ISUB-18 Self-Powered Subwoofer System
Widescreen Equipment Review of the ISUB-18 Self-Powered Subwoofer System
by Gary Reber
The ISUB-18 Self-Powered Subwoofer System used in both Bag End professional and home theater product lines originated featuring technologies licensed from E.M. Long and Long/Wickersham Labs. This includes Time-Aligned™ monitor loudspeakers and ELF™ (extended low-frequency) subwoofer systems. Long and Wickersham believe, and Jim Wischmeyer, the President of Bag End, agrees (as do we at Widescreen Review), that time and phase are important performance parameters for the accurate reproduction of the input signal by a loudspeaker. The ear/brain system may be even more sensitive to time cues than it is to amplitude variations, and attention to both factors in the design of a loudspeaker is necessary to fully preserve the original sound recording.
The time response characteristics of a subwoofer are very difficult to measure and have been virtually ignored by most manufacturers. The result is apparent: most subwoofers don’t integrate well with the main loudspeakers. Bass often sounds disassociated from the midrange, and the basic character of all sounds is subtly changed when a subwoofer is added to a system; not necessarily for the better. Back in the 1980s, Bag End decided that they could improve this situation by implementing the Time Align and ELF technologies. Evidence of their success can be found in the professional world where Bag End subwoofers are known for their quick, tight, and musically connected sound.
Since the early 90s it’s noteworthy that film sound quality has improved substantially, especially in the bass region. A likely reason for this improvement is that some of the world’s best post-production companies, where the best-sounding soundtracks are actually created, have abandoned their old THX®-approved, ported boom-box subwoofers and replaced them with systems from Bag End.
Today the newer INFRA systems are an updated version of the original ELF systems. Utilizing much of the same technology and the same fundamental thinking, the INFRA systems include updated SMD (Surface Mount Device) technology and additional driver and cabinet configurations.
Bag End subwoofers do sound different, with a resonance-free clarity and startling pitch definition that is unmatched by most other designs.
A subwoofer in a vented enclosure goes through a substantial phase change and produces two impedance peaks right in the middle of the subwoofer’s operating range.
Adding a conventional low-pass filter creates system delay. Typical subwoofers can never be completely time-integrated with the main loudspeakers because their phase position in their pass band is a moving target, always changing with frequency. This is why the bass from many subwoofers sounds like it is lagging behind the sound from the main loudspeakers or coming from another place away from the main source. Some designs offer a “phase” control, but since the phase is a moving target there is no correct position for the entire range. The best this seems to offer is some subjective improvement under some circumstances.
An INFRA system utilizes a loudspeaker in a sealed box and operates it primarily below the system resonance of the driver and cabinet combination. Like all sealed box designs, it has a single resonance, and in an INFRA system this resonant frequency defines the upper roll off of the systems’ frequency range. In fact, the acoustic output is down slightly at, and rolling off above, the system resonance. This dramatically reduces the influence of the resonance on the sound being reproduces. By definition, resonance is the systems’ “favorite note.”
It’s the frequency that takes the least energy to excite, and the system favors this frequency over others. When a complex transient signal enters a loudspeaker system it excites all frequencies… but at resonance the system plays louder and persists more in time as compared to the other frequencies. This is why it is desirable to reduce the influence of the resonance so that the loudspeaker is not creating additional sound that is not in the recording. It is useful to operate any transducer outside of its resonance, as is typically the case with good loudspeaker, phono cartridge, and microphone designs. A strong resonance in a loudspeaker can blur and mask the details in the original sound…
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At Widescreen Review we use the flagship 18-inch INFRA subwoofers in two of our reference systems.