Transmission Line and Quarter-Wave Enclosures
By Michael Suding · Published 14 September 2026
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In brief
In a transmission line, the rear of the cone feeds a long, damped line. Practical designs emit sound at the end of the line. Tuned as a quarter-wave resonator, the line supports the deep bass below the driver's resonance.
The principle
Link to section “The principle”The ideal transmission line would be infinitely long and damped so that the rear sound of the cone is completely absorbed. Practical enclosures are shorter: at low frequencies, sound comes out of the opening at the end of the line.
Section through a folded transmission line: the rear of the cone feeds the damped line, which opens to the outside at the end.
Quarter-wave resonator
Link to section “Quarter-wave resonator”One variant is the quarter-wave enclosure. The line is long enough for a standing quarter wave to form somewhat below the driver’s resonance frequency Fs; the opening then supports the deep bass.
The length follows from the wavelength: a quarter wave at 40 Hz is a good 2 m long. For such a long line to fit into an enclosure, it has to be folded.
Assessment
Link to section “Assessment”Proponents of this design attribute a clearer bass to transmission line and quarter-wave enclosures than to bass reflex enclosures, because individual frequencies dominate less. This is a widespread view, not a generally proven property.
Our database does not yet contain a transmission line plan with reliable documentation.
Calculators and terms
Link to section “Calculators and terms”- Calculator Calculate wavelength – with quarter wave and temperature Calculates the speed of sound, the wavelength and the half and quarter wavelength from frequency and air temperature.
- Glossary Fs Fs is the resonant frequency of the driver without an enclosure: the frequency at which the moving mass and the compliance of the suspension resonate together.
- Glossary Wavelength The wavelength λ is the distance between two points of equal phase in a sound wave. It results from the speed of sound and the frequency: λ = c / f.
Sources
Link to section “Sources”-
Tier B · secondary source
Wikipedia · accessed on 14 September 2026
Enclosure types: sealed (12 dB/oct.), bass reflex (24 dB/oct.), 4th- and 6th-order bandpass, transmission line/quarter wave, horn, tapped horn, isobaric.
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Tier B · secondary source
Wikipedia · accessed on 13 September 2026