Designing a High Performing DIY Modular Subwoofer

Some of the content of this article is from the SHC SUB15 DIY Home Cinema Subwoofer Guide.

Defining the Ideal SHC Modular Subwoofer

The first step in designing a product like this is always to figure out what your intentions and goals are. Here were our initial design goals for the SHC SUB15:

1. With the limitations of many home theater setups in mind, aiming for a subwoofer with a maximum depth of 25cm to match the depth of the LCR speakers, allowing them to fit into shallow cabinets behind the screen.
2. Matching up the sub configurations with the LCR10’s to form an infinite baffle wall.
3. Designing the SHC system to not only be expandable, but also reversible, allowing users to customize their setup based on their evolving needs and preferences.
4. Prioritizing a driver that is readily available to most consumers. After all, Simple Home Cinema is about accessibility, and expensive boutique drivers would not be ideal for everyone.
5. Achieving a smooth in-room response that extends down to 20Hz.
6. Minimizing group delay, as it is crucial for preserving clarity in bass frequencies. Therefore, all subwoofer configurations need to utilize sealed designs, ensuring group delay stays below 1 cycle for any frequency.

When you are in the initial design phase of a product, you always start by aiming high, then scaling back your ambitions based on what is actually achievable. The SUB15 is no different.

1. We quickly realized that the primary constraint for this design was its size, which limited the bass extension we could realistically achieve. As a general rule of thumb, larger enclosures tend to be more effective for subwoofers, particularly when aiming to reach frequencies of 30Hz or lower. Additionally, due to the limited depth available, we were unable to install a driver with an exceptionally long excursion that would fit in out 25cm limit. Consequently, this limited the raw frequency range to 25Hz, with room gain playing a more significant role below this.
2. To be able to use the same driver for all configurations, the response is not perfectly flat out of the box and will need some minimal pre-EQ. We have included suggested EQ settings in the guide that should bring the raw response of the 50L and 102L enclosures to near-flat, which can be inputted into a digital signal processor (DSP) before conducting room correction.

Tackling the challenge of creating a high-performing subwoofer that is also as modular as we envisioned was tough, but I believe we accomplished most of our goals with this first design. Needless to say, with the goal of always improving on future designs.

Your Audio, Your Way

No two home theaters are the same, and here at Simple Home Cinema, we recognize that. That is why we wanted to make our system as flexible as possible, and provide you with all of the options and data to support your decision on the best speaker configuration for you and your room. We did not want to assume that one speaker configuration was better than another just because it functioned better for us in our rooms.

In addition to presenting all of the options, we wanted to make the designs truly modular. No matter which configuration you choose to build first, you can always adapt it to match any of the other configurations. This allows the system to be both upgradable and genuinely modular. A key benefit of this design approach is that it allows you test different configurations without having to discard your existing setup, which saves you both money and time.

SUB15 Configurations

The SHC SUB15 system is built around a single versatile box that can be arranged in various configurations to suit your preferences and needs for subwoofer capability, output, and size.

SUB15 50L

Dimensions (WxDxH): 60cm x 25cm x 52cm / 23.6in x 9.8in x 20.4in

The SUB15 50L has an internal volume of 50 liters with a near field F3 at 35Hz and an F10 at 25Hz. However, that F3 is lowered to 27Hz in a 5m room once pre-EQ is applied with the response only being down 5dB around 10Hz – so there is usable output down to 10Hz!!!

This configuration is suitable for smaller rooms where space is at a premium. It can also be used in conjunction with the half height 30L LCR10 to build out a half meter tall baffle wall.

This configuration serves as the fundamental building block of the SHC SUB15 system. All subsequent configurations adhere to this standard box build, allowing for adjustments to either decrease its F3 or increase output according to your preferences and requirements.

SUB15 102L

Dimensions (WxDxH): 60cm x 25cm x 104cm / 23.6in x 9.8in x 41in

The SUB15 102L has an internal volume of 102L liters with a near field F3 at 32Hz and an F10 at 25Hz. However, that F3 is lowered to 22Hz in a 5m room with the response only being down 6dB around 10Hz using our suggested EQ.

The 102L configuration is built from 2 identical 50L boxes stacked on top of each other with the top cover removed and aligning the two inner braces. Once sealed together, this configuration effectively doubles the internal volume. This helps the driver to be more efficient under 35Hz.

