Transmission Line Enclosures

Transmission line or T-line enclosures have several interior walls that are used like a wave guide for soundwaves leaving the enclosure.

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A perfect transmission line loudspeaker enclosure has an infinitely long line, stuffed with absorbent material such that all the rear radiation of the driver is fully absorbed, down to the lowest frequencies. Theoretically, the vent at the far end could be closed or open with no difference in performance. The density of and material used for the stuffing is critical, as too much stuffing will cause reflections due to back-pressure,[dubious – discuss] whilst insufficient stuffing will allow sound to pass through to the vent. Stuffing often is of different materials and densities, changing as one gets further from the back of the driver’s diaphragm.

Consequent to the above, practical Transmission Line loudspeakers are not true Transmission Lines, as there is generally output from the vent at the lowest frequencies. They can be thought of as a waveguide in which the structure shifts the phase of the driver’s rear output by at least 90°[dubious – discuss], thereby reinforcing the frequencies near the driver’s Fs. Transmission lines tend to be larger than ported enclosures of approximately comparable performance, due to the size and length of the guide required (typically 1/4 the longest wavelength of interest).

The design is often described as non-resonant, and some designs are sufficiently stuffed with absorbent material that there is indeed not much output from the line’s port. But it is the inherent resonance (typically at 1/4 wavelength) that can enhance the bass response in this type of enclosure, albeit with less absorbent stuffing. Among the first examples of this enclosure design approach were the projects published in Wireless World by Bailey in the early 1970s, and the commercial designs of the now defunct IMF Electronics which received critical acclaim at about the same time.

A variation on the transmission line enclosure uses a tapered tube, with the terminus (opening/port) having a smaller area than the throat. The tapering tube can be coiled for lower frequency driver enclosures to reduce the dimensions of the speaker system, resulting in a seashell like appearance. Bose uses similar patented technology on their Wave and Acoustic Waveguide music systems.

Numerical simulations by George L. Augspurger and Martin J. King have helped refine the theory and practical design of these systems.

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