SB29 DOS 28L Subwoofer

[VilleKn]

Hello fellow diy and audio enthusiast!

I am first time poster, but a long time lurker, and here to announce new project that I've started. It is a humble compact dual opposed closed subwoofer box using SB29SWRNX-S75-S drivers. At this point I've pinned down and purchased the drivers, amplifier and box design. Also the 24 mm birch ply in precut pieces have been ordered. The amp will be Crown XLS1002 that is good for 700 W to 8 Ohms bridged. EQ will be handled by a MiniDSP 2x4 that I've had for long time.

The target for this sub is supplement a two way speakers with one 7" inch midbass (Seas E18RNX) for mostly music. We live in a condo, so I have to be mindfull of the neighbors meaning that SPL target is not high.

I chose the SB29SWRNX driver mostly due to price and decent Xmax of +/- 11 mm. I could purchase these locally in Finland at ca. 125,00 €/pcs which seemed like a bargain. Also the drive is seems robust, is heavy and the color is black.
https://sbacoustics.com/product/10in-sb29swnrx-s75-6-norex/



The internal volume will be about 28 L. VituixCad simulation yields Qts 0,71 and Fb 47,7 Hz. With little help from Linkwitz transform Vituix shows the below curves. Max SPL is 103 dB at 40 Hz and 97 dB at 20 Hz within Xmax limits. With room gain this shuold meet the target. The curve below has sub sonic and LP filter applied.



Box will be made out of 24 mm birch ply that I can source precut to specified size. The rest of the woodworking will be made at the local community college woodworking class. There will be more bracing running top to bottom not shown below.



Thanks for reading this far and more to follow.

-Ville

[Evil Twin]

This appears to be a well conceived project- of course, in condo, 103 dB plus boundary reinforcement may get noticed by the neighbors. But my experience is that European flats and condos have much thicker and heavier walls than in America.

😂

It is fortunate you will not need your patio to do the wood working! I have tried to so similar things in the back of my Imperial shuttle, but the saw dust gets everywhere...

[VilleKn]

The walls are ca. 20 cm concrete and the volume shall dependent on the master volume setting not set to max all the time.😉

I have used the patio for woodworking before, but at this time of year it is +/- 0 degrees Celcius, dark, damp and raining water, wet snow or plain snow. These are not the ideal conditions for quality work.

-Ville

[VilleKn]

There is some progress to report.

I made a few minor change to desing to simplify the construction and hopefully for enhanced looks:

• The drivers are not countersunk -> just a trough hole to cut
• The baffles are inset by 5 mm -> this will allow for a foam bad, with hole the size of a driver, and wrapped in fabric of some stylish cover to be installed (this is an idea in my head, not tested for funtionality).
• Chamfers were reduced to just ca. 2 mm.

The ordered cut to dimension birchply pieces habe arrived and passed incoming QA for square cuts and size tolerances.

In the end, with the simplified woodworking and an available workspace in our building, allowed me to do the construction at home without too much complaints from my missus or the neighbours.

Driver holes and the chamfers were cut with my trusty Biltema router.


Glue up was done in our study. The bessey right angle clamp make it easy to glue pieces without sliding and in the correct place.

Now the sub is fully assembled, and ready for sanding and finnishing. Test "woofes" have been sampled. I will post a picture later...

-Ville

[VilleKn]

One question: Should there be a small hole for pressure equalization to outside in these types of small closed subwoofer enclosures?

[1Michael]

No. That is not needed.

[theSven]

One question: Should there be a small hole for pressure equalization to outside in these types of small closed subwoofer enclosures?

I would say no from the few sealed sub enclosures that I have built. Others who have more experience can share their thoughts on this. There is lots of sub building from years ago in this forum. You can try searching to see if this was asked previously. I've not seen it though from all the threads I have been updating through the year.

[VilleKn]

I found two sources to recommend a pin hole for small closed box subwoofers to allow pressure equalization.

I Will make a small hole to the bottom and plug it with felt.

1. https://www.linkwitzlab.com/Pluto/subwoofer.htm
"Box construction The subwoofer box is assembled out of 3/4" multi-layer plywood. For maximum stiffness use glued and screwed butt joints or other wood joinery. The internal volume is 15.4 liter. No stuffing is used. A small pinhole equalizes the static air pressures inside and outside of the box."

2. https://sound-au.com/linkwitz-transform.htm#guidelines
"The use of a small (say 5 mm) vent stoppered with felt to present a significant resistance to airflow is also a good idea - especially if the woofer does not use a vented polepiece (via the dustcap). This allows air pressure to equalise slowly, since you will have to expend considerable effort to make sure that the box has no air leaks. If present, any leaks may whistle or make some other equally undesirable noise when the subwoofer is in use. It is unlikely that you will be able to blame the dog for these noises (in case you thought you might get away with that excuse). "

[JonMarsh]

I would say no from the few sealed sub enclosures that I have built. Others who have more experience can share their thoughts on this. There is lots of sub building from years ago in this forum. You can try searching to see if this was asked previously. I've not seen it though from all the threads I have been updating through the year.

This technique has been used in the past by some manufacturers, and if a woofer has a cone and surround that is genuinely gas tight, I'd be inclined to endorse it IF it is likely that the system will be moved around, to different elevations. It's difficult to get a true gas tight enclosure, but not impossible.

[theSven]

Learning from the masters here on the forum!

[JonMarsh]

In the end, with the simplified woodworking and an available workspace in our building, allowed me to do the construction at home without too much complaints from my missus or the neighbours.



-Ville

I sympathize with your issues for construction space- while I have lots of room now, the major build I did in 2019 was on the patio of my rented condo.

[tktran]

I was never quite sure what the late (and great) SL was concerned about.

Perhaps he was referring to change in pressure difference due to cone movement? ie. volume displacement of subwoofers with large amounts excursion (Vd = Sd x excursion) will result in higher pressure as the cone moves in, and lower pressure as the cone moves out (Boyle's Law)

Or was he concerned about static pressure differences of an airtight box vs outside box atmospheric pressure, which as you may know, varies slightly from day to day, and is dependant on temperature and humidity etc. That this would create unequal pressures between the inside of the box and the outside, resulting in the driver being displaced from it's rest position.

I never left any hole, and never had any problems. And I have two 12" subwoofers per cabinet, and 2 cabinets in the room.

But it would be interesting to see some empirical evidence to see what the effect would be.