Bass Trap Port Size

[Introspector]

I am a few weeks away from moving into a new home that has a finished basement that I plan to renovate into a dedicated home theater. The previous owner, and finisher of the basement, was very thoughtful in his execution. Among other nice touches, he framed in the walls of the finished basement roughly 18" in from all of the concrete foundation walls. His motivation for this was to provide future accessibility for wiring and plumbing repairs. And while I certainly appreciate the opportunity for easy cable runs, what I really see is the opportunity for very large and very invisible bass traps.

Based on a number of calculators, I will be in need of substantial low frequency absorption. My design concept is to fill the wall cavities with rockwool or fiberglass (after running the wiring of course). To make the best use of the bass traps, I was thinking about creating ports along the floor. One idea was to cut away the drywall for some height, just above the baseboard, running the full circumference of the room, and then covering the opening with acoustic fabric. My thinking is that this would convert the existing crawl spaces into glass filled bass traps that are 18 inches deep, 8 feet high, and run along at least two of the four walls of the final home theater space.

Assuming that my general approach is valid, my KEY question is, how large should the port be into the bass trap? Will a 12" opening from the baseboard up, running all the way around the theater, be sufficient for the lowest frequencies to be absorbed? Would a smaller port be sufficient? Do I need to provide a port at all?

As a quick follow up question, how much of the wall space do I need to fill with fiberglass to get the most efficient use from the material? I'm guessing that there is a diminishing rate of return as the fill ratio approaches 100%.

 

[bpape]

If you do this, you'll completely blow any isolation you have into and out of the room - unless you box in the absorption.

You'd definitely want to keep the openings large to avoid it acting more like a narrow Helmholz resonator. What you may likely find though is that there are still other places in the room that require broadband bass absorbtion. Your plan could work well from a decay time standpoint but usually, the rear wall around ear level is a good candidate for broadband bass only control. Many times, this is the cause of some of the deepest nulls in frequency response.

Bryan

 

[Introspector]

I appreciate your concerns about sound leakage. My plan is to cover the "ceiling" of the crawl spaces with MDF and seal any gaps with Green Glue.

And I share your concerns about primary axial nodes, especially along the longer front to back axis of the room. I originally considered one long port extending around the baseboard of the room primarily for aesthetic reasons. But you've got me thinking...

What if instead I cut large ports 2' x 4' in various places in the wall and then cover them with fiberglass acoustic panels? Appearance wise it would still look low key, like any other acoustic panel. But it would then provide low frequency ports into the huge bass traps behind. I was thinking maybe a 2'x4' or 4'x4' port right at the center point on the front and side walls. (At the rear center is a double glass sliding door with heavy draperies.) Maybe another couple 2'x4' ports behind each of the mains on the front wall.

This would provide a much more traditional look to the room (typical acoustic panels) while providing better axial node absorption by locating ports at the wall centerpoints.

Bryan - What do you think about this approach? If instead of one long port along the baseboard, I switch to 2'x4' or 4'x4' ports, where would you recommend locating the larger rectangular ports for optimal absorption. Obviously I would do some data capture with SMAART to confirm the presence of specific nodes, but where do you predict the greatest value to be found?

 

[bpape]

You'll still want to box in any cavities you have outside the shell. Sound will flank right around if you don't. You'll not get the MDF sufficiently sealed to the concrete around to stop it.

If you can't do center of the rear wall, then around the perimeter at the bottom is likely best. Vertical corners in front would also be good. I don't think I'd do right behind the speakers as that would minimize any tweaking of position to help with SBIR.

If you can do say 1' high all around the perimeter at the bottom and enclose it all outside the studs in MDF, that'd likely be cheaper than the MDF/GG at the ceiling and be more effective. You'll still want to insulate the cavity outside the box though to allow the wall structure to absorb some.

Bryan