DTQWT mkII
DOUBLE TAPERED QUARTER WAVE TUBE
Copyright 2010-13 © Troels Gravesen

DTQWT mkII replaced by new DTQWT mkIII
- but read this article anyway for background story. 

THE DRIVERS     CROSSSOVER     CABINET     DTQWT MODE OF ACTION      MEASUREMENTS     

LINKS

INTRO

150 liter brutto volume is what you have to be prepared for should you decide to follow. From the front it seems fairly modest, 28 cm wide and 105 cm high, but depth is 50 cm and it will require some 20-25 cm minimum to the rear wall to breathe. Overall sensitivity is 95 dB/2.8 volts and having a minimum impedance of 6 ohms, it can be driven by low-wattage SET amps. I do not recommend 2-4 watts SET amps; these amps need 98-100 dB speakers, no matter what manufacturers may claim. I have no trouble driving the DTQWTs from my 8 wpc 300B SET amp. Deep bass - and that very special 300B midrange. With a proper 300B SET amp you can play these speakers for hours and hours without listening fatigue, a sign of low distortion and proper balancing of drivers. 20 watts do even better in terms of bass slam and punch. For a DTQWT amp review, please read here.

In many ways life gets easier when we make big speakers. We have more drivers, bigger drivers and distortion may all of a sudden be reduced significantly as none of the drivers have to move much to produce high sound levels, and we don't have to rely on drivers being capable of large cone excursions either.
We may all have experienced a 6" driver being driven to high SPLs and doing its best to pump air in order to produce deep bass notes - and at the same time trying to produce a smooth upper midrange/lower treble, the 640-2560 Hz range. It's not an ideal situation but a compromise most of us have experienced and as long as we don't push a "six + one" too hard, things go well.

The DTQWT construction is based on the TQWT construction supplemented by two 10" Eminence Deltalite II 2510 bass drivers added, providing a total membrane area exceeding a 15" driver. The front drivers and crossovers are the same as for TQWT. The rear bass drivers are fed from a simple 1st order crossover. It is thus possible to up-grade an existing JA8008/TQWT design by building new cabs and buying four bass drivers, 2 coils, 2 caps and 2 resistors, that's all.

So, what's a double tapered quarter wave tube (DTQWT)? Well, nothing new under sun. The quarter wave tube, transmission line - or Voigt pipe - came up around 1930 thanks to Paul Voigt. A tube having a length of 25% of the driver's resonance frequency, i.e. if we have a driver with Fs = 40, we would need a pipe of 34400/40/4 = 215 cm length (34400 = velocity of sound in air in centimeters/second). This doesn't necessarily mean we must use a Fz = 40 Hz driver to 40 Hz horn. We can load a horn with whatever we want except that drivers with a very high Qt may perform better in a traditionally stuffed transmission line. The DTQWTs were tried with 10" drivers having a Qt = 1.2 (brought in by DTQWT customer) and the bass simply became too boomy (too low electrical damping). 
To learn more about transmission line speakers I suggest reading here at Martin J. King's website: http://www.quarter-wave.com/. If the tube is tapered it's a horn, in this case a conical horn. Horns come in all shapes and sizes depending on actual application, from the deepest bass to the highest treble.

Classic Lowther examples of two drivers loading the same horn.

Loading a tapered tube or transmission line can be done in numerous ways and nothing prevents us from using multiple drivers loading the horn - even from drivers having different size. Same thing can be seen in some bass reflex designs where we find e.g. 10" and 12" in same cabinet, sharing the same port.

