A WAW and MLTL Part 1.
I will have some free time on my hand this Christmas so I was considering a new project. Finishing my 3WC build has left me with a pair of unused Tang band W5-2143 from the speakers they replaced. I really love the sound of the W5-2143. I initially had them in 12 liter ported enclosures tuned to around 55 Hz. The midrange was so good that I initially accepted the flaws of a wideband speaker. I tried to mitigate the lack of bass by adding sub-woofers. It kind of worked but I was still a bit worried about the limited Xmax of the W5-2143 in a ported box tuned low. Limited SPL was also a problem when not used near field or in a smaller room.
This is when I attempted my first WAW (Woofer Assisted Wide-band) aka FAST (Fullrange Assisted Subwoofer Technology) build. The result was quite bad because I made a couple of bad design decisions. I put the W5-2143 in a small chamber (4 liter) and crossed the speaker a bit too high (320 Hz). The sound wasn't terrible but the midrange magic was gone. I therefor wanted to build a new pair of speakers with the W5-2143 and the magic. My first thought was something horn loaded like the DECWARE DNA, Mileva or Demetri. But the lack of SPL was nagging in the back of my head and there it was again, the pipe dream of a WAW!
My 3WC build also left a couple of 8 inch subwoofers (the "stands" making them look like droids). I made them out of a pair of sub-woofers I bought second hand. I bought them cheap mostly for the plate amps so I kind of got the elements for "free". The sub-woofers, Argon Sub 840, was sold by HiFi Klubben in Sweden under their own brand name Argon. The elements are marked Model 8YD-ARGSUB840-4. I have not been able to find any T/S-parameters for these elements so I had to measure them myself. Rod Elliott got a pretty good guide with a handy Excel spread sheet to help.
The parameters I measured for the 8YD-ARGSUB840-4 was:
Re = 5.66 Ohms
Fs = 40 Hz
Qms = 3.72
Qes = 0.70
Qts = 0.59
Vas = 19.14 L
Sd = 210 cm2
Those parameters can then be used to derive the following parameters:
Compliance = 0.31 mm/N
Moving mass = 50.04 g
Vmax = 168 ml
Sensitivity = 85.98 dB
Now the big question, what can I build with these element and the W5-2143? The W5-2143 doesn't measure exactly as specified. These are the measurements by xrk971 at DiyAudio:
Sensitivity = 90.5 dB
Xmax = 4mm
Sd = 91.6 cm2
Fs = 69.3 Hz
Mms = 6.75 g
Qms = 3.23
Qes = 0.572
Re = 5.97 Ohms
Va s= 9.20 L
Bl = 5.54 Tm
Le = 0.187 mH
The W5-2143 is best suited for a ported box but I will run it in a closed one to save some space and to give it a bit of protection if I am going to run it low in a WAW configuration. Simulation software like Basta!, WinISD and BassBox Pro all point to a volume of 6 to 8 liters for a closed box with a Q around 0.71. F3 would be around 100 Hz and the sensitivity around 90 dB.
The sensitivity of the ARGSUB840 is only around 86 dB. Not a problem as long as they are driven by their own plate amps with a bit of EQ. But what if I want to use them in a 2-way without the plate amps? Is there a way to increase sensitivity? My first thought was a tapped horn but I haven't seen one designed for an 8 inch element. I would also like to keep the enclosure as small as possible. Low bass, high efficiency, small box, I know - I am already setting myself up for failure...
I stumbled across The Subwoofer DIY Page in search for a compact and efficient sub-woofer design. I have never thought of MLTL for a sub-woofer before but The Boom Unit looks promising. Time to fire up Hornresp and crunch some numbers. I want to compare efficiency so lets start with W5-2143 in an 8 liter closed box.
It requires a bit of imagination and experience to interpret the results in Hornresp. What I take away from this simulation is F3 = 101.4 Hz and an efficiency of 91 dB.
Lets compare it to the ARGSUB840 in a TL alignment. A good place to start is Quarter Wavelength Loudspeaker Design and make sure to grab the Alignment Tables Spreadsheet by Keith Webb (Excel spreadsheet). Also check Designing Mass-Loaded Transmission Line (MLTL) Speakers and Transmission line speaker design – Acoustical labyrinth. I figured that a good staring point would be a tapered TL with twice the Sd at the start and half the Sd at the terminus with a speaker offset of 40 cm from the start.
A bit of "fluff" added to the beginning of the line.
Not a bad start with an F3 of 36.7 Hz and a sensitivity of almost 90 dB with a line length of only 160 cm.
This is just a first cut with a simple two segment tapered TL. It looks promising enough to continue the work and that is what I am planning to do in my next post...
