Here's another option. This drawing shows an asymmetrical baffle treatment and puts the woofer center line as shown by the dashed line, but it does put the woofers closer together at 30" center to center. As the drawing shows the woofer cabinets are 11" apart. It would be possible to make the baffle lip symmetrical but that would separate the woofers by two more inches.


 

I would vote for making the horn assembly symmetrical so the midrange driver is centered in the MTM. If we need the woofers closer together I could eventually build some new baffles for the prototype boxes that allow the drivers to go to the edge of the box. It's not really an MTM if the tweeter isn't in the middle (ignoring the 045 for convenience).
 

Moving the woofer to the edge won't change much as the chamfered area above the horn uses up most of the space.

I will make the composite horn rig symmetrical and build it as tight as possible. It will probably yield a test set up where the woofers are just under 32" center to center.
 

The addition of 2" thick OC fiberglass on all panels is sufficient to reduce the standing wave ~ 350 Hz. Impedance drops to "normal" and the objectionable sound is greatly attenuated.

This doesn't necessarily have anything to do with the project. I just thought it might be interesting to some that the standing waves generated in a cube are definitely objectionable and are viewable in an impedance run.

Some closed box data:

driver mounted normal
4.0 cu ft gross
no fill
Qtc = 0.56
Fc = 49.8

driver mounted normal
4.0 cu ft gross
2" OC fiberglass on all panels except baffle
Qtc = 0.54
Fc = 47.8
 

I ran the TS parameters using BB6P just for fun and here is what I got.

One can recreate the whole thing by using the \Test\Driver feature and entering the following:

Manufacturer: JBL
Model Name: 1500AL
Serial Number: 1500AL-01686
Resistor: 982 ohms
Piston Diameter: 13.18 in
Voice Coil Resistance (Re): 5.4 ohms
Voltage (V1): 10.04 volts
Frequency (Fs): 27.9 Hz
Voltage (V2): 1.514 volts
Frequency (F1): 21.4 Hz (.289 volts)
Frequency (F2): 39.5 Hz (.289 volts)
Volume (Vb): 4.195 cu ft (includes inverted cone volume and enclosure cutout volume)
Frequency (Fc): 48.0 Hz
Voltage (V2): 1.47 volts
Frequency (F1): 40.7 Hz (.285 volts)
Frequency (F2): 58.3 Hz (.285 volts)

results

Mechanical Parameters
Fs = 27.9 Hz
Fc = 48 Hz
Qms = 8.072
Qmc = 14.07
Vas = 248.6 liters
Cms = 0.226 mm/N
Mms = 0.144 kg
Rms = 3.128 mohms
P-Dia = 13.18 in
Sd = 880.2 sq.cm

Electrical Parameters
Qes = 0.305
Qec = 0.549
Re = 5.4 ohms
BL = 21.13 N/A

Electromechanical Parameters
Qts = 0.294
Qtc = 0.529
no = 1.704%
1-W SPL = 94.46 dB
 

They "look good" in 4.0 to 5.0 cubic foot volumes tuned to ~ 28 Hz.

This volume/tuning should respond very well to room placement and/or EQ. Just a few dB at 28 Hz should do wonders in the right room.

It is easy to see why Tim thinks sealed sounds decent too. I would imagine room gain would fill in the bottom nicely. Again, a few dB of VLF/LF EQ with the sealed system could be very interesting. Excellent transient character!

I have yet to determine the port length for the prototype boxes for this tuning. I did unpack one this weekend to look at it. It sure isn't as thick as a cardboard mail tube or PVC pipe. I'm holding off cutting the requisite holes in the box until I do a few more 15" driver measurements (1500SUB, 2234H, and 2235H).

I also worked on the low pass filter mounting boards. I hope to have the low pass filters built and shipped to you by the weekend. No switches, you will need to change the damping by hand. I'll include the extra resistors which easily screw into the terminal strips. I'll have the default damping already installed. I'd like you to do sealed box measurements with the filters first before venting the enclosures. All the components on the boards attach via terminal strips and tie wraps so all the components are easily removed and replaced. Very "prototypical" as opposed to PC board mounted and such.

 

I have received all the inductors, capacitors, and resistors for the prototype networks as of last night.

Big, heavy, expensive, TOTL stuff...

I'm going to go looking for nylon bolts and brackets today.