At the heart of the Sierra is OPPIX™.
OPPIX stands for Optimized Paradromic Phase Integrated X-Over (crossover). This sophisticated variable slope crossover precisely aligns the acoustic phase response of multiple transducers at the exact point where the response of one transducer crosses over to another, with reference to a specific point in space. This precise alignment allows the transducers to mimic a single point source radiator, allowing for symmetrical dispersion even with an asymmetrical transducer layout while maintaining a linear frequency response and greatly reducing off-axis phase and response anomalies. It also avoids the undesirable frequency response anomalies commonly found with even the best coaxial drivers.
As an example, examine the off-axis measurements of these 2 popular high-end 2-way loudspeaker using a common tweeter-woofer driver layout (see below). Notice the dramatic difference in the off-axis frequency response above and below the speaker (vertical symmetry). Also compare the horizontal response to the vertical response. This is a typical asymmetrical response, and since what we hear in a room is a combination of direct-sound and reflected-sound from all directions, even though the on-axis response might be flat, the asymmetrical loudspeaker will deliver a less accurate response, since the speaker performance varies dramatically off-axis.
Competing 2-way loudspeaker
Another Competing 2-way loudspeaker
Now notice how symmetrical the Sierra’s off-axis response measurements are (see below).. Symmetry is maintained up until the point where the length of the sound waves become less than the distance from the source to the edges of the baffle. In the case of the Sierra, precise response symmetry is maintained from 40Hz up to approximately 10kHz, at up to 25 degrees off-axis. This translates to accurate in-room performance, a “correct” soundstage with precise imaging, and allows the Sierra-1 to perform to spec when positioned vertically or horizontally. OPPIX™ also allows the Sierra to perform exceptionally well as a vertical or horizontal main and a vertical or horizontal center. I recommend three Sierras in the front (left, center, right) for the ultimate in timbre matching and soundstage consistency.
The Sierra-1
Another feature of the Sierra crossover is maintaining extremely linear electrical impedance. From 1 kHz up through 20 kHz, impedance varies by only ±3 ohms, from 2 kHz up, by a remarkable ± 1 ohm. You will also notice that impedance does not rise as frequency increases. This translates to an easy load for any amp and consistent performance from one amplifier to another. We have tried the Sierra on a host of receivers and amplifiers and it delivered consistent performance from all of them.
The Sierra-1 Impedance + Electrical Phase
This advanced crossover also features:
- High-performance metalized polypropylene capacitors in the direct signal path, these offer lower dissipation than the more common polyester based film capacitors
- Air-core inductors to reduce saturation effects combined with heavy gauge windings to reduce resistance and improve efficiency
- Low inductance resistors to reduce distortion and provide accurate transient information
- Heavy gauge internal copper wiring
- Linear impedance response for consistent performance regardless of amplification source
- Isolation mounting, whereby the crossover is attached to the cabinet using isolation feet, which reduces the transfer of resonances from the cabinet to the crossover components
Optimizing this crossover and developing the technique and circuit topology to accomplish this was my most rewarding and challenging aspect of this loudspeaker…
Woofer and tweeter tomorrow 
Sierra - The Crossover
