AudioSource SW Four Subwoofer
AudioSource says it designed the SW Four powered subwoofer to be compatible with a wide variety of loudspeakers, and consequently it is equipped with a full complement of adjustments. Physically, however, it is an almost featureless black box meant to be placed on the floor in (usually) an inconspicuous location.
The subwoofer is raised 2 inches off the floor by four sturdy feet, and its 12-inch downward-facing driver radiates its output through that gap. The driver itself has dual voice coils that effectively sum left- and right-channel inputs to mono. The ducted enclosure is filled with a sound-absorbing material, and its port is close to the woofer on the bottom of the cabinet.
Also on the bottom is a panel with the SW Four's inputs, outputs, and operating controls. The controls include small knobs that enable continuous adjustment of the crossover frequency, from 40 to 180 Hz, between the subwoofer and the main speakers and of the subwoofer's output level. The SW Four also contains a 150-watt amplifier whose heat-radiating fins are at one end of the enclosure.
Connecting the subwoofer to an audio system is a simple process, with two basic options: high-level input (through insulated binding posts) from the system amplifier's loudspeaker outputs, or line-level input (through standard phono jacks) from preamplifier outputs. Either way, the incoming signal is filtered through the crossover circuits, which send the bass frequencies to the subwoofer amplifier and the higher frequencies to the main loudspeakers or to the main speaker amplifiers.
The remaining controls (both slide switches) are a three-position power switch and a phase switch that introduces a 180-degree phase shift (a waveform polarity reversal) into the subwoofer output to compensate, if required, for the unavoidable difference in arrival time of sounds from the subwoofer and the main speakers. In some cases such reversal will yield a better blend of the outputs from the subwoofer and the main speakers. The power switch, in addition to on and off settings, has an Auto position that is its normal recommended setting. In Auto, the subwoofer switches on automatically when a signal is received and shuts off after no signal has been present for several minutes.
The SW Four's somewhat sparse specifications include an overall frequency range of 20 to 250 Hz and a signal-to-noise ratio (S/N) greater than 90 dB. Its instruction manual is complete and clearly written and includes interconnection diagrams that cover any likely application.
We measured the SW Four's frequency response with close microphone spacing at each of the marked crossover frequencies - 40, 80, 120, and 180 Hz. The response from the cone was broadly peaked, with its maximum level occurring between 60 and 100 Hz at the higher settings and dropping to the range of 60 to 80 or 90 Hz as the crossover frequency was reduced. The output level from the speaker also fell off rapidly as the crossover frequency was reduced, measuring 10 dB lower with the 40-Hz setting than with the 180-Hz setting.
Although that behavior may seem odd, it is actually of little significance to the user. The adjustment of these controls must, in any case, be done by ear, so one will normally turn up the level control to compensate for any reduction in output when the crossover frequency is lowered.
We also measured the output from the port, which proved to be very strong and had the effect of flattening out and extending the system's low bass range.
Using the 180-Hz crossover setting and with the output set for a 90-dB sound-pressure level (SPL) at 1 meter from the cone (with the subwoofer upside down), the SW Four's distortion was between 1.2 and 2.5 percent from 55 to 450 Hz. At lower frequencies the distortion increased, reaching 9 percent at the subwoofer's effective lower limit of 40 Hz.
These results have to be interpreted in light of the SW Four's intended function, which presumably is to obtain a worthwhile low bass output in a system whose main speakers lack that ability. We operated the SW Four in conjunction with a pair of small, inexpensive speakers whose response fell off below about 70 Hz. When we adjusted the subwoofer level and crossover frequency by ear, the results were quite satisfactory, with a respectable degree of low-end "body."
The other side of the coin (as with any subwoofer) is that the SW Four can easily be misadjusted to produce the kind of boomy bass often associated with speakers having inadequate woofer systems. The calibrations of its controls are rough approximations at best, and they may even be worthless for establishing the desired system response with some main speakers. Also, as a general rule, a subwoofer crossover frequency in excess of 100 Hz may cause undesirable effects, such as localization of the subwoofer position.
Bearing those cautions in mind, however, we found the AudioSource SW Four subwoofer to be an inexpensive, versatile, and effective means of enhancing the low bass performance of a modestly priced speaker system.
