. If this isn’t enough, the pro amp will power a second sub easily, but you will need to contend with the noisy fan and come up with an external subwoofer crossover.

Which one should I choose?

Firstly, consider your performance requirements. For high output, the pro amp option makes sense – you have all the power you could want from the start, and if you want more you already have another amp channel. Be prepared to do something about the fan noise and getting a crossover. This all involves extra effort and hassle, but for those who really want the extra punch, it’s worth it.

Most won’t need this and will appreciate the simplicity of a plate amp. If you want a small budget home theatre sub, choose the vented Shiva with the 300w plate amp. Be prepared for some extra challenge with the vent. If it seems too much, then spend a bit more and get the Tempest with more power. If small size and simplicity is critical, then pair the Shiva with the 500w plate amp which has EQ included unless you are happy with a music only sub or are sure you have enough room gain to achieve more bottom end.

While all this might sound confusing, the decision mainly comes down to size, performance and cost. All of the above can perform well or poorly.

More than any other audio component, a sub requires proper integration. If you are serious about doing it right, a measurement microphone and mic pre along with software are essential tools. This equipment will allow you to choose the best location, use the correct settings and include the best eq settings.