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In-floor subwoofers - 8x8" drivers each, highest WAF factor, lowest THD

11294 Views 17 Replies 6 Participants Last post by  T-Dub
In-floor subwoofers - 8x8" drivers each, highest WAF factor, lowest THD

I built in-floor sealed subwoofers consisting of 8 x 8" drivers each (Dayton Audio DCS 205-4). Those were the only 8" drivers which I found to fit in the subfloor cavity with a box built around them. Each sub has the cone surface area of 2 x 15" subs and 1 x 15" sub displacement. The drivers were not the reference type but I had to compromise since they were the only ones which would fit. The whole build was designed around the subfloor cavity restriction.
The purpose of the subs was to use them below 40Hz. The box size gives each driver Qtc=0.9, Vb=5 Ltr, , Fb=82Hz .
When the house was still renovated and did not have the drywall installed yet I tested them both with 120Hz x-over at full displacement and they played really loud - I would say around 120dB at 120Hz. The garage floor concrete slab was shaking :surprise:.
Each subwoofer has 2 boxes - the first one is the "drivers head", the second box is the "cavity box". Assembled total length is 10.5ft, height 9.25", width 14".
The subwoofer boxes are bolted together and have a removable cover for bolts access. They were installed in the basement ceiling - between the floor joists. The boxes are separated from the floor structure and don't transfer vibrations to the house. The drivers are mounted opposed to each other to cancel any vibrations too. The boxes are suspended by multiple metal straps (like a swing cradle). For safety there are aircraft cables spun across the bottom too (not tensioned, slightly loose and taped to the boxes to prevent rattling).
The subwoofer ports are covered with the same covers used for the heating ducts in the house and you can't tell that they are there.
When there was no furniture in the house yet you could feel the floor bending from the air pressure whenever the subs played.
Now when I have them set up below 40Hz they are enough for movie playing. For this open floor plan and the music levels I enjoy occasionally though I would need 4x that displacement. That's why there is another project on the go for few years already - but that's another topic altogether :wink2:.


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More pictures of the build..


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.. and some more pictures..


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That was a very innovative and remarkably well executed solution you created. Impressive work!
Very cool!

Couple of questions:
1. Do you have more detail on the business end of the box? Specifically the port that feeds into the room.
2. What are using to power these?
3. Do you have any drawings of this?

I have seen some other applications of under floor band-pass subs, but yours is pretty elaborate and well made.
Here are the in-floor subwoofer design drawings..
I'm using Crown XLS 2500 to power both subs. You can use Crown XLS 1500 too. Plenty of power to drive them both to Xmax.
The combined cone surface per sub equals 21" sub. The excursion is 8mm only though. Displacement equals 15" sub each subwoofer box.


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The subwoofer port is 10" cavity at the end of the subwoofer with grill mounted on top
Thanks for sharing!

Last question, how are the 8 drivers wired? Series/parallel or some combination?
You can wire them based on you amp power and amp speaker Zmin.
I wired them for 8ohm : (2) parallel groups, each group consist of (4) in series.
Brilliant concept and fantastic execution! I’m a big proponent of putting tactile transducers in seating risers, so when I saw the title, I had to know the whole story. I’m curious- you seem to have gone to a lot of trouble to decouple these from the floor; did you consider letting them do some “bass shaking”? Or is that a natural side effect even with all the decoupling?

Kudos on a VERY cool idea!
The corner of the room is the stiffest area of the floor. Besides that the EQ needed for floor shaker given the floor structure and resonances and the EQ needed for the Subs would be different and impossible to get equal response.
That's why I decided to: 1. Cancel out all subwoofer drivers vibrations by opposing them in pairs; 2. Decouple the subwoofer boxes from the house framing so no direct sound gets transferred to the walls, floor and nearby rooms.
The sound effect is generated by the subwoofers pressure alone. After certain sound level you start feeling it in the floor. Whenever a massive flying object in the movie hits the ground it feels like it landed right in front of you.
Again it's a main floor room with basement floor below and although I doubled up all the floor joists to make it stiffer the floor is still responsive.
It's obvious you put a lot of time and thought into this system. Hopefully it does everything you expected it would.
Awesome build!
Today I did some THD measurements of the in-floor subs using REW and calibrated UMIC. The readings were taken in the Listening Position which is 12ft away from either sub.

The room is open concept, 45ft deep, 28ft wide, 10ft ceilings. Open to both basement and second floor through stairs. No treatments on walls, lots of hard surfaces, very little furniture, lots of windows. There is a horizontal bass trap in the back upper corner.

The normal listening level peak is 105dB Pink Noise (2Hz-20Hz noise spectrum) in Listening Position.

That is the max level before LF starts rattling the windows, patio door, floor, walls.

Any louder than that and it gets too much LF and the room can not take it.

There is no EQ on the subs.

The subs are xovered at 40Hz.

The subs were drawing no more than 400W each side (-20dB LED on CROWN XLS1500 is solid on; -10dB LED never came on; subs drivers wired for 8 Ohm)

I measured THD starting with the Noise Floor and then continued with the 8Hz - 37Hz band.

I knew the subs were clean sounding even before I started.

The THD measurements really surprised me though. THD% was around 1% anywhere in the 8Hz-37Hz band.

That is a mid-bass driver THD territory, not a subwoofer THD territory.

That shows the advantages of using multiple smaller subs vs one single sub even when the displacement is equal.

Some of the advantages are increased sensitivity (add extra +3dB each time number of subs is doubled; here that means 9dB more sensitivity - which means the total power for all 8 subs is 1/8 of the power needed for 1 large sub if big and small driver sensitivities are equal), reduced thermal compression (the 1/8 power is further dived between the individual drivers - in my case that's about 40W/driver; heat dissipation is a lot easier when the heat load is spread out to multiple drivers) , reduced THD.


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This is a fascinating sub build. Thank you for sharing! Excellent work I hope you are enjoying the **** out of it.

I have one question. Why cross them so low at 40hz? With such sensitivity & low distortion/compression why not cross higher (80-120hz)?
Sorry I guess that’s two questions haha.
There are 2 main resonances within the subwoofer box :

1. 60Hz standing wave (box length 10ft)
2. 90Hz subwoofer head cavity 1/4 wave length resonance (40in head unit cavity)

That's why the sub is xovered at 40Hz.

Main towers take over above 40Hz.
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