The 8c's have two high-excursion woofers in the back of their cabinets. These woofers operate below 100 Hz only. Inspired by the work of the late speaker designer Roy Allison, the 8c is designed to leverage Boundary Coupling. Boundary Coupled Bass (BCB) offers three main advantages:
- Boundary coupling avoids the destructive interference between the speaker's direct sound and the boundary reflection.
- Low-frequency headroom is increased by up to 6 dB.
- Increased directivity in the bass, for a good directivity match between the Boundary Coupled Bass and the cardioid midrange, which takes over above 100 Hz.
The 8c's have pre-programmed settings that let the 8c's easily adapt to a specific distance between the back of the 8c and the front-wall (in accordance with the professional and scientific literature, we call this wall the front-wall: listener-orientation is the reference, not the speaker's). The best results are obtained by placing the 8c’s between 10 and 50 cm from the front wall.
The distance presets in the 8c assume a solid wall. Only if the 8c is placed in front of a solid and rigid wall, does Boundary Coupled Bass work exactly as predicted: the 8c’s bass drivers acoustically couple to the front-wall, and the 8c and the wall effectively combine to become one single source. This combined 8c/wall-system radiates sound in a phase-coherent hemispherical pattern. The preset that corresponds with the actual distance between the back of the 8c and the wall, does two things: 1) it assures that the 8c/wall-system and the cardioid midrange driver in the 8c's front baffle are time-aligned; and 2) it makes sure that the 8c together with the wall has flat frequency response.
For the purpose of boundary coupling, most walls you commonly see in homes and studios work well. Their divergence from perfectly solid and rigid is small enough for the boundary distance presets to do a good enough job. 'Perfect' boundaries are for instance concrete and brick wall, but even double-pane glass behaves like a very good boundary.
Greater divergence from a solid wall will lead to less predictable boundary coupling results. Less predictable boundaries for instance are single-pane glass windows, dry-wall, and bass-traps that work below 100 Hz. These kinds of walls let some bass energy pass through, dampen some energy, and reflect the rest. Depending on how great the divergence from solid is, Boundary Coupling may or may not still be reasonably effective. However, the distance presets will not work exactly as predicted.
The wall will reflect some frequencies more strongly than others and some frequencies will experience a greater amount of phase-shift than others. This will affect the frequency response and the phase response of the combined 8c/boundary-system.
In general, if a wall is not very solid, as a result of some phase-shift of the reflected sound you should expect the apparent acoustic distance between the 8c and the wall to be a bit larger than the actual distance. In practice, this means a front-wall distance preset of say 10 or 20 cm more than the actual distance may work a bit better. Use in-situ acoustic measurements to see which preset renders the highest output at, and slightly above the 100 Hz crossover frequency between the woofers in the back and the cardioid midrange driver. But the difference between getting it right and getting it almost right is subtle. If you’re not sure which one to pick, simply go with the preset that corresponds to the actual physical distance, because - unless you have a room that is beyond reproach - not much is lost if you don't perfectly nail this setting. Slight imperfections in boundary time-alignment are simply dwarfed by other room effects, such as reflections and room resonances. Most likely, you'll be hard-pressed to say which setting is best based on the acoustic measurements. Optimize the front-wall distance setting if you can, but there's no reason to fuss over it if you can't.
Also, because the wall doesn't reflect all bass energy that hits it, you should expect to see less than the full 6 dB of reinforcement of the bass. You might want to increase the level of the 'SUB' in the 8c user interface (lanspeaker.com / app) to compensate for this, or use parametric EQ for more frequency-specific compensation. Again, in order to apply the appropriate equalization of the response, it is advised to use in-situ acoustic measurements, as explained in this article: https://support.dutchdutch.com/rew/. Whereas it's not very useful to worry about getting the boundary distance settings exactly right for the purposes of time-alignment, the effects of getting the frequency response right are much more obvious and thus important. We highly recommend you take the time and put in the effort to properly match the 8c's to your room, as described in the aforementioned article. If you're not sure how to do this, your dealer might be of assistance.
Finally, less predictable Boundary Coupling does not necessarily mean less good overall audio performance; it simply means that the boundary coupling does not work exactly as predicted. With careful measurements and adjustments of the distance settings and EQ, great results can be obtained regardless. You could argue that it's actually preferable to have a front-wall with some internal damping, because as a general rule it is good to have some low-frequency absorption in a room. And indeed, I've heard very good bass performance of the 8c in rooms with lots of bass-traps, including right behind the 8c. But as is often the case when there are conflicting forces at play, it's difficult to generalize with great certainty here. There simply are too many degrees of freedom.
My personal belief is that the 8c probably comes into its own most if it is placed in front of a more or less solid boundary, and to have most of the bass-traps elsewhere in the room. This way, you get all the benefits of Boundary Coupled Bass, as well as the even bass-response that comes with having sufficient acoustic damping at low frequencies.
Note: Resistive absorbers (such as rockwool panels) and diffusers are practically acoustically transparent at low frequencies, it’s as if they’re not even there. Ignore them when you're deciding which front-wall distance setting to use. Simply measure the distance from the back of the 8c to the actual wall.