This configuration is suitable for medium sized rooms as the increased box size helps with increasing output under 35Hz. If you plan on pushing this sub to the limit, a high pass filter will need to be set at 20Hz with a 6db/octave roll off to ensure that distortion is kept to a minimum.

SUB15 102L Dual

Dimensions (WxDxH): 60cm x 25cm x 104cm / 23.6in x 9.8in x 41in

This configuration mirrors the construction of the 102L single driver setup. However, instead of sealing the upper box, a second driver is inserted into the box, with both drivers wired in parallel.

This setup effectively doubles the xmax, resulting in a potential increase of +3dB to the maximum SPL; assuming the same 500W power source used for the single driver version. If the power is doubled to 1000W shared between the two drivers, this would add an additional +3dB, totaling a +6dB output increase compared to the single driver 102L configuration, while maintaining a similar response curve as the 50L setup.

This configuration is ideal for larger rooms or for individuals seeking increased output, as the addition of the second driver effectively doubles the air volume moved. However, it is important to note that since both drivers share the same internal air space, they cannot be independently controlled. If individual control over the drivers is desired, the next configuration will provide that capability.

SUB15 Dual 50L

Dimensions (WxDxH): 60cm x 25cm x 104cm / 23.6in x 9.8in x 41in

This configuration involves two identical 50L configurations stacked on top of each other. Therefore, it has the same F3 as the 50L configuration, but with the added benefit of being able to increase output by 6db over the single driver 50L.

This configuration is ideal for larger rooms or for individuals seeking increased output, as the addition of the second driver effectively doubles the power and air volume. This configuration can also be used with active room treatment technologies as it gives you the ability to control each driver independently.

Room Sizing

ACHIEVING REFERENCE LEVEL
Please note that reference level playback is very loud for most environments. The program level for the LFE channel required for reference playback is 85dB, while dynamic peaks (e.g., explosions) need to be played back at up to 115dB. However, if the main speakers are bass managed and there is a bass contour applied, the required SPL could increase to 122dB.

With this in mind, if you are trying to reach reference level, it is recommended that you support the baffle subs with additional subs placed inside of the room. We also recommend this to help even out the bass around the listening area.

NORMAL LISTENING LEVELS
People tend to listen to soundtracks at home at least 5-10dB below reference. 4-5dB below reference is also close to Dolby’s recommendation when listening to movie soundtracks mixed for smaller home cinemas, as it is equally as LOUD as the Reference level played in a large auditorium. Keep this in mind when deciding how much bass SPL you need.

The below table lists both the program SPL and the Max SPL that each different sub configuration can reach, as well as when 2 sub stacks are placed in the baffle wall.

*For information only. Single subs or single sub stacks should not be used in a baffle wall.
**SPL measurements taken at -30dB using the LFE channel at a distance of 3.5m from driver.
***Max SPL measurements taken using the LFE channel at a distance of 3.5m from driver until average distortion was 10% in a 7m room (25-200Hz).

Room Gain

Room gain is especially beneficial for sealed subwoofers like the SUB15. Unlike ported subwoofers which have a sharper drop-off in low frequencies, sealed subwoofers typically roll off at a slower rate; around 12dB per octave in the case of the SUB15. This characteristic allows the room’s natural boost at low frequencies to have a more significant impact. Even though the SUB15’s specifications might list a low-end extension (F3) in the low 30s, the room gain will extend the bass response much lower in reality.

The table provides an estimate of how much lower the in-room bass extension might be for the SUB15 based on room size only.

It is important to remember that these are just estimates of the amount of gain provided solely by the size of your room. The actual amount of room gain can vary depending on several factors such as room construction, room shape, subwoofer placement, and listening position. For example, in my 7m room, I have two single driver SUB15 102Ls installed in my baffle wall, and I have a flat response down to 19Hz. This is not only due to room gain, but also boundary gain.

Experimenting with subwoofer placement in your specific space is crucial to finding the sweet spot that maximizes the positive effects of room gain and minimizes any unevenness in the bass response.

More to Come

I will cover the performance of the SUB15 in more detail in follow-up articles, as I finish up the construction of my theater upgrade. From the testing I have done with rough calibration, I am very pleased and satisfied with the output of these subs and how well they sound installed in the baffle wall.

We are also excited to announce that we are designing and testing a high-excursion variant of the SUB15. This variant doubles the excursion and power while fitting into the same SUB15 box, albeit with some slight modifications and a slightly deeper overall design. This change will increase the maximum SPL and lower the F3. Stay tuned for more details in the upcoming months.

SUB15X Coming Soon…..