I read some concerns about the measured distortion levels, and questions regarding what causes this, and to what extent this affects sound quality.
As I have my own speaker designs using not the same, but somehow similar technology with acoustic ports to control directivity, I have an interest in explaining how this works - what causes this distortion, how does it affect sound quality.
The D&D speaker tested here looks like a very well engineered design, where they made good choices for the final product. What they did, how they did it, and why they ended up choosing they way they did, does not necessarily match any of my own observations or solutions.
My designs are different form the D&D, but they share some common principles, such as the directivity control in the upper bass - lower midrange frequency range using acoustic ports with acoustic resistive damping in those ports, as well as inside the cabinet.
My speakers also show the same behavior with increased distortion with lower frequency, levels may be different, but same tendency.
As for sound quality, see post #1 where the reviewer
@hardisj described the sound as excellent, and that there were no indications of high levels of distortion, even when playing very loud.
This corresponds well with my own findings - distortion is not a problem, even at very loud volume. A clue to understanding why this distortion is not as huge a problem as one can initially suspect from looking at the graphs, can be found when looking deeper into the data. Then you see that the distortion is not caused by some hard limit - similar to amplifier clipping or a cone bottoming out, it just gradually increases with increasing spl. And the distortion is (mostly..) dominated by low order harmonics. At higher frequencies this increase in distortion disappears (clue - volume flow velocities decrease..).
As to what causes this, it is a combination of several factors. The distortion from the drivers is not the only source of this distortion, and choice of acoustic damping materials, their placement and internal design of the enclosure all affects the end result. In such small speakers, it may prove to be quite difficult to get rid of completely, and I stopped working on improving this after getting the distortion levels down to something reasonable. After all - there was indeed nothing wrong with the sound.
Future state-of-the-art speakers will all have some sort of directivity control like this, the impact on sound quality is quite significant, especially in rooms with less than ideal acoustic properties.