XBL² speaker technology
ADI Technology has developed a patented technology for linearizing loudspeaker drivers: extreme BL linearity, or XBL²™ (US patent 7,039,213). This technology greatly flattens the BL curve (motor strength) over the majority of the driver's usable excursion, with typically no net increase in production cost. A flatter BL curve means lower distortion (see the work of Dr. Wolfgang Klippel).
A flatter BL curve
ADI compared the BL-versus-excursion of several driver motor topologies with FEA: the traditional short-gap/long-coil (SGLC), the newer long-gap/long-coil (LGLC), the traditional underhung (UH) and the XBL² motor topology. Each motor was optimized for a typical subwoofer Xmax of 15.4 mm one way, linear.
- XBL²
- less than 5% BL decrease over 24 mm of total excursion
- Other topologies
- the same 5% decrease over 12 to 18 mm
- At −10% BL for XBL²
- the other motors are down 18 to 23%
- Distortion at ±10.8 mm
- XBL² has less than 30% of the distortion of the other motors
The XBL² motor offers much flatter BL. In fact, it shows less than a 5% BL decrease over 24 mm of total excursion, while the other topologies range from 12 to 18 mm for the same BL decrease. When XBL² has decreased 10%, the other motors have decreased between 18 and 23%. XBL² motors offer significant advances in BL linearity, meaning shorter coils and top plates can be used when targeting a specific operating range. Typically an XBL² motor uses the shortest voice coil of any topology for a 10% BL variance.
Compared to overhung designs, the shorter coil means more mechanical clearance in the motor and a higher tolerance of rocking, so dual spiders are not required for high excursions. Combine this with a flatter BL curve, lower inductance and lower moving mass, and the advantages of XBL² over overhung topologies, in both performance and price, are readily apparent.
Lower distortion
Keeping the BL flat means a more constant motor force at excursion, which means better tracking of large-signal inputs. Asked to follow an ideal ±12.7 mm excursion signal, the XBL² motor tracks the input best while the target excursion is still well within the usable excursion limits of the driver (a rated Xmax of 15.4 mm): its output stays much closer to the input signal.
This advantage translates into directly measurable and audible reductions in distortion. For the example motors analyzed, the distortion figures are given for ±10.8 mm excursion (70% of Xmax): the XBL² motor has less than 30% of the distortion present in the other motors.
Not only for woofers
XBL² motors are useful for all audio transducers. While this example has focused on woofers, the advantages also apply to midranges and tweeters. The low inductance and moving mass of this motor are extremely beneficial in wide-band transducers: typically, XBL² will halve the inductance of a comparable-excursion overhung motor, which can add a full octave of extension at the high end. Add the lower moving mass of the short coil, and the driver designer has a greater degree of flexibility.
XBL² represents a major step forward in loudspeaker design. To show its production viability, Adire Audio has implemented the technology in several loudspeakers, including the Brahma series car audio subwoofers, with a DUMAX-verified Xmax of ±27.3 mm from a voice coil only 1.1" long.
Most of our products at RAW Acoustics use this technology from ADI: the SDX7 woofers in the HT3i, HT8i, TL7 and Apex designs, and the 4.5" XBL² drivers in almost every model.