1st Sub - Looking for feedback.

[dewhite]

I'm a new member, and have been reading several threads over the past few weeks trying to get some ideas. I am about to finish a center channel speaker project (ZD3C) which I was doing to refresh my woodworking skills before taking on this sub. I have posted the thread shown below over at the Parts Express Tech Forums and AVSforums, and I thought I would repost here to get some additional responses...

----------REPOST----------

I posted here a few days ago about my center channel project and was impressed with the quick and helpful responses. Unfortunately I had already started construction on my project so I didn't get the full benefit of everyone's advice (although I think I made changes that will drastically improve my finished project).

I thought with my planned subwoofer project I might post BEFORE starting this time.

My room is about ~4,000 cubic feet, but opens up on two sides into probably another 10,000CF of the house. I use my setup for about 95% HT use, and am not a critical music listener. I currently have a Velodyne VRP-1200 that struggles along from it's position in the front right corner of the room. The idea is to use (2) RSS390HO-4 drivers downfiring in a ~7CF vented box, with an ep2500 amp.

I've read in several forums that using a subwoofer enclosure as a coffee table is a bad idea because it doesn't allow for optimal placement, but I think if I want to get a box this big into my room it's the only way. Ignoring the placement problems, please give my drawings a quick look and let me know if I'm on the right track. Most of my dimensions were determined by looking at an existing coffee table and trying to stick with pieces that can be cut out of 24"x48" "handy panels"...

I've neglected bracing in my drawings, but realize it will be important to brace heavily based on the size and weight of my design. I'd plan on using probably 4 window braces across the short axis of the enclosure, and one or two spaning the length of it. If I figure 3/4 of a cubic foot for bracing and driver displacment, and use WinISD beta to model the enclosure with 6.25CF @ 25Hz it looks like (2) 4" ID ports at a 14 inch length should work nicely.

One question I do have, is if the ports' area displaced inside the box (about .32 CF) should be removed from the internal displacement of the enclosure and remodeled in WinISD (which would change each port's length by about inch or so)?

I'm also wondering how much difference it will make if I face the ports towards the couch, or oppositely towards the open end of the room facing the rest of the house...

For anyone who is interested, I plan to use 3/4 inch MDF veneered in red oak to build the subwoofer enclosure, 4" PVC for the ports and legs, and birch plywood for the base plate and the tray on top, which would be filled with 12"x12" granite tiles (much cheaper at $4 per tile than a granite slab). I'd like to keep the top tray unattached to the enclosure (it's weight should be enough to keep it on there straight) so that if I move it can be removed to split the weight up. Also if it ever gets damaged, or doesn't match my interior decoration chioces I don't have to start over from scratch.

Questions, comments, thoughts, links, ideas, rants, etc are all(ways) appreciated.

[ThomasW]

http://techtalk.parts-express.com/showthread.php?t=194154&highlight=downfiring+RSS390

http://techtalk.parts-express.com/showthread.php?p=1414577&highlight=downfiring+RSS390#post1414577

[dewhite]

http://techtalk.parts-express.com/sh...nfiring+RSS390
http://techtalk.parts-express.com/sh...90#post1414577

Based on your input, and from comments on other forums I'm probably going to scrap the coffee table / downfiring woofer idea.

I'd like to figure out what would yield the most impact (overall performance improvement) for about $4-600 in amp and driver(s) over the Velodyne I have now.

I would be placing it in the front corner of the listening area depthwise parallel to the front wall of the room and front facing towards the open side of the room (sitting in the front-right corner facing to the left).

I could do a box as large as 18w x 30h x 38d (outside dimensions) but would rather build something a little smaller (like 16w x 26h x 30d), put a granite slab on top, and pass it off as an end table.

I'm starting to get overwhelmed with all the choices and things that need to be factored in. I'm wondering if any of you have any thoughts on which proven designs might be the most applicable. I remember that discussion of Rythmik products is looked down upon here, but wondered if anybody could say if their 15" vented design with the A300 amp is a better or worse idea for my needs/wants.

