True TL (transmission line) speakers have a folded labyrinth tunnel behind the woofer cone that comprises a damped wave guide that reverses the phase of a low-frequency sound pulse from the rear of the cone, emerging from the terminus end of the labyrinth in-phase with the front of the woofer cone. A proper TL design relieves compression back-pressure, cancels internal slap-echo back through the cone and, most important, can produce lower bass than any other design using the same driver.
- Skill level:
Things you need
- Woofer with low Fs, Qts, and Qes values
- 1/3-octave real-time analyzer (RTA)
- Pink noise generator
- Calibrated microphone
Design the speaker according to the largest infinite baffle volume possible for the driver with which you are working, using Thiele-Small parameters. Make sure that you have full parameter documentation for your driver. Within the confines of this volume, draw out solid panels between the right and left walls, that describe a rectangular folded tube tapering down from behind the woofer cone to the terminus slot. The total length of the tube should be a quarter wavelength of 4/7 the free air frequency of the driver you are using. Use 1,125 divided by that frequency, then divide by four to determine the labyrinth length. This will result in a design barely wider than the woofer, but tall and deep to accommodate the labyrinth folds.
Build your TL so that one side wall is open before sealing the cabinet. Line all walls with egg crate mattress pad material. Pack the area behind the woofer densely with Dacron pillow stuffing. Place Dacron loosely in all other parts of the TL. Staple acoustically transparent cloth over the inside of the terminus slot.
Tune the TL with one side panel temporarily fastened in place with only a few screws. Test with pink noise and an RTA, with the microphone placed 1 meter from the speaker, equidistant from the woofer and the terminus slot. To tweak the system for highest output at the target lowest frequency, experiment with stuffing density at the terminus slot. The mass of the air column together with the woofer mass will actually act to shift the woofer resonance down.
Tips and warnings
- Fs is the speaker's resonance in free air.
- Qts is the speaker's total mechanical and electrical Q: how it will behave within a given closed box versus no box expressed as a ratio (think of it as reluctance versus compliance).
- Helmholtz resonance and tuned columns are what make pipe organs work.
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