Thats a big box to go unbraced.
Can you pick up a 4" or larger hole saw and drill from HomeDepot/Lowes? You could probably get them to cut the brace panel to size.
Another option could be to get 1.25"ø or 1.5"ø diameter poplar dowels (HD/Lowes), arrange in a offset "+" cross brace (X axis should not intersect at the center of each dowel), where one individual dowel is set on top of the other to form the "+". I'd place the "+" braces in three locations unevenly spaced from upper to lower a bit above the slot vent. You'd want to screw the front to back dowel's to the rear panel, facilitate panel removal to alter stuffing.
Or, if located near Philadelphia you're welcome to stop over and use my home woodworking shop/tools.
Can you pick up a 4" or larger hole saw and drill from HomeDepot/Lowes? You could probably get them to cut the brace panel to size.
Another option could be to get 1.25"ø or 1.5"ø diameter poplar dowels (HD/Lowes), arrange in a offset "+" cross brace (X axis should not intersect at the center of each dowel), where one individual dowel is set on top of the other to form the "+". I'd place the "+" braces in three locations unevenly spaced from upper to lower a bit above the slot vent. You'd want to screw the front to back dowel's to the rear panel, facilitate panel removal to alter stuffing.
Or, if located near Philadelphia you're welcome to stop over and use my home woodworking shop/tools.
I wasn't content with mine (Pensil 10.2) until I removed about 2/3rds of the stuffing I had in them and made some other changes.
Since then, I have tried several other damping methods and the following is what ended up working best (for my room and taste):
1) Top 1/3rd of cabinet: lined on top, back and one side with 2.5" pink fiberglass - no polyfill or other stuffing
2) Middle 1/3rd of cabinet: stuffed with standard polyfill - I didn't weigh it, but the amount is about 1/4 of the total amount I had in each cabinet to start with
3) Bottom 1/3rd of cabinet: empty
Having said all this, I am still very likely going to be building something different for my Alpairs. Why? I tried them in a 2.3 cu.ft. standard bass-reflex box (my Clairtone Console project) tuned close to Fs and the bass response was shocking. That is what I was looking for, and the Pensil just wasn't designed to do it. I wish I had known that it was designed to only go to the mid-50s. Live and learn.
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iirc some had to block the Pencil 10.2's vent for more focused Bass http://www.diyaudio.com/forums/mark...estern-electric-300b-bravo-3.html#post2469781
I have to wonder where's the trade off with the Pencil vs ported box, i.e., bsc box losses efficiency/sensitivity, "tube