|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.
By it's shape TQWT is a conical horn with relatively high cutoff frequency. The driver is not placed at the apex of a horn as usual but rather at 1/3 of horn's lenght. Nevertheless there is a degree of hornloading in lower midrange and upper bass. The TQWT therefore adapts well to fullrange or midbass drivers, which don't have very large excursion. Generally drivers somewhat between the extremes (around 0.4 Qt) seem to be ideal for these. Such drivers require some reinforcement in the lower mid/upper bass to counteract the baffle loss and hence benefit from the partial and slight horn loading provided. Proper mouth size should be 1-2.5x Sd depending of the author, which should be closed down a bit to get the best sound.
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.
The port is used to adjust the rear flow according to the driver type, internal damping and desired response. The height of the port should also be adjusted through both measurements and listening sessions. System response will be primarily determined by the respective line-length and vent resonant frequencies; the driver position being used to create destructive interference of the higher frequency stuff we don't want coming out the vent (that is, it shouldn't be at an even fraction of the acoustic length of the pipe).
During the tune-up, it's a good idea not to close definitely a side of the box to allow easy modification of the internal damping.
|The TQWT is really good at a few things. It manages
very well to combine a horn and transmission line (and
also some reflex loading). TQWT's TL behavior offers a
nice acoustic load for the driver over most of it's
operating range and in addition the bass reflex
reinforcement can be tuned pretty low so that a pretty
deep and tight bass can result.
The main problem of the TQWT is that it will produce a combfilter effect due to line resonances at lower frequencies, certain bass notes would be near inaudible, other would boom way too much, a very uneven and lumpy bass response. The only way to reduce these line resonances is to stuff the line severely, which changes air speed and therefore acts as prolongated line, but at the same time kills efficiency. The notches are caused by line harmonics cancelling the output from the drivers front. They correspond to impedance curve peaks which can be easily seen on a plot.
There is fortunately another option. Instead of troublesome tuning, two or more pipes can be employed in one enclosure, possibly even with different drivers. One pipe should be tuned exactly 1/2 octave below the other, once the minimal stuffing in the throat (the narrowes point in the tapered tube) is taken into account. The trick is to have a major port resonance at the lowest frequency desired (essentially the driver's resonance). The pipe resonance will likely be somewhat higher than that. The port then needs a second "parasitic" (pipe type) resonance at the second harmonics of the line resonance. Normally this (2nd harmonic) resonance is out of phase with the driver's front radiation and hence creates a cancellation, the source of the deepest and lowest notch. This means the "parasitic" and normally unwanted resonance of a classic port is desired. What it does is to simply remove energy from the pipe at the the 2nd harmonic. In any case, now one pipe will have a "peak" wherever the other has a dip. If this is well calculated such enclosure will deal well with the resonanes, having a much flatter impedance curve and a fairly flat frequency response. This procedure should provide a deep and tight bass with a pretty even response.
There is a comprehensive study on TQWT with additional helmholz resonators writen by Jack Jakulis (Word 97 format).
Another possible improvement is use of multiple drivers placed at different positions along the length of the TQWT (SuperWhammodyne) to encrease efficiency and prevent cancelation problems with single driver TQWT. Check revisited edition of TQWT on Lowther Holland page, where another driver is added to cure uneven response of a single driver TQWT.
Usually TQWT produces excesive midrange output. In order to lower it's output lining and stuffing must be used. There were many dicussions where this should be aplied.
One of the ways to reduce line resonances is to stuff the line severely, which "elongates" (changed speed of sound) the line and prevents midrange output, but at the same time it decreases efficiency. This method is usually applied in transmission line designs, where theoretic principle derives from long lossy pipe.
Lining of the TQWT's sides is useful for prevention of reflections from side to side of expanding pipe (horn).
Instead of a smart talk, here are measurements of different stuffing strategies.