Traditionally, the rule of thumb has been that crossing to a woofer as low as possible is best. In some respects this is a good rule because it helps avoid breakup and "grunge" from the woofer, and can improve off-axis dispersion. However, some care must be given to low-crossover-point designs to avoid "hot spots" in the off-axis response that can occur as the tweeter begins to pick up in the critical upper midrange region. This extra off-axis energy, usually in the 2-5 kHz range, can be refl ected back to the listener and can quickly create an overall system balance that is harsh and fatiguing. During the crossover design of this system, I paid extra attention to off-axis measurements to assure that the overall in-room response of the system was well balanced.

The crossover between the woofers and tweeter is at about 1,650 Hz. This low crossover point required using steep slopes to help protect the tweeter from overexcursion. The tweeter uses a relatively standard fourth-order electrical fi lter and a small padding resistor to achieve its eventual target 4th order roll-off. The woofer uses a third-order electrical crossover plus a conjugate network to achieve its 4th order roll-off. A small inductor in series with the shunt capacitor acts as a trap for the upper end woofer breakup, which ends up about 50 dB below the reference level. Baffl e step compensation is built into the third order low-pass filter.

The overall frequency response of the speaker is quite flat; though a somewhat exaggerated BBC dip was built into the on-axis response to achieve the desired off-axis performance. Despite the depression around 3k, the overall apparent response of the speaker in a "real" listening room should end up +/- 1.5 dB or so throughout most of the operating range.