Definition Transmission Line

One of the famoust TML's: IMF TLS50

A transmission line enclosure is a waveguide in which the structure shifts the phase of the driver's rear output by at least 900, thereby reinforcing the frequencies near the driver's Fs. Transmission lines tend to be larger than ported enclosures, 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.

A variation on the transmission line enclosure utilizes 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 resulting in a seashell like appearance. Most notably Bowers & Wilkins have used this approach in their flagship Nautilus speaker as well as the use of smaller straight tapering tubes in many of their other lines.

Recently, numerical simulations by several researchers (eg, George Augspurger and Martin J. King) have brought a degree of order to the theory and practical design of these systems.

The Transmission Line has its design roots in the Stromberg-Carlson acoustic labyrinth (1930).
It first consisted of a log pipe, (open at one end and the driver mounted at the other) with a cross-sectional area about the same as that driver.
The line length was made about 25% of the driver resonance's wavelength.
Working with the same basic concept in the early 1960s, A.R.Bailey expermented with different damping materials and techniques in folded labyrinth lines.
This work has sinds become the basic bible for most TL designs.
With TLs, the enclosure functions as a low-pass filter with a 90degrees phase shift, absorbing all the rear wave energy of the woofer except for frequencies below about 75Hz.

TLs can be characterized by:

  • Low cabinet resonance
  • Relativley loud deep bass (below 50Hz).
  • Highly damped impedance peak.
  • Decreased cone motion in the 40Hz region.
  • Low degree of mid-bass coloration.
  • Rather low overall efficiency.

Line Length and damping material

The basic line length criteria for TLs is: 25% of the wavelength at, or just above, the driver resonance.

The following are the traditional, and definitely "proven by performance", criteria for TL construction:
  1. Make the line length 1.80-2.4 meter (6-8 feet) this corresponds to 1/4-wavelength of an undamped line 35-45Hz.

  2. Make the cross-sectional area of the line starting dirctly behind the driver to be at least 25% greater then the driver's radiating ares Sd.
    The strating cross-sectional area of the line should then taper to the port opening, which should be equal to Sd. This generally means that the last taper section, or the last 60 - 90cm (2-3 feet) of the line.Beginning cross-sectional areas of the line can vary from about 1.25 to 2.5 times the Sd. This defines a taper ratio, beginning area/Sd, which will have a direct effect on the subjective quality of the bass and midbass of the TL.
    Low ratios generally give the "lean" and "tight" sound quality associated with TL's.
    As the ratio increase past 1.4 - 1.5, the emphasis is placed more on the low bass area, and a better sounding midbass. This criterion depends upon critical damping of the line, and results will tend to vary with a combination of both line taper and stuffing.

    Table 1.0 gives the different ranges of the beginning cross-sectional area for the line with different diameter woofers.

  3. Damping material, in order of preference:
    a. Long Fiber wool
    b. 50% Dacron, 50% long fiber wool
    c. Darcon
    d. Fiberglass

  4. The line should be stuffed with an average of about 0.5 lbs to 0.7 lbs of acoustical stuffing material per cubic foot of line volume.
    Decreasing the quantity of the material near the port area will change the midd-bass quality.

More very good information about Transmission Line Loudspeakers:

Table 1.0 - TL Line Requirements for differten Line Tapers

Port Area
Port Area
Beginning line area
(Tapper Range:
2.5 - 1.25)
15 0.0855 858.1 133 333-166
12 0.0450 451.6 70 175-88
10 0.0330 335.5 52 130-65
8 0.0215 212.9 33 83-41
6.5 0.0158 161.29 25 63-31
5 0.0089 90.3 14 35-18

Table 2.0 - Line Length vs. Frequency

Frequency Undamped
c = 344 m/s
c' = 123 m/s
Hz 1/4 L 1/4 L
20 Hz 4.3 meter 1.55 meter
25 Hz 3.44 meter 1.25 meter
30 Hz 2.86 meter 1.04 meter
35 Hz 2.47 meter 0.88 meter
40 Hz 2.16 meter 0.76 meter
45 Hz 1.92 meter 0.67 meter