Thanks!

[ThomasW]

There are drivers more suitable for down firing applications. I'm a little pressed for time so I'll post more on this later..

[fbov]

There are really two questions here:
- what subwoofer configuration do you need for good, flat low frequency response across the listening area you plan to use.
- what would be the best thing to add to your current sub to achieve this goal?

The problem is that low frequencies interact strongly with room dimensions to create large (20-40dB) changes in perceived sound level with varying room position - bass can disappear. No amount of additional power will overcome a cancelling room mode; since the added power makes both positive and negative waves equally louder, they still cancel.

There are a number of approaches, and until I figure out how to set up my room, I won't comment further, except to say I think my next step is to get out the mic and start running sine waves to see if I can identify my room modes. Then, I'll try placing the sub where it'll cause the least damage, or likely build a second to give me enough flexibility to find a good combo.

So, perhaps consider making a Velodyne clone, or other complementary addition rather than a replacement. The coffee table is a bit on the large side for this; I'm considering end tables.

Have fun,
Frank

PS listen to the curmudgeon; he got me straightened out (enough).

[dewhite]

Happy to listen, looking forward to learning some things.

When I get home today I'll snap a photo of my room so I can make references to specific locations instead of trying to describe everything in relative terms.

I'd hoped to give the VRP1200 to my brother for his low budget HT setup. I helped him hang an LCD a few weeks ago and promised him my old Sony floorstanders and center channel as I'm replacing them with Zaph's ZDT3.5 and ZD3C. He's planning to use his old bookshelves as surrounds and I thought the Velo would round out the hand-me-down 5.1 setup.

If that little guy is really part of a solution for my room though, I'm sure we can come up with a project to build for his room on the cheap...

[don9146]

Is that the only position you can place your sub? Audioholics has a good article (with measurements) demonstrating how moving a sub in a room can change the sound a the listening position. It is amazing how changes in sub placement will change the sound.

http://www.audioholics.com/tweaks/get-good-bass/subwoofer-placement-the-place-for-bass-part-1

Later,
Don

[brent_s]

Modelling software assumes net volume as there's no way for the program to know your starting gross volume. So, you would subtract bracing, ports, and driver displacment from your gross internal volume and use that number for the cabinet volume in WinISD. Also, make sure you're using WinISD Pro 0.50a7, that's the latest as far as I know.

WinISD shows a sag of 4.7%, putting it at the limit of the recommended range. Considering that downfiring is never going to do good things for a driver's suspension, why tempt fate. That number actually surprised me, given how stiff the 315HO I have is...it takes some real force to move the cone by hand.

Given the budget of $320 for a pair of 390s, would $395 for the Maelstrom-X be too much of a stretch? Sag is only 2.5%. In 7 ft^3 net tuned to 20Hz with 1000 watts, I'm seeing nearly identical output between one Maelstrom and two 390HOs. The 390HOs in that model just brush Xmax while the Maelstrom is only using about 18mm with Xmax=33mm.

I'm assuming a coffee table sub implies nearfield placement...like 3'-4' to most listeners. This will go a long way towards solving output and room interaction problems. You won't get the "free" output from boundary gain using corner/wall placement, but you shouldn't need it. Given that type of placement, you might want to consider using the Maelstrom sealed to give you more size flexibility in that cabinet...Kevin recommends as small as 4 ft^3 while WinISD is showing 7 ft^3 to get a Q of .706. EQ to taste.

Do you have any idea of your actual SPL requirements? I get by just fine with a single ported DVC15 in a 3400 ft^3 room 13' from the sweet spot with typical movie playback at -15 below Reference...will produce peaks around 100dB, give or take on most movies. Even the little 1 ft^3 sealed 315HO in my living room (easily 6,000 ft^3 plus of space) will go louder than I care to listen for very long. Point being, even a single 15", thinking something like the Tempest-X or SDX15, might be all you need, especially if you decide to stick with the coffee table nearfield placement. I know, blasphemy for those think there's no such thing as excess, but there are economic and decore realities to consider.

-Brent