FWIW here's what Dave says about customization on his site's forum (and I noticed my waveguides pop out btw).
"Time to discuss the Sierra tweeter...
In order to realize our goal of reducing the negative acoustic effects caused by each loudspeaker component, the Sierra would require a special tweeter. In addition to my typical requirements (linear on- and off-axis response and excellent extension), I demanded an exceptionally clean spectral decay combined with very low distortion. Of course, our good friends at SEAS of Norway were once again up to the task.
SEAS engineers recommended a new tweeter currently under development that I could customize to suit Ascend’s needs. Of course, this new tweeter took a bit longer than expected, but the first samples finally arrived and the results were very impressive. This new tweeter has a few unique features. It contains an integrated slightly elliptical wave guide made of flexible and absorbent elastomer. The subtle shaping helps control the off-axis response and aids in the reduction of diffraction, while the elastomer material helps absorb stray high-frequency wavelengths. The wave guide insert also has the added benefit of covering the magnet assembly screws, thus eliminating the pesky reflections these cause.
The tweeter also contains an extremely wide integrated surround (the part of the tweeter that attaches the dome to the faceplate). This serves to better dampen the dome, thus reducing various resonances and break-up modes.
We added our own customizations to this already superb tweeter to further improve performance.
The final result is a tweeter with an extremely impressive spectral decay. Both harmonic and IM distortion are extremely low and the frequency response is smooth and linear both on- and off-axis. This tweeter is incredibly natural sounding, with a clear and detailed character that is crisp without ever being fatiguing. It literally oozes that magical “airy” sound that is so elusive yet critical to a high-end loudspeaker.
Tweeter Features:
- Fully manufactured by SEAS of Norway
- 26mm high-definition pre-coated fabric dome
- Fully integrated slightly elliptical wave guide made from elastomer to reduce diffraction and control off-axis response
- Integrated wide surround for reducing resonance and break-up modes
- Low viscosity magnetic fluid for improved power handling and damping
- Pole piece damping chamber to absorb back-wave reflections and lower the resonance frequency
- Exceptionally clean spectral decay and low distortion for greater detail and improved accuracy"
