A custom in-wall home theatre system project

So, some time ago, a real estate development company in Taiwan commissioned me to design and build a custom in-wall concealed multichannel speaker system, which will be installed in the basement multipurpose room of one of their new projects. This project has been very challenging, because the space is not a dedicated home theatre. It also serves as a place for residents to practice musical instruments and hold small performances (there’s even a piano stored in the corner of the front wall that can be pulled out). On top of that, the interior design style leaves very limited room for acoustic treatment and speaker installation. The acoustic requirements for cinema use and musical performance are also somewhat contradictory, so the design needed some clever thinking. Overall, this project has been a lot of fun.

Although cinema use is not the room’s top priority, the space is quite large (9.2m x 4.7m), and the listening distance from the speakers is considerable. Therefore, the system needed high sensitivity and output capability to achieve the results I had in mind. After discussions with the architect, we decided to reserve up to 18.5cm depth in the walls for the speakers, and I based my design on that constraint. One of the trickier parts of designing in-wall speakers is that you need a 2π measurement environment to obtain proper data. That means either embedding the speaker in the ground outdoors or using a specialised 2π anechoic chamber (usually only available at driver manufacturers). Luckily, I had previously purchased the 2π measurement module for my Klippel NFS, so I was able to carry out these measurements myself.

The two-way speakers responsible for each channel consist of a horn-loaded tweeter and a 7.5-inch Textreme diaphragm neodymium midbass. Ever since I used Textreme for the midrange/tweeter in the development of my three-way speaker Cadentia, I’ve loved this material. Brands like Rockport and Perlisten also make extensive use of it. It’s lightweight yet rigid, and it delivers excellent transparency. The enclosure is sealed, 4 ohms impedance, with a sensitivity of 93dB/2.83v/1m (under 2π conditions). Dimensions are 36cm tall, 24cm wide, and 18.5cm deep.

Unlike conventional speakers, in-wall designs lack the baffle effect that helps control directivity. With the midbass operating in an infinite-baffle condition, the transition from omnidirectional low frequencies (nearly 180°) to beaming at higher frequencies is more abrupt. If you simply followed the usual 2–3kHz crossover point into a waveguide or horn tweeter, the system’s power response would have an unnatural "step" shape, producing tonal imbalance. To achieve a smoother power response roll-off—without making the on-axis response too odd—the crossover point must be set lower, and the horn carefully shaped so its directivity gradually narrows at the right frequency to match the midbass. The most time-consuming part of this design process was simulating and prototyping the horn acoustics (I absolutely love my 3D printer for this work, haha). In the end, I was quite satisfied with the results achieved within the deadline. The figure below shows the horizontal polar response of the small speaker.

Thanks to the relatively large effective diaphragm area and 1-inch compression tweeter, this small speaker has impressive output capability. Measurements confirmed that at 102dB/1m (2π conditions), harmonic distortion remained below 1% above 120Hz. Since the system will apply a high-pass filter around 100Hz to these speakers via the surround amplifier, low bass extension was not a major priority. That said, after finishing the design, I realized that if I reduced the cabinet depth to about 15cm and increased height and width slightly, it could easily be turned into an on-wall speaker (similar to the KEF Q4 Meta I designed previously). If I have time in the future, it might be worth exploring as a new standard product.

The subwoofers use dual 12-inch drivers in sealed enclosures measuring 120cm tall, 45cm wide, and 18.5cm deep. For these drivers, this is quite a small volume, so naturally deep bass extension is limited. As with many sealed in-wall subwoofers on the market, the system requires a dedicated DSP and high-power amplification to correct low-frequency response. The setup will run the surround processor’s sub-out to a MiniDSP Flex, then feed into a multichannel power amplifier, handling both crossover and low-frequency EQ. The chosen 12-inch driver is one of my favourites—it features dual demodulation rings in its motor structure, delivering excellent low-frequency distortion performance.

You may notice from the photos that there are four subwoofers, even though this is only a “.2” multichannel system. Why four? That goes back to the room’s acoustic treatment challenges. As mentioned earlier, the multipurpose room is large, with long reverberation time and obvious low-frequency room mode (standing wave) issues that would harm cinema sound quality. In a typical theatre, you’d simply add as much absorption as possible. But here, not only is space limited due to interior design requirements, the room also needs to support musical performance—so over-damping would be undesirable. My solution was to install hybrid diffusion/absorption panels wherever possible. Near the piano, the design emphasises diffusion, preserving more early reflection energy. On the sofa seating side, absorption was prioritised to suppress flutter echoes and control reverberation above 100Hz to a reasonable level. For the deeper bass problems, I employed CABS (Controlled Acoustic Bass System).

The principle of CABS is straightforward: place subwoofers at specific positions on the front and rear walls to suppress certain lateral room modes. Then, by inverting the signal of the rear subwoofers, adding delay, and adjusting level, the rear subs cancel the forward bass waves when they reach the back wall. This eliminates longitudinal modes and greatly reduces low-frequency reverberation. In this system, two of the four subwoofers are dedicated to this task—it’s essentially a DIY “lite” version of Trinnov. In-wall subwoofers are ideal for this approach, so this project provided the perfect opportunity to try it out.

At present, all the design work is based on measurement and theory. Once the speakers are installed, they will be hidden behind acoustically transparent fabric, and final sound adjustments will still need to be made on-site. Toward the end of the year, when the construction project is nearly complete, I’ll personally travel to Taiwan to carry out acoustic measurements and set up the multichannel system along with the CABS. I’m really looking forward to hearing the final results in the actual space!