This article examines the Jensen Bass Ultraflex enclosure designs as published in Jensen Hi-Fi Speaker Systems (1956). Rather than treating the Ultraflex cabinets for 15-inch, 12-inch, and 8-inch loudspeaker units as separate constructions, they are considered here as a single design family, scaled to different driver sizes.
Jensen defines the Bass Ultraflex as a duct-loaded bass reflex enclosure in which the rear radiation of the loudspeaker is coupled to the listening space through internal air chambers and rectangular ducts. This underlying principle remains consistent across all three cabinet sizes. Differences between the variants are primarily matters of scale—total internal volume, duct cross-sectional area, and intended placement.
Jensen HI-FI Speaker Systems – p. 19
The purpose of this article is to summarize how Jensen itself defined and implemented the Ultraflex concept in 1956, using the original manual text, construction drawings, and design notes as primary sources.
Original illustrations from the Jensen manual are used throughout the article, as they are integral to understanding how these enclosures were meant to be built and understood at the time and because the photos and illustrations help explain the ideas.
Taken together, the Bass Ultraflex designs represent a unified geometric system rather than a set of independent cabinet plans.
The Bass Ultraflex principle
Jensen presents the Bass Ultraflex as a distinct acoustic principle, positioned deliberately between existing enclosure categories.
In the Ultraflex enclosure, the loudspeaker is mounted conventionally on the front baffle. The rear radiation of the cone is directed into a large internal air chamber, from which it is coupled to the outside environment through rectangular ducts formed by fixed internal panels. These ducts are integral to the enclosure structure and are important for the performance of the loudspeaker, allowing the backwards movement of the voicecoil to contribute to the performance.
Jensen is careful to define this relationship explicitly. One of the clearest statements in the manual reads:
“It can be termed a duct-loaded bass reflex enclosure since the operation is similar but with ducts connecting the opening ports to the inner chamber.”
This sentence establishes the Ultraflex as an extension of bass reflex principles rather than a departure from them. The enclosure does not introduce a new acoustic category, but instead replaces simple port openings with ducts whose length, cross-sectional area, and placement are treated as essential design parameters.
Immediately following this definition, Jensen draws a firm boundary to prevent misinterpretation:
The Ultraflex is explicitly stated not to be a folded horn.
Jensen HI-FI Speaker Systems 1956, p. 12
Although the internal layout—particularly in the larger enclosures—may visually suggest horn-like pathways, Jensen rejects this classification outright.
The ducts do not provide progressive acoustic impedance transformation, nor are they intended to function as waveguides. Their role is limited to coupling the rear chamber to the listening space in a controlled and repeatable manner.
Across all Ultraflex variants, Jensen identifies three acoustic constants as fundamental to correct operation:
- Total internal enclosure volume
- Total duct cross-sectional area
- Fixed duct length
Jensen HI-FI Speaker Systems 1956 – p. 13
These parameters are treated as fixed by design. The Ultraflex is presented as a complete acoustic system whose performance depends on accurate execution of the specified geometry rather than measurement and calculations.
This emphasis on internal geometry over external form explains why Jensen allows flexibility in cabinet shape and placement, particularly in the larger designs. Provided that internal volume and duct dimensions are preserved, the enclosure may be adapted for corner placement, sidewall installation, or built-in cabinetry without altering its fundamental acoustic behavior. In this framework, the enclosure is conceived first as an acoustic system and only secondarily as a piece of furniture.
Compatible loudspeaker systems
Jensen distinguishes the three Bass Ultraflex enclosures primarily through driver compatibility.
The 15-inch Ultraflex is shown with several high-performance loudspeaker systems, including large coaxial and multi-way units. Jensen treats this enclosure as a general low-frequency platform rather than as a cabinet intended for a single driver type.
The 12-inch Ultraflex is associated with a more limited range of systems. Jensen illustrates it mainly with coaxial and two-way configurations. The enclosure is presented as a defined match for specific 12-inch systems rather than as a universal platform.
The 8-inch Ultraflex is tightly coupled to small two-way system kits. Jensen does not suggest alternative driver types or more complex system arrangements for this enclosure.
This progression reflects Jensen’s treatment of enclosure volume as a constraint on system complexity. As cabinet size decreases, the range of compatible systems is reduced accordingly.
Jensen HI-FI Speaker Systems 1956 – p. 18
Expected performance and intended use
Jensen does not provide numerical performance data for the Bass Ultraflex enclosures. There are no frequency response curves, efficiency figures, or tuning frequencies given for any of the three designs.
Performance expectations are communicated indirectly through enclosure size, intended loudspeaker systems, and placement within Jensen’s product range.
One of the most striking aspects of the Bass Ultraflex designs is not what Jensen includes, but what is deliberately absent.
Nowhere in the 1956 material does Jensen provide frequency response curves, efficiency figures, tuning frequencies, or measurement-based performance claims. There are no calculated alignments, and no attempt to formalize the enclosure as a system defined by numbers. Likewise, there is no suggestion that the user should adjust, tune, or experiment with duct dimensions. The Ultraflex enclosures are presented as fixed acoustic systems, defined by geometry and proportion rather than by optimization.
This omission is consistent across all three sizes. Performance expectations are communicated indirectly through enclosure scale, intended loudspeaker systems, and relative positioning within Jensen’s own lineup. The assumption appears to be that correct construction, airtight assembly, and adherence to dimensions are sufficient to achieve the intended result.
