Unveiling the Science Behind In-Wall Speaker Sound Excellence

L-Acoustics recently launched in-wall versions of its best-selling Soka, X4i loudspeakers and SB6i and SB10i subwoofers to underpin our growing global presence in the premium home and yacht markets. As in-wall loudspeakers command a majority share of the premium residential installation market, we looked carefully at our range of professional and commercial solutions to identify the models most suitable for reengineering into in-wall designs for home cinemas, media rooms, and bespoke entertaining areas.

Professional audio loudspeaker technologies from L-Acoustics, including colinear array driver configuration and coaxial mid/HF drivers, are already ideally suited to in-wall installation as both designs offer a controlled and predictable dispersion pattern. With their compact cabinets adding flexibility to the integrator and interior designer, Soka and our ever-popular X4i were the obvious choices for developing in-wall variants.

These speakers were particularly well suited to in-wall installation as all the acoustic energy is projected forwards into the room with no rear venting, and their interior volume is fixed, so their sound output, balance, and maximum SPL are predictable almost regardless of the wall construction, wall cavity volume or cavity damping. The sealed cabinet design also massively reduces the sound energy traveling rearwards into other rooms compared to open-backed or non-sealed cabinet in-wall loudspeaker designs.

There are several other design considerations in in-wall loudspeaker design compared to on-wall or freestanding models, with the physics of sound propagation offering some advantages and some challenges when using the entire wall as the speaker's front baffle. The wall where the speaker is installed transforms the wavefront at omnidirectional frequencies from a sphere into a half-sphere, with a potential gain in sound pressure level of up to +6 dB at low frequencies.

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Boundary Gain & Room Modes
When a speaker radiates sound, it disperses some energy in all directions. If the speaker is near a boundary, such as flush to a wall, some sound waves will quickly hit that boundary and reflect into the room. If the direct sound (from the speaker) and the reflected sound (from the boundary) are in phase, they will combine and reinforce the wave, leading to louder sound. This 'boundary gain' is most prominent with bass frequencies, which have longer wavelengths. 

Practical implications of this boundary gain include a noticeable boost in bass response as compared to freestanding speakers of the same design/cabinet volume. This might be desirable depending on the room acoustics and sub-integration and must be considered at the design stage. Boundary gain does not come without challenges for integrators, though. The effect can interact with room modes—specific frequencies that are naturally reinforced or attenuated based on the room's dimensions. This can result in more significant peaks and dips in the room's frequency response, which may require further acoustic treatment or equalization.

Traditional in-wall installation speakers often have switchable filters as part of the onboard crossover electronics that attenuate a broad spectrum of LF frequencies from the output, usually at -3db or -6dB. These analog filters are a very broad brush across the LF spectrum. They can easily attenuate frequencies that are not getting much boundary gain, leading to an equally lumpy but much lower volume bass response. Not ideal.

The Solution: Dedicated Speaker Presets & DSP to the Rescue
The advent of modern DSP Room-EQ processing has made onboard boundary gain compensation filters all but redundant for premium installations. As such, we decided not to put bass compensation filters on the in-wall versions of our loudspeakers. The potential boundary gain of each model has been measured in our laboratories and is adjusted by dedicated speaker presets on our amplified controllers. In addition, the DSP capabilities and onboard algorithms in these amplified controllers allow the integrator to compensate for any deviation in the frequency response curve during calibration of the installed speaker. 

Phase and Time Incoherence
Driver technology and configuration also play a pivotal role in the sound performance of any in-wall loudspeaker design. Traditional two-way speakers with a bass/mid driver and a tweeter mounted separately on the same baffle will invariably suffer phase and time incoherence as they radiate two distinct dispersion patterns, which are offset by the distance between the two drivers. The sonic effect is challenging for parametric EQ solutions to address fully.

The Solution: Well-Designed Coaxial Drivers
Coaxial loudspeaker drivers, with the tweeter mounted deep inside and at the center of the mid/bass unit, offer a genuinely spherical dispersion and time alignment wavefront, negating many of the issues of traditional separate driver designs. The output from a well-designed coaxial driver is considerably easier to smooth in DSP EQ and subjectively offers a much more coherent sound with better imaging. Our diminutive X4i has been our best seller for residential and commercial installations for this reason, and its coaxial driver benefits are now incorporated in the new X4r in-wall version. 

Soka is one of the loudspeaker models in the L-Acoustics range that uses a colinear driver array – technology derived from L-Acoustics line-array speaker technology for large venues and stadiums. The colinear system and the DOSC waveguide give us greater control over directivity and energy going forward (so less energy is reflected from the baffle wall), right down to much lower frequencies than traditional tweeter plus woofer loudspeakers. This guarantees a more consistent sound result irrespective of the materials used to cosmetically finish the baffle wall, from reflective polished hardwood to heavily damped fabric coverings. 

In-Wall Speaker Design Considerations
The physical consideration of in-wall loudspeaker design is almost as important as the acoustic engineering inside. Main installation speakers can be no more than approximately 100mm deep to ensure they will fit in traditional 4-inch framework and can be no more than 400mm wide to ensure they fit between typical 16-inch vertical studs. The standard X4i and Soka models were designed at 100mm deep as standard, with Soka just 99mm wide and X4i only 116mm wide, making these models perfect for reengineering into in-wall variants. 

We have also ensured the acoustic waves do not propagate backwards through the wall on the in-wall variants by developing a front grille that forms an acoustic seal against the wall. Mounting these models in-wall is super-easy with templates included, and fixing options include rigging kits for free ceiling suspension and 600x600m tile mounts. 

Another shift in in-wall loudspeaker design and installation has been the advent of DSP EQ to smooth output and advanced modeling software to ensure you get the speakers as near to ideally situated as possible in the first place. Both our in-wall and standard model loudspeakers can be modeled in our Soundvision software to guarantee results before installation commences. This allows the integrator to import room drawings from the most commonly used CAD platforms, and insert our loudspeakers into the space to validate the acoustic result in the listening area long before anyone has taken a saw to the plasterboard. For the ultimate in-wall speaker system performance, we also provide an acoustic measurement solution comprising our Network Manager software and our P1 Milan AVB processor platform, which enables the result to be EQ optimized and validated.

The take-home message is that, despite the 'fit-and-forget' reputation in-wall loudspeakers have attained in the home and yacht market, best-in-class models have evolved to become highly sophisticated and innovative devices backed by cutting-edge software development. As arguably the most critical equipment choice in any audio integration project, it is critical to ensure the in-wall loudspeakers you specify can deliver premium sound for your prestige projects.

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