M-Audio BX Guide D'utilisation page 6

11. Variable Crossover: You can use this subwoofer in a variety of monitoring contexts, from two-
channel to multichannel (e.g., 5.1, 7.1) systems, and as an accompaniment to practically any
direct-field monitors. To this end, we have provided a versatile yet efficient way to tailor the
subwoofer's response to work well in any typical monitoring system.
For a two-channel system, there is an in-line low-pass filter control that lets you adjust the
crossover frequency. You can set it anywhere from 50 Hz to 200 Hz. It splits the signal by
routing everything below that frequency to the subwoofer and everything above it to your main
monitors. By using a fourth-order Linkwitz-Riley topology, the circuit enables you to achieve the
flattest net response through the crossover point.
For multichannel systems, this subwoofer can reproduce just the output of the LFE (low-
frequency effects) channel, or it can reproduce the bass from some or all of the main channels
using conventional multichannel bass management schemes. In this application, you can
simply "open up" the subwoofer fully by setting it to 200 Hz (for use with external bass
managers), or you can set it to 120 Hz (to comply with the Dolby
band limit).
12. Phase Selector: Because low-frequency signals have long wavelengths compared to the
dimensions of most rooms, subwoofers can cause phasing or cancellations that can be quite
dramatic. For this reason, we have included a phase inversion switch, which causes a 180°
phase shift in the subwoofer's output. This gives you more freedom in choosing where to locate
the subwoofer (and consequently, the crossover frequency phase alignment) in your monitoring
environment. Note that this function affects only the signal going to the subwoofer amplifier,
not the signal going to your direct-field monitors.
13. Bass Reflex Port: This subwoofer has a vented enclosure containing a custom bass-reflex port
tube with a circular cross section on the rear of the enclosure. The port effectively gives the
subwoofer a smooth response down to 20 Hz, and since its low-frequency contribution
complements that of the front-firing woofer driver, the woofer stays linear and distortion-free
throughout the passband. The port has also been designed using Failure Mode and Effects
Analysis (FMEA) software in order to maximize output and minimize audible port air turbulence
noise.
14. Power Amplifier (not pictured): To generate a powerful, tight bass response, this subwoofer
uses a 240-watt amplifier with customized discrete transistor architecture. The amplifier has a
carefully designed damping factor which, in conjunction with the mechanical and
electromechanical systems built into the driver, helps eliminate the natural tendency of the
driver to "ring" after input signal has ceased. This results in an extremely accurate and
controlled bass response. You will notice that kick drums and staccato bass notes reproduced
by the subwoofer are tight and solid, as though coming from a large acoustic suspension
system.
15. Enclosure (not pictured): Like the other components, the subwoofer's enclosure has an
important role in shaping the overall sonic response. To provide more stable performance, this
subwoofer employs a special high-acoustic-efficiency medium-density fiberboard (MDF) and
unique interior reinforcement designed to reduce sonic coloration and absorb vibration
generated by the subwoofer's high-SPL signals and transients. Additionally, the cabinet uses
carefully placed acoustic foam to absorb internal reflections and dampen standing waves. This
effectively makes the enclosure behave as though it were larger, resulting in a system more
capable of reproducing very low-frequency signals naturally and evenly.
16. Thermal Protection (not pictured): The subwoofer's thermal protection protects it from
overheating. If a critical internal temperature is reached, the subwoofer's amplifier will
temporarily shut off until the temperature returns to a lower level. While in this protection mode,
the LED Indicator will illuminate solid red, and no audio will be heard from the subwoofer.
Once its internal temperature has dropped sufficiently, audio output will resume automatically,
and the LED Indicator will illuminate blue again.
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Digital standard for the LFE