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    Transmission Line

A transmission line enclosure is a waveguide in which the structure shifts the phase of the driver's rear output by at least 90°, 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 1/4th 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. Bowers & Wilkins have used this approach in their flagship Nautilus speaker as well as smaller straight tapering tubes in many of their other lines.


Have also a look at Bailey's Transmission Line Calculator




The criteria for selecting a woofer for a TML loudspeaker box are still not clearly defined.
There is not (yet) a design methodology which is similar to the Thiele / Small - system with parameters for closed boxes, bass reflex speakers and systems oassieve membranes.



Below is a list of woofers that have been successfully used in TM systems:

DynaudioKefFocal
21W54B-13910K515
30W54B-1108K516
 B-200S8K011db




  Driver

Cabinet

  fs : Hz Sd : cm2   Inside width : cm
 



 
Classic tuning
  Cabinet height [h] : cm
  Depth behind driver [a] : cm
  Depth at TML port [c] : cm
  Depth at base [b] : cm
  Length of TML duct [LTML] : cm
  Resonant frequency [fb] approximately : Hz at 275 m/s
  Enclosure volume approximately : liters
  Classic: the unfold length is 1/4 of the resonance frequency's wave length

Low tuning
  Low tuning [h] : cm
  Length of TML duct: LTML : cm
 
  Resonant frequency [fb] approximately : Hz at 275 m/s
 
  Enclosure volume approximately : liters
 
  Low tuned: the unfold length is 1/3 of the resonance frequency's wave length


Half of transmissionline is to be filled with damping material (dark gray in the picture)

Therefore, the velocity of speed in the TML duct drops by 20% to approx. 275 m/s

Calculate change of Speed of Sound in fibrous material

Take a look at 'Definition Transmission Line'