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Old 15th April 2015, 03:16 PM   #1
JamesE is offline JamesE  United States
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Default Under bed sub

I need more bass in my theater/bedroom. The room is large--10,000 cu. ft. Currently I have 4-15 inch drivers setup as an infinite baffle sub with a MK 350.

The target freq. for this new sub is 40 Hz and below. If I build 4 subs with 18 inch drivers that are 12x28x37 they will fit under a queen sized bed with a 4 inch high toe space. They would be down firing.

I've looked at some speaker cabinet design software. And don't really understand what I am looking at. Am I making any big mistakes? What can I do differently to have a better sub? Is this a good driver for this application? Dayton Audio UM18-22 18" Ultimax DVC Subwoofer. Should the subs be filled? How much bracing if 1 inch MDF is used?
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Old 16th April 2015, 01:26 AM   #2
GM is offline GM  United States
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Well, there's 'no replacement for displacement' in a sub system, so with only ~5.716 ft^3 based on using 19 mm/13 ply Baltic Birch or Appleply plywood before subtracting out the drivers and bracing, you theoretically can just make it to ~15 Hz at THX reference.

For sub duty, 1" MDF requires too much bracing to = the stiffness of the plywood plus the relatively minor bracing it requires.

Stuffing is probably desirable to help dissipate the high heat of a power hungry driver.

Based on published specs, the UM18 is suitable for down firing.

Divide the room's longest dimension by ~565 ft to find where room gain begins in earnest. The sub system ideally needs to be ~flat to this frequency and let any room gain fill in a bit below this point.

The mains and bed/sub should each be at one of the room's odd harmonics and the distance between them ideally <1/4 WL of the XO point or ~282.5 ft/40 Hz = ~7.06 ft, though some find up to 1 WL acceptable down this low or ~1130/40 = 28.25 ft.

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Old 16th April 2015, 04:24 AM   #3
JamesE is offline JamesE  United States
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Thank you for the reply.

I could make 2 cabinets, ~11 cu ft. with 2 drivers in each.

The room is a geodesic dome and the widest point is 27 ft. The volume of the room is ~5,000 cu ft. I just got done building a bass trap around the outside of the room and am very pleased with the results so far.
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Old 16th April 2015, 08:53 AM   #4
bjorno is offline bjorno  Sweden
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FYI:

b
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Old 16th April 2015, 09:40 AM   #5
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FYI:

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Old 17th April 2015, 03:42 PM   #6
JamesE is offline JamesE  United States
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Thank you for info on the domes. I've looked but have never found anything. I know of no other HT in dome homes.

I gave up on the idea of the subs under the bed. Leaning toward 2 ported subs with 2 18 in drivers in each. I guess bigger is better. Fb-14.7, Vb-18.8cu ft. Now I just need to learn more about ports and internal bracing.

This is a waterfall plot of how the room in looking and a picture of the bass trap that I completed which extends all the way around the room.
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File Type: jpg Bass Trap-1.jpg (998.6 KB, 102 views)
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Old 18th April 2015, 03:19 PM   #7
JamesE is offline JamesE  United States
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Another question. Do I need 2 subs with 2 18 in drivers for this room or will 2 subs with 1 driver each be sufficient?
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Old 18th April 2015, 06:15 PM   #8
GM is offline GM  United States
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Quote:
Originally Posted by JamesE View Post
Thank you for the reply.

I could make 2 cabinets, ~11 cu ft. with 2 drivers in each.

The room is a geodesic dome and the widest point is 27 ft. The volume of the room is ~5,000 cu ft. I just got done building a bass trap around the outside of the room and am very pleased with the results so far.

Another question. Do I need 2 subs with 2 18 in drivers for this room or will 2 subs with 1 driver each be sufficient?
You're welcome!

OK, so ~flat to at least ~21 Hz.

How loud do you want to go at low distortion at the listening position [LP]? This will determine what size/qty. of drivers, amp power is required.

Room construction normally needs to be factored in also when one wants to go below 20 Hz at THX reference or ~live concert, etc., i.e. my house is a basic Deep South lapboard sided stick built on a suspended floor/crawl space and learned the hard way that repeated 120+ dB sub 20 Hz organ sustains and bursts down to the corner loaded dual Servo-Drive Contrabass subís 14 Hz tuning [four long throw 15Ē/four 18Ē PRs] during various action movies, culminating with a depth charge sequence in the U-571 film that literally brought the house down, dropping the living room ceiling/insulation and some empty [fortunately!] storage boxes on us and shattering one of the concrete columns supporting the floor between the subs and LP along with splitting just about every drywall tape seam in the house.

Fast forward ~14 yrs and just found out that there was some roof structural damage also to explain the [mysterious] tape joint splits in the attached garage some years later that added a couple of grand to an otherwise routine re-roofing. Apparently my houseís Fs was a lot higher than I assumed.

I assume your dome is quite rigid and if on a concrete slab will probably handle whatever you want within reason, but better to factor in building construction instead of assuming like I did.

Not sure though how your bass trap is going to impact the sub systemís infra/sub-bass response though and have to wonder if its design wasnít a bit premature. If I understand the plot correctly, it seems like it will absorb a lot of any sub systemís lower BW.

GM
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Old 18th April 2015, 07:07 PM   #9
miklos is offline miklos  Hungary
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Man, this is wanderful, good readig. Thanks GM.
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Old 18th April 2015, 07:32 PM   #10
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You're welcome!

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