Ron,
Based on how big this box is and how low the fs of the A12 is, I would imagine that it should have no problem reaching 50 Hz. At the risk of offending the designers of the Pensil - but no offense is intended - all I can suggest is that I think that the vent does not have enough resistance to "pull" the tuning frequency low enough. If I apply my "Accidental MLTL Design" to this box, I get a longer vent length than just the thickness of the front baffle. Using WinISD: for a 40 in tall x 10.5 in wide x 14-7/8 in deep box (approx 104 liters volume) with a 2.0 in tall x 10.5 in wide vent, if I set the tuning frequency at 46 Hz, the vent length should be 2.55 in. This may be something you can easily try - make a 2.55 in x 10.5 in piece of stiff cardboard, or foam core board and tape that to the top lip of the vent to make a shelf vent. See if that improves the bass response. Very easy to do and reversible - you have nothing to lose. This should impose a little more resistance and improve the bass extension. A 0.75 in long vent would indicate a tuning freq of only 53 Hz according to WinISD. And if the lip is radiused, the freq is even higher due to lower resistance to air flow.
http://www.diyaudio.com/forums/full-range/231951-accidental-mltl-technique.html
Based on how big this box is and how low the fs of the A12 is, I would imagine that it should have no problem reaching 50 Hz. At the risk of offending the designers of the Pensil - but no offense is intended - all I can suggest is that I think that the vent does not have enough resistance to "pull" the tuning frequency low enough. If I apply my "Accidental MLTL Design" to this box, I get a longer vent length than just the thickness of the front baffle. Using WinISD: for a 40 in tall x 10.5 in wide x 14-7/8 in deep box (approx 104 liters volume) with a 2.0 in tall x 10.5 in wide vent, if I set the tuning frequency at 46 Hz, the vent length should be 2.55 in. This may be something you can easily try - make a 2.55 in x 10.5 in piece of stiff cardboard, or foam core board and tape that to the top lip of the vent to make a shelf vent. See if that improves the bass response. Very easy to do and reversible - you have nothing to lose. This should impose a little more resistance and improve the bass extension. A 0.75 in long vent would indicate a tuning freq of only 53 Hz according to WinISD. And if the lip is radiused, the freq is even higher due to lower resistance to air flow.
http://www.diyaudio.com/forums/full-range/231951-accidental-mltl-technique.html
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Ron, i would've guessed a stuffing problem, not enough fill, type of fill? Did you also line the walls with felt or such before the stuffing?
Let us know how you make out with Bob B's MLTL, seems interesting enough.
I'm not exactly clear on what a MLTL does differently then Resonant Air Coupler (Pencil), i.e., aside from physical size does one or the other yield higher efficiency, less extension, impedance differences...
- xrk971, Curious, can you run that WinISD program for 52.35L 35.5"x9"x10" standard Pencil design?
Let us know how you make out with Bob B's MLTL, seems interesting enough.
I'm not exactly clear on what a MLTL does differently then Resonant Air Coupler (Pencil), i.e., aside from physical size does one or the other yield higher efficiency, less extension, impedance differences...
- xrk971, Curious, can you run that WinISD program for 52.35L 35.5"x9"x10" standard Pencil design?
Ok like i said, confused. Frugal-phile box library says: "Mark Audio Pensil Series (complete set) Resonant Air Couplers updated 10-june-11" Frugal-phile | Box Library / Mark Audio
Pensil's and Bob's design (obviously i can't see the innards and he hasn't posted the box plan), look like a simple vented box, one slot loaded the other with a tube.
I'm not getting where the simple rectangle equates to a Mass Loaded-Transmission Line aspect of the two designs? two designs?
