DIY 18" Subs for Atmos Theater Build

I'm about to start building our new house. the land is titled and we have a design that suits us with a sizable Theater Room - (L5.7m x W4.5m x H2.55m). I'm using Kef speakers for all round. But I'm unsure if I should buy a 2nd SVS PC13-Ultra or go DIY Custom 18" Subs. However although I'm mechanically very handy and can get some or all tools required to complete the job I just dont know hat would suit my room, current equipment, where I buy the drivers, whats a good driver, what design is good, how many would I need. etc etc etc. Basically I have no idea what I'm doing in regards to 18" DIY subs. Also I live in Geelong. At the moment I have some time to build these things and would love to get some help and also share the progress here with you.

Rough Sketchup


The plan - 26th Feb 2016

Front L/R - Floor Standing Kef Q900
Surround L/R - Floor Standing Kef Q900
Surround Back - Kef Q300
Centre - Kef Q600c
Atmos Ceiling - 2x Kef Ci200RR-THX - Will buy another 2 to make 4 total
Screen & PJ
Projector - Epson tw9200
Screen - 120" 16:9
AVR - Denon x7200 (Need to buy still)
AMP - Rotel 1585 5 x 200w
Blu Ray - Oppo 103D
Satelite TV - Foxtel
Game console of choice - PS4
PC - 5960x + 2x GTX Titan X

Building Materials & Stuff
So far its Rockwall insulation and Sound check plaster and some special glue with 90mm noggins.
The Behringer iNuke range is a solid choice. I'd recommend getting the DSP version(s).

Corner loaded, a single 18" Ultimax will keep up with your main speakers (120dB max for the pair) down to 20Hz, in a sealed box. If you went ported (and its gonna be big), you could get to 16Hz or so with a single driver, and still keep up with your mains.
However, some people like the bass to be 6-10dB louder than the rest of the setup. For 6dB, you'll need another sub. For 10dB, another again.

I'd probably set off with a pair of 24" cube-shaped enclosures, and go from there. If they're lacking (and I doubt they would be), you can either add more, or build some big ported boxes.


just a guy

2006-05-12 6:59 pm
Above the THX +10 dB reference? Few homes around here could take the abuse of 121-125 dBs in the bass. For sure mine didn't with pipe organ symphonies. :(


+10 db over THX spec is pretty boring these days. The average system on avs forum can do that. And a lot of them can do 125+ db at single digit frequencies, way beyond THX spec for spl and low frequency extension.

Here's a new build thread, eight sealed 21s added to the existing two Othorn tapped horns, over a ton of subwoofers in this room, and it appears he is getting just under THX spec with a single sealed 21. So with the other seven 21s and the two Othorns playing he's going to easily do well over 125 db down to single digit frequencies. Measurement of one driver - 8x sealed Incriminator Audio Judge 21" build - Page 2 - AVS Forum | Home Theater Discussions And Reviews

Video of this system playing 10 hz bass song - not sure how many of the drivers he has going here but he says he is severely amp limited - not sure how loud it is either but it's a fun song with fun lyrics - 8x sealed Incriminator Audio Judge 21" build - Page 4 - AVS Forum | Home Theater Discussions And Reviews

And one more video from another guy in the same thread saying he is hitting 129 db at the listening position -

Now I'm not sure how true any of this is, bass at that volume usually sounds like a lot of popping distortion so either these mics are very good or these guys are not producing the spl they say they are, but either way, their systems are fairly impressive.

One other fun fact from that thread - apparently Oprah rocks a pair of Danley TH221s. 8x sealed Incriminator Audio Judge 21" build - Page 4 - AVS Forum | Home Theater Discussions And Reviews

There are LOTS of systems like this on avs, IIRC MKTheater is doing something like 140 db at 5 hz or something like that in his basement HT.
Last edited:
Under a WCS (worst case scenario), the signal sent to the subwoofer system can require >125dB at the headrest of your seat in order to accurately reproduce the signal. This is with a 7.1 system with redirected bass from the 7 channels sent to the SW in addition to the LFE channel and assuming all channels are coherent with the same signal and the overall playback level is set to the level used to calibrate the channel levels (75dB at the seat with PN using the procedure internal to the processor). This playback level tends to be quite loud for most. Mostly from the midrange and upper frequency content. The bass content sent to the SW could be anything from 3-120Hz. A ton of movies have had the soundtracks analyzed and so far the loudest signal was about 2dB under a worst case scenario.

Obviously this requires quite the system to maintain this type of headroom from say 15-120Hz.


2016-03-07 7:15 pm
Hi! I am new to this page, just wanted to check out your project!
I also in progress of build new dedicated cinema room, about same size as yours.
I can recomend Dayton Audios subs, i have listen to many setups with Dayton, it can never go wrong actually.
And great value for money also!
I have 4 pcs of Dayton Ultimax 15DVC for my cinema, but already planning to buy 4 pcs more!
I am looking for best enclosure for these woofers now, and will soon start build.
Most people use them in sealed box, its easy and always works good. Sure, but i think vented enclosure will rock harder. Ofcourse it will be bigger enclosures ,but space i have a lot of.

Good luck to your house build!