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First described by Paul Voigt in 1930, TQWTs allow a good quality-to-price ratio. They are also easy to build.
Similar to transmission lines, the TQWT is different by its shape, a kind of horn is
used as back load but the driver is not placed at the beginning of the cone as usual but
on its side.
The port is used to adjust the rear flow according to the driver type, internal damping and desired response.
The resonance frequency depends only on the length L of the cone and the driver position is calculated by the formula indicated on the schematic.
During the tune-up, it's a good idea not to close definetely a side of the box to allow an easy modification of the internal damping.
The height of the port should also be adjusted through both measurements and listening sessions.
It's not advisable to select a very low frequency cutoff as a not very refined bass could be obtained.
The TQWT adapts very well to full-band drivers or bass-medium drivers of small
The folded horn gives a very practical loudspeaker with a driver positioned at the right height without any support.
Example for a 170mm driver. The lower cut-off is at 45Hz. May be enhanced with a tweeter and a simple passive crossover.
Here is another version for smaller loudspeakers:
Example using a 130mm driver like the Fostex FE127 but could use also smaller 100mm drivers like the Fostex FE103 or FE103 Sigma.
By Gérard Chrétien, published in L'Audiophile # 38 (Spring 86).
It uses a Triangle T17FL (170mm) as woofer-medium and a tweeter Technics 5HH10.
As the T17FL is no longer available, you can replace it by a Triangle TP16PP122o (Fs=53Hz, 92dB).
The tweeter can be replaced by the Triangle TZ20 (93dB) with probably no resistive network.
This very simple filter operates at 8KHz/6dB and the level of the tweeter is reduced by around 5dB to match the woofer.
A big work was done to obtain the best damping.
During the damping tuning a side is not definetely closed.
The loudspeaker (nearly) finished.