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

Will P · « **on:** 18 Aug 2020, 03:09 pm »

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

Will P · « **Reply #1 on:** 18 Aug 2020, 03:17 pm »

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).

Will P · « **Reply #2 on:** 18 Aug 2020, 03:24 pm »

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

Will P · « **Reply #3 on:** 18 Aug 2020, 03:28 pm »

Will P · « **Reply #4 on:** 18 Aug 2020, 03:33 pm »

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.

Will P · « **Reply #5 on:** 18 Aug 2020, 03:34 pm »

You can definitely feel the air flow coming out of the vents. I placed the tower speakers so they stay on top of the vent covers otherwise the covers would rattle.

I measured the THD with the vent covers in place. Surprisingly the pressure drop around the vents did not create significant distortion.

No chaffing sound is coming from the vent covers for those test levels.

I did not measure the excursion of the drivers but it seemed around 0.8 Xmax. The subs are capable of around 6-8dB more before hitting the Xmech. The sound quality remains almost the same until it hits the Xmech.

The main strengths of that system are:

1. VERY VERY LOW THD - around 1% from 8Hz - 37Hz.

2. The subs are totally out of view and wife really happy about that.

OMG, I still can't believe those numbers given whatever the average mass market subs THD measurements are (10% and up).

I did the readings few times over just to make sure. Than I pulled out the RatShack meter and its SPL reading was +/-1dB from the REW readings - so the UMIC was OK.

The bass sound is Clean and Detailed - no matter if the music is playing at 100dB or 70dB - you can always hear the bass standing out especially when the sub sound starts/stops.

It sounds effortless, quick, clean, with authority, no lingering after the sound is gone.

-

JCarney · « **Reply #6 on:** 18 Aug 2020, 05:53 pm »

Very impressive build and install Will. Looks like it was both challenging and a blast, congrats on completion.

JCarney

Will P · « **Reply #7 on:** 18 Aug 2020, 06:17 pm »

Thanks!

Yes, it was one of those things you build once and you know you"ll have no chance of modifying it later.

.

Folsom · « **Reply #8 on:** 18 Aug 2020, 07:18 pm »

soooo, does it sound awesome?

Woodsea · « **Reply #9 on:** 18 Aug 2020, 08:01 pm »

Very nice! I will have to remember this for my retirement home. WAF factor and then dampen the room a ton so the resonance doesn't travel.

Will P · « **Reply #10 on:** 18 Aug 2020, 08:25 pm »

Quote from: Folsom on 18 Aug 2020, 07:18 pm

soooo, does it sound awesome?

Yes, it does sound good

Since the low distortion doesn't give away the subs playing - inaudible harmonics levels below room noise floor SPL - the subs frequencies are as much heard as they are sensed.

Really smooth low end.

mresseguie · « **Reply #11 on:** 18 Aug 2020, 08:53 pm »

This is seriously cool! Unfortunately, there's no way in hell my wife would ever allow such a thing.....unless I did it while she was visiting her parents 5000 miles away (which she does periodically).

Kudos!!

Will P · « **Reply #12 on:** 18 Aug 2020, 09:30 pm »

Thanks!

RolandButcher · « **Reply #13 on:** 18 Aug 2020, 09:49 pm »

Nice work! I went down the infinite baffle subwoofer rabbit hole a few years ago but I don't have the space or the skill to make it work. Looks like you did an amazing job. Congrats

Will P · « **Reply #14 on:** 19 Aug 2020, 01:17 am »

Thanks!

lokie · « **Reply #15 on:** 19 Aug 2020, 02:25 am »

Congrats. Cool project.

I also have IB woofers and couldn't be happier.

Will P · « **Reply #16 on:** 19 Aug 2020, 03:31 am »

Those are a step below IB - they are sealed subs since no access to outside space was available.

The sealed boxes are placed between the floor joists.

IB was my preferred choice but no way to duct them to the outside though.

Will P · « **Reply #17 on:** 19 Aug 2020, 03:34 am »

Will P · « **Reply #18 on:** 19 Aug 2020, 03:34 am »

Letitroll98 · « **Reply #19 on:** 19 Aug 2020, 12:05 pm »

Awesome job. What amplification are you using?