Bag End S21E
Sound & Vision Subwoofer Review
Way Down Deep II Bag End S21E
Bag End’s is a unique approach to subwoofing. All other subs in this survey – and indeed all others known to me – are designed for as low a fundamental resonance of the driver/enclosure system as possible, the “usable response” being generally considered the area above that point. Bag End inverts standard practice by designing the resonance to occur as high as practical, around 63Hz in this case, and then operating the subwoofer below that point. The putative advantage is that a sub’s output below resonance may be weak and growing weaker with declining frequency (at the rate of 12dB/octave, this being a sealed-box system), but the shape of the rolloff is quite smooth and predictable, lending itself to correction by a processor with the mirror-image response just before the power amplifier.
Bag End’s ELF-1 is a 2-channel processor designed to supply just that kind of correction, as well as integration with the main speakers, subwoofer driver protection, and “concealment” of audible distortion. This concealment is achieved with a sliding filter circuit that kicks in at the very lowest frequencies under high-level conditions to prevent audible distortion or even driver damage.
The ELF-1’s rear panel contains the AC power-cord socket and six XLR connectors for inputs and outputs. A bank of switches on the left side of the front panel enables the user to control various bass parameters, including the low-frequency cutoff (the lowest being 8Hz), contour (low-end boost of 1-7dB), gain, polarity, and the threshold at which electronic concealment engages. Another bank of switches controls various highpass parameters, including crossover frequency, gain, and polarity. shortly before this article went to press, the ELF-1 was supplanted by the Infra-M processor, which is said to offer the same sound quality in an easier-to-configure package. However, I used the ELF-1 for my testing.
The 21-inch-diabeter driver used in the S21E was the largest in the survey, offering nominally 36% more radiating area (“bore”) than the 18-inchers that will appear in Part 3. In this case, the increase in sheer driver real estate does no necessarily transfer directly into pants-flappage factor. In a system that rolls off at 12dB/octave throughout its operating range, it takes a considerable low-frequency boost to produce the same sound pressure at the lowest frequencies that 1W produces at 64Hz. At 32Hz, the power required to perform this feat is 16W; at 16Hz, the power required is 256W and at 8Hz the power is a sobering 2kW-well beyond the manufacturer’s recommended limit of 800W. And at frequencies where one feels bass as much as hears it-below roughly 25Hz-the aforementioned electronic boost, and the ear’s natural insensitivity to bass at low to moderate playback levels, combine to impose excursion demands that a singer driver-even a 21-incher-simply can’t meet.
Measurements and continuation on pdf.