Powered Subwoofer

Overview

A powered subwoofer reproduces only the bottom two to three octaves of the audio band, roughly 35 to 110 Hz, and does so with hardware that no full-range cabinet can spare: an 18-inch long-excursion driver, a 1500 W amplifier, and an enclosure sized around the physics of long wavelengths. In a portable PA it sits on the floor, where boundary loading adds up to 6 dB of free output, and carries a satellite top cabinet on a pole, forming a system in which each box handles only the band it is built for.

The acoustic engine is the LF Driver Assembly, loaded by the ported Ported Enclosure. Electronics live on the Plate Amplifier, a single module whose finned plate seals into the rear wall; the DSP Section performs crossover and protection duties, the Switch-Mode Power Supply feeds the amplifier from any mains voltage, and the I/O and Control Panel distributes signal to the rest of the system. The External Hardware set, casters, handles, pole socket, acknowledges that a 42 kg box gets moved every night.

How it works

Line-level audio enters through the balanced Balanced Input Stage and is digitised by the ADC/DAC Codec. The Audio DSP applies a low-pass crossover, selectable at 80, 100, or 120 Hz, so the subwoofer reproduces only its band; the complementary high-passed signal is sent back out on the I/O and Control Panel to feed the satellite tops, relieving their woofers of bass excursion and raising the whole system's clean output. The DSP also runs the protection layer: an RMS limiter modelling voice-coil temperature and a peak limiter modelling cone excursion, each tuned to the specific driver. A Polarity Switch flips polarity so sub and tops add rather than cancel through the crossover region, the correct setting depends on relative placement and is found by ear or measurement.

The processed signal drives the Class-D Power Stage, a full-bridge switching amplifier. Subwoofer duty is the friendliest case for class-D: the audio band is far below the 400 kHz switching frequency, so the Output Filter Choke filter removes the carrier easily, and efficiency above 90 percent lets 1500 W of capability cool through the passive Amplifier Plate. Upstream, the PFC Boost Converter stage holds a regulated 400 V bus from any mains voltage and the LLC Resonant Converter converts it to the amplifier rails; Bulk Capacitors buffer the kick-drum transients whose instantaneous demand far exceeds the average draw from the wall.

Driver and enclosure

Moving air at 40 Hz requires displacement, and displacement is the product of cone area and excursion. The 18-inch Subwoofer Cone provides about 1200 cm² of area; the suspension, a Triple-Roll Surround with multiple rolls and a Dual Spider that resists rocking, allows over 20 mm of peak-to-peak travel. The 100 mm Voice Coil is a 100 mm four-layer winding rated near 1200 W; windows in the Vented Cast Basket and vents in the Motor Structure let the moving coil pump cooling air through the gap, because at full output the coil dissipates more heat than a domestic space heater.

The enclosure is a bass-reflex design tuned near 40 Hz. Each Slot Port is formed by the cabinet walls themselves, giving a large mouth area that keeps peak air velocity below roughly 20 m/s, above that, port turbulence becomes an audible chuff. Around the tuning frequency the port radiates most of the output while cone excursion drops sharply, which is why a vented box plays 3 to 4 dB louder at the bottom of its band than a sealed box of the same size. Below tuning the cone unloads, so the DSP applies a steep infrasonic high-pass filter as part of its protection set. The 18 mm birch plywood shell with internal bracing keeps panel resonances out of the passband, and the Driver Baffle is doubled where it carries the 12 kg driver.

System use

In a typical compact rig the subwoofer takes the stereo mix, sends high-passed signal up the pole to the tops via its outputs, and a distance rod threaded into the M20 Pole Socket places the top cabinet at head height. Larger deployments ground-stack multiple units or arrange them in cardioid configurations, DSP presets that delay and invert a rear-facing cabinet to cancel bass energy behind the stage. The Transport Casters and Recessed Handles handle the load-in; everything else is one mains cable and one signal line.