DTQWT mode of action
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DTQWT is in principle a 2-way with integrated subwoofers, thus no high-pass filter on the main front driver. This has the indisputable advantage of not needing any series capacitors, which would be excessively expensive if of good quality and sonically inferior to no caps, no matter how good the caps are. 
Loading a horn helps keeping front driver membrane in place due to high acoustic impedance compared to a vented enclosure or open baffle and this is further assisted by the mode of action of all drivers.
DTQWT features an 8" front driver and two 10" rear mounted drivers, all connected with positive polarity. They are placed opposite to one another and the rear drivers are fed through a huge coil, thus only adding weight to the lower octaves. All drivers load a center horn and you may ask whether the large bass drivers won't have an impact on the front driver from sharing the same center horn and the answer is yes - and this is intentional. Air goes where pressure is low, thus when the rear bass drivers move inwards, they will push air into the large center horn and by doing so they will suck air from the front horn and pull in the membrane of the front driver. Obviously the front driver will do the same thing for the bass drivers. 
By fine-tuning expansion of front, center and rear horns while measuring impedance, a balance can be found allowing the best possible power handling of the main front driver. The subjective experience from this is a cleaner midrange, distortion has been lowered (see measurements below) at high SPLs and yes - we have more bass. In fact a lot more bass compared to TQWT. The typical quality of the bass from a horn is quite different from conventional high mass/low efficiency bass drivers in small vented enclosures. The DTQWT delivers deep, dynamic and dry bass without the typical vented enclosure overhang.
In principle we can load the horn with any 10" bass drivers regardless of Fs, but there are limits. I found drivers with Fs ranging from 30-60 Hz suitable and without any noticeable difference unless the driver at the same time has high Fs and extremely low Qt, e.g. Faital 10PR300, which on paper looked suitable but in reality didn't perform well. Actually Fs was closer to 65 Hz and Qt = 26. The Eminence Deltalite 2510 II proved just ideal with Fs = 49 Hz and Qt = 0.39 (own data).
The length of the horn determines the lowest frequences you can expect from these kind of systems. This is not a function of the driver's Fs as many may think. Mind you: You can have a 5" high-Fs driver doing 30 Hz; you only have to built a humongous exponential horn to make it happen.
DTQWT is designed to do 40 Hz and from the impedance graphs below you can see this is the tuning frequency of the horn making two impedance peaks like a vented system.

Despite the description above, many people have trouble understanding why we don't need bass drivers with huge Xmax and low Fs. Please compare the combined membrane area to an e.g. 10" bass driver in a vented cabinet with the DTQWT. Here we have some 2 x 330 cm^2 from the bass drivers and 220 cm^2 from the front driver, in total 880 cm^2. This is 2.6 times the membrane area of a single 10" driver. If a single 10" bass-reflex loaded driver has to move +/- 5 mm, we only need 5/2.6= +/- 1.9 mm. Next, all drivers in the DTQWT loads a horn, an acoustic transformer. Wikipedia: A horn loudspeaker is a complete loudspeaker or loudspeaker element which uses a horn to increase the overall efficiency of the driving element, typically a diaphragm driven by an electromagnet. The horn itself is a passive component and does not amplify the sound from the driving element as such, but rather improves the coupling efficiency between the speaker driver and the air. The horn can be thought of as an "acoustic transformer" that provides impedance matching between the relatively dense diaphragm material and the air of low density. The result is greater acoustic output from a given driver.
The DTQWT horn works quite different compared to a bass reflex enclosure and we do not need excessive panel thickness and I wish you could all experience how the e.g. side panels feel when I play bass even at thunderous levels. You won't feel much.
Some also think the damping panel that can be added to the rear will dampen the bass output considerably. It doesn't. Not at all. It only dampens some midrange output from the bass drivers and may be useful if you have the speakers close to the front wall. I never used them despite having the speakers only 30 cm from the wall.

Initially the DTQWT was built with a single 10" bass driver, providing a system impedance of ~4 ohms minimum, not something that should make most decent low-wattage amps uncomfortable considering the overall system sensitivity. My 20 wpc PSE mono-blocks (6AS7 triodes) did well, but adding a second unit in series with the first driver left no doubt that the amps felt better about ~6 ohms minimum. Actually they felt much better and I admit I was a bit surprised by the difference. Distortion was significantly reduced at (very) high levels and the grip on the bass drivers significantly improved. Possibly the reduced cone motion from having two bass drivers also reduces back-EMF (back-electro-motive-force) and SET amps have low damping factor with poor grip on bass drivers compared to push-pull valve amps or solid state amps. Obviously my solid state amps didn't care at all whether seeing 4 or 6 ohms.

Some of the components used for the DTQWT

The Drivers
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The story behind the front drivers can be read here: TQWT

Left: Audax TW034XO-P47N tweeters. Right: Waveguide specially designed for TW034.
Click images to view large.

This is diy, so you have to do a little more yourself: Center the tweeter on the waveguide as seen on photo.
Mark for holes on waveguide and add a 10 mm mark on the drill. Make 3 x 10 mm holes in waveguide,
check if dome is properly centered and fasten with the screws supplied with the kit. That's it!

SEAS T35C002 option

Go to SEAS T35C002 option

Should you want to try out the new SEAS T35C002 tweeter in replacement of the Audax dome, click above link and read my comments and instructions.
The DTQWT T35 version includes T35 tweeter and waveguide with four threads fitting the faceplate mounting screws of the T35 tweeter.

