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True labyrinth versus infinite baffle
True labyrinth versus infinite baffle
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Old 22nd April 2008, 01:32 AM   #1
Tubamark is offline Tubamark  United States
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Default True labyrinth versus infinite baffle

Hello, all.

I am building my basement listening room/office, and am faced with a decision . . .
I have a front corner in which I can install either a large box of some sort (projecting into adjacent utility room) or simple infinite baffle in same location. Simple grille will be all that's visible in-room.

I love IB bass, and have suitable high-Qts woofers (Hawthone 15" Augies). The rub is this: the added cost of soundproofing the utility room (for the benefit of the rest of the house) is just not practical.

The solution I am considering is an anti-tuned "true" labyrinth behind the driver designed to simply absorb the backwave as much as possible before it vents into the utility room. -40 db or more seems possible, but I'm looking for a precedent . . . Can the MJK calculations handle this?

There are existing mini versions of this (Nautilus mid-treble or Linkwitz Pluto midbass). Mine would differ by having an aperiodic terminus.
Mine would function like a too-long & overdamped transmission line, or a very large aperiodic box.

I have a rough strategy in mind for construction/stuffing/testing of said labyrinth, but would love some modeling or literature (been hard to find so far) to guide me.

Yep - I've considered tuned alignments (like tapped horn, or TL) in the same spot, but IB simplicity that allows driver changes and slow rolloff is really what I'm after. Tuned alignments are driver sensitive, and this box will be kinda permanent once the wall is up.

Box could be up to 18" x 25" x 72" exterior dimensions MAX. Smaller or none would be better, of course.

Thoughts? Precedents?

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Old 22nd April 2008, 05:03 AM   #2
GM is offline GM  United States
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Location: Chamblee, Ga.
Default Re: True labyrinth versus infinite baffle


Well, the ideal would be a plane wave tube (PWT), i.e. a 1/2 WL terminated pipe that acts acoustically as an infinite horn with the same throat area as the area of the tube (CSA). For a Sd = CSA, a 1st order roll off (Qtc = 0.5 aka critically damped) would begin around 106Hz, completely damping the driver's Fs impedance peak, ergo it would have a nominally flat electrical phase through its passband. IOW the theoretically perfect IB.

It wouldn't be small though at ~36.47 ft^3 net Vb/driver to take it down to ~Dc, so it would need to be a false wall built in and then there's the lack of any real LF output without digital boost as the trade-off for having such an accurate reproducer. Yes, these and other simple aperiodic alignments can be simmed up to a point using MJK's software.

WRT other high SQ alignments, the driver's high Qts and relatively high Vas means that your max cab size is acoustically small for one driver, so two makes it doubly so.

For instance, a max flat impedance TL (aka max flat aperiodic) would have a much flatter BW to a lower F3 with a 'close enough' flat impedance, phase response as the PWT with the trade-off of a 2nd order roll off (Qtc = 0.707 aka max flat), so can't even get in the ballpark WRT your output abatement requirements. At ~14.14 ft^3 net Vb though, one driver should fit in your cab's max dims.

Obviously, less is always less in bass reproduction, so with decreasing cab Vb, F3 rises and stuffing density increases and since the latter rapidly reaches a point of diminishing returns, ergo so goes Vb reduction.

Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.
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Old 27th April 2008, 02:06 AM   #3
Tubamark is offline Tubamark  United States
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Default Re: Re: True labyrinth versus infinite baffle

Thanks for the feedback. Seems like my High-Q/Vas free-air driver is the limiting factor, and the design could become complicated after all . . .

My room's longest dimension is 22', so room gain won't kick-in fully until <26 Hz.

I guess I should just consider a different driver (with mid-Qts & very low Fs, and go for for a simple critically-damped (read: big box) sealed alignment. Many driver choices would suit that approach, and still meet my objectives for backwave attenuation, deep response and insensitivity to (reasonable) driver selection.

Let me know if any other alignments come to mind that would meet that criteria!

-- Mark
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