Tapered Quarter-Wave Tube

First described by Paul Voigt in 1930, TQWT/QWT uses like a transmission line design pipe lenght of 1/4 wavelenght of driver's resonance frequency. TQWT and QWT stands for (tappered) quarter wave tube and are also refered as Voight Pipes.
TQWT can't be seen only as a combination either helmholtz resonator or TL. It's behavior is a complex physics of 1/4 wave helmholtz resonator, bass reflex loading and horn loading. Therefore, TQWT cannot be successfully represented by only one of factors involved.

Since the TQWP is an oscillation horn, rather than compression, the point of maximum oscillation occurs at the port.
This oscillation is what moves the air in the port.

  • This page calculates dimensions based primarily on Fs of driver used, which is obviously not ideal.
  • Line tuning is set to Fs of driver to critically damp it at that frequency.
  • Calculations assume use of 19mm (0.75in) material for walls and interior baffle.
  • Dimensions are internal.
  • Height of intenal baffle should not extend more than the center point of the driver as mounted.

  • This calculator is not expected to give perfect results, but hopefully will be a good starting point.

Qt of the driver used in a TQWT should be between 0.2 and 0.7, best results can be expected if Qt = 0.4 .. 0.5.

The length of the TQWT should be agreed on about 1.0 to 1.2 x resonant frequency of the speaker.
The initial cross-section Sa of the TQWT is 75 to 100% of the membrane surface, the end cross section Se is 225 to 300% of the membrane surface.
The center of the speaker should be about 40 to 45% of the total length.
The diameter of the tailpipe Dv should be about half the effective membrane diameter, the length of the tailpipe Lv is about 1.5 x Dv

Driver parameters:

Driver resonant frequency Fs  Hz
Driver diaphragm surface Sd  [cm2]

H[cm] - Interior height of the TQWT
W[cm]- Inside enclosure width
T1[cm]- Initial interior at the beginning of the TQWT
T2[cm]- Distance Front/interior divisor at the top
D[cm]- Entire interior depth of the TQWT
T3[cm]- Distance of the loudspeaker from the beginning of the TQWT
T4[cm]- Lenght of the divider
Dv[cm]- Diameter of the final pipe
Lv[cm]- Length of the final pipe
Vb[l] - Net volume of the box in litres (without volumes of the reflex pipe)
F3[Hz]- Lower critical frequency of the TQWT ( -3dB )

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