You didn't stuff the entire box by any chance? cogitech did that with his A10.2 Pencils and complained about the poor bass performance. When he removed some of the stuffing, the missing low bass emerged.
jeff
It has nothing to do with shape of vent but fact that vent serves to constrict the opening which would otherwise be fully open cross sectional area for a quarter wave tube. The constriction makes the resonant freq of the quarter wave tube act as if it was longer giving a lower resonant freq. Analogous to how adding mass to tips of tuning fork will bring its quarter wave freq down. So a vent makes it "mass loaded". A mass loaded TL is close to a bass reflex cabinet but elongated, if you collapse aspect ratio back to a classic golden ratio box, the mass loaded operation goes away and you go back to bass reflex. You can read more about this at Quarter Wavelength Loudspeaker Design
Yes, +1 on this, make sure you follow stuffing plan. But typically no more than top 2/3 of pipe should be stuffed or 26 in down from top and the less you stuff, the more boomy the bass. If you stuff all the way and stuff densely, it becomes aperiodic and you get no bass enhancement.
The pensils are not designed to extract the maximum possible extension from the drive unit. If that is what you want, then you need a different enclosure. I did such a box for a friend in Scotland; it's not been released on-line, nor will it be since I know Bob is doing a similar one, and I try not to cover the same turf. The pensils are a specific alignment tuned to provide pro-audio style broadband gain to the mid 50Hz regions that self-corrects for step-loss in most practical situations, while the box is stuffed throughout like a regular TL, providing a near-flat impedance load and linear drive unit deflection. This is a part of the design; however, as is made clear on the pdf's, a removable back to the enclosures is recommended to allow the damping to be adjusted to suit amplifier output impedance, room response and personal preference.
The A12P, excellent unit though it is, cannot be tuned quite as low as the original A12 in a vented box, be it an MLTL, BR or whatever. Mark traded off a little bottom-end extension for a couple of extra dB efficiency, greater HF extension and lower HD. Personally, I think this was a wise choice since the current model is significantly superior to the original in all areas but outright LF extension.
The A12P, excellent unit though it is, cannot be tuned quite as low as the original A12 in a vented box, be it an MLTL, BR or whatever. Mark traded off a little bottom-end extension for a couple of extra dB efficiency, greater HF extension and lower HD. Personally, I think this was a wise choice since the current model is significantly superior to the original in all areas but outright LF extension.
An MLTL is a vented box. Bass reflex design assumes a uniform air particle density in the enclosure and no standing waves. An MLTL deliberately generates and uses standing waves in the box. The transition point from one to the other could be said to occur when one dimension is stretched sufficiently for the resulting eigenmodes (standing waves) to shift Fb for a given set of vent dimensions, or otherwise alter the FR from what you would obtain / be indicated for a given Vb under purely Helmholtz resonant conditions.
Good, they shouldn't be lined. To an extent, it depends on the amplifier & room conditions; that's why I designed them to have a degree of adjustability with a removable back & the damping. As an initial step, reduce overall damping density by 1/3 throughout; also make sure the damping isn't crammed up against the driver; too much too close will mass-load the cone & distort it under dynamic load conditions, preventing it from oscillating as intended.
As I say, the A12P based pensils are never going to be bass-monsters; that's not what they, or the driver were designed for, but they are quite efficient boxes and and not particularly reactive, so distortion levels are reasonably low and they're an easy load for most amplifiers, short of 500mW spud amps of course.
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The stuffing might be too close to the driver. I didn't know it needs to be 26" down from the top of the cabinet. Thanks!
That's because it doesn't. They're designed to a default uniform damping density. Xrk presumably isn't aware of the specifics of the alignment used in these boxes. You can of course adjust to taste; that's why a removable back is strongly recommended. As I say above, as an initial step, try reducing total damping density by 1/3. Then adjust as you see fit / prefer. This may be necessary in any event, since dacron hollow-fibre pillow stuffing is assumed, but there's dacron & then there's dacron. There's a lot of different types, and they often have somewhat different damping properties which you don't know about until you try it unfortunately.
Yes, if resistance is high enough. 12ga is frankly overkill for internal wiring, at least in terms of low resistance levels. You could try shifting to a pair of 24ga conductors pulled from a run of Cat5. The additional series R won't ammount to anything significant over short internal runs, but at least it's solid core.
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I used this for my PolyFill: Amazon.com: Fairfield Poly-Fil Premium Polyester Fiber, White, 1 Box, 5-Pound: Arts, Crafts & Sewing
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