Well, the day finally came so here is my build thread. Thank you to everyone in this wonderful forum for all the help provided, both direct and indirect.
As you can see from the title I chose an 18” Maelstrom-X and 2 18” passive radiators. The form factor is a 25” cube and the tuning is somewhere between 15.5-16.5Hz depending on how much fill is added (according to BassBox). Double-thick front and side baffles give it an internal volume of 6.4ft^3 after taking into account the drivers and braces.
The main reason I opted to spend the extra money on the PRs vs. a sealed design is explained by the following figure.
I chose AE PR18s with a 2100g mass. Out of curiosity I calculated the equivalent port dimensions and it is 22in diameter by 48 feet long!
Here is a excursion sim from WinISD including a SSF (4th order 13Hz) and the BFD and EP2500 rolloff. This is my approximation of worst case conditions as it is 2000W and the coil temp rise is 100K.
Here is the design as drawn in google sketchup. I used excel and measurements of the driver and PR frames to make sure there was enough internal clearance. The spreadsheet allowed me to vary the back-to-front offset of the passive radiators and it would redraw the figure shown on the right.
I settled on moving the PRs 1” towards the back w.r.t. being internally centered.
And the cutsheet diagram
Due to workspace and skill limitation I decided to have the box built for me rather than go the tool borrowing/rental route. I had the box made by Nik Brewer from this thread.
The box arrived last weekend and it was a beauty. Here is a picture of it as I was putting the hurricane nuts in.
inside the box showing the driver/pr and braces clearance.
For fill I purchased standard cheap pillow polyfill. I think it may be a little less dense than “speaker grade” polyfill. In a rush to hear it for the first time I just put two bags in for a total of 40oz. I will be adding more soon but will need to find a way help it stay in place, particularly at the top of the box, while keeping it away from the moving parts.
For initial testing I mounted the drivers without the rubber gasket because I’m saving my socket head screws for the final product. Also the machine screws I’m currently using make it difficult to use the gasket because of the tight flush-mounting.
Here a link to a post where I show my final mounting solution
http://www.hometheatershack.com/for...rom-2xpr18-prebuild-questions.html#post160267
Measured in-box impedance (including about 0.1ohms total between amp and driver)
This was pretty close to the BassBox and WinISD predictions. The two peaks are just a little closer to each other than the simulations. The result shows that I really shouldn't have to worry about thermal issues over much of the low end because of the high impedance and cooling benefit of high excursion.
This graph places the tuning at about 15Hz (0 degrees phase crossing). But eyeballing the excursion on test tones makes me think it is closer to 16Hz.
Here is my first REW measurement. It is not in the final room. It is placed 1' from the wall and 5' from the corner in a 11.5'x25.5' room in the basement which opens up to a hallway and stairs. I used a mic calibrated down to 10Hz
It was quite easy to smooth out to fit a 60Hz LPF target using the BFD.
Listening impressions.... I don't know what to write here, it's just ridiculous - particularly stuff at 30Hz and 16-20Hz.
edit: shortcut to the finished result
As you can see from the title I chose an 18” Maelstrom-X and 2 18” passive radiators. The form factor is a 25” cube and the tuning is somewhere between 15.5-16.5Hz depending on how much fill is added (according to BassBox). Double-thick front and side baffles give it an internal volume of 6.4ft^3 after taking into account the drivers and braces.
The main reason I opted to spend the extra money on the PRs vs. a sealed design is explained by the following figure.

I chose AE PR18s with a 2100g mass. Out of curiosity I calculated the equivalent port dimensions and it is 22in diameter by 48 feet long!
Here is a excursion sim from WinISD including a SSF (4th order 13Hz) and the BFD and EP2500 rolloff. This is my approximation of worst case conditions as it is 2000W and the coil temp rise is 100K.

Here is the design as drawn in google sketchup. I used excel and measurements of the driver and PR frames to make sure there was enough internal clearance. The spreadsheet allowed me to vary the back-to-front offset of the passive radiators and it would redraw the figure shown on the right.


I settled on moving the PRs 1” towards the back w.r.t. being internally centered.
And the cutsheet diagram

Due to workspace and skill limitation I decided to have the box built for me rather than go the tool borrowing/rental route. I had the box made by Nik Brewer from this thread.
The box arrived last weekend and it was a beauty. Here is a picture of it as I was putting the hurricane nuts in.

inside the box showing the driver/pr and braces clearance.

For fill I purchased standard cheap pillow polyfill. I think it may be a little less dense than “speaker grade” polyfill. In a rush to hear it for the first time I just put two bags in for a total of 40oz. I will be adding more soon but will need to find a way help it stay in place, particularly at the top of the box, while keeping it away from the moving parts.
For initial testing I mounted the drivers without the rubber gasket because I’m saving my socket head screws for the final product. Also the machine screws I’m currently using make it difficult to use the gasket because of the tight flush-mounting.

Here a link to a post where I show my final mounting solution
http://www.hometheatershack.com/for...rom-2xpr18-prebuild-questions.html#post160267
Measured in-box impedance (including about 0.1ohms total between amp and driver)

This was pretty close to the BassBox and WinISD predictions. The two peaks are just a little closer to each other than the simulations. The result shows that I really shouldn't have to worry about thermal issues over much of the low end because of the high impedance and cooling benefit of high excursion.
This graph places the tuning at about 15Hz (0 degrees phase crossing). But eyeballing the excursion on test tones makes me think it is closer to 16Hz.
Here is my first REW measurement. It is not in the final room. It is placed 1' from the wall and 5' from the corner in a 11.5'x25.5' room in the basement which opens up to a hallway and stairs. I used a mic calibrated down to 10Hz

It was quite easy to smooth out to fit a 60Hz LPF target using the BFD.
Listening impressions.... I don't know what to write here, it's just ridiculous - particularly stuff at 30Hz and 16-20Hz.
edit: shortcut to the finished result