Main front and bass drivers

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Left: JA8008 extended range, high efficiency driver. Right: Eminence DeltaLite II 2510 bass drivers for DTQWT.
Click images to view large.

DTQWT mkII crossover
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Changing your mkI crossover to mkII, download image here.

DTQWT drivers and crossover Parts
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Kit replaced by DTQWT-mkIII

Jantzen Audio 8 mm felt and 30 mm polyester foam.

Board for front drivers = 200 x 160 mm; board for bass section = 200 x 160 mm.

Please notice that the orientation of an RC circuit (1.0 uF + 15R) in the MT section can be either way. RC or CR, doesn't matter, does the same thing.
This also goes for the 10 uF + 15R in the bass crossover, it can be either way, RC or CR.
I have this question regularly as the photo below is opposite to the drawing above.

Crossover pics

Crossover for the JA8008/TW034 front drivers is the same as the for TQWT construction.

Right: Three resistors here for tweeter attenuation, 1R8, 2R2 and 2R7. My default is 2R7. Adjust to personal taste. Click images to view large.
I have learned that some people get confused by the three resistors on crossover board for tweeter attenuation. Some builders have connected all three.
Only one of the resistors should be soldered to the 5.6 uF capacitor. The resistors provide three levels of treble, low value = high level, high value = low level.
Click images above to view more closely.

Click image to view large (2000 pix width)

Crossover for bass drivers

Crossover for the two bass drivers is simplicity itself: An 18 mH coil in series with the drivers and 10 uF + 15 ohms across
output and ground provides flat impedance for the 1st order network. Good thing about these large cabs: Lots of space for crossovers.

Cabinet Construction

Click heading or images to go to cabinet construction page.

Initially only one bass driver was used, but there is only one thing better than a 10" bass driver: Two! Total membrane area = ~988 cm^2 from 2 x Beyma @ 768 cm^2 plus 1 x JA8008 @ 220 cm^2. A typical 15" driver is around 830 cm^2.

Click image to view large -
and see what Kjetil in Norway has been thinking about possible front panel designs.

Measurements
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Presenting proper images of bass capability from this construction is difficult due to front and rear mounted drivers. Normally we would take nearfield readings of both drivers and "port" (horn mouth) and splice the readings at e.g. 100 Hz and at 350 Hz (JA8008). The port output is usually presented separately. This approach doesn't work because a significant part of the lower bass comes from the horn and in order to measure lower bass we need a measuring window so wide ( > 100 milli-seconds) that we measure everything; that is 1) what comes from the drivers and port - and 2) what is reflected from room boundaries.
Feeding the speaker 31 Hz warble tones leaves no doubt this speaker has potentials in lower octaves, thus I placed the speaker in 6 different locations in my workshop (12 m^2) and used a 320 millisecond window in the CLIO MLS mode, allowing a lower frequency reading of 3 Hz. My workshop is anything but ideal for this kind of measurements, but I think the graph provides a useful sense of bass extension down to around 35 Hz.

Left: 320 ms reading of speaker in six different locations. ½ octave smoothing. Right: SPL @ 1 meter, 2.8 volts, left and right speaker. The left graph also tells how enormously important room placement is for the response coming from a loudspeaker, and this goes for any speaker, not only the DTQWT seen here.
Basically the speaker display the exact same performance as the JA8008/TW034/TQWT construction because the front drivers and the crossover are exactly the same. View here. What's more interesting is the impedance plot seen here:

Left: As can been seen from the graph above, we have a minimum 6 ohms impedance (red line inserted) and as stated in the beginning of this article, 8 watts from a SET amp can drive this speaker to significant levels before running into trouble. In fact much louder compared to average home-audio listening level, which is around 85 dB. This does not mean 20 wpc won't do better if you want to play even louder. The DTQWT can play enormously loud before significant distortion occurs. Remember, with 1-2 watt input we may have ~95 dB at 1 metre distance - and this is loud, really loud, but it doesn't mean I recommend 2-4 watts being sufficient. It isn't. Small amps may soon get into clipping no matter what manufacturers may claim.

Right: Distortion (THD) measured at 0.25 meter distance to 8008/TW034 drivers, 1/48 octave intervals, 2R7 to tweeter, 2.8, 5.6 and 11.2 volt input equivalent to 95, 101 and 107 dB at 1 meter. At normal listening level, e.g. 90 dB, we're below 0.5% THD from 500-20000 Hz. Doesn't get much better than this for home audio.  

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Builders' response on TQWT and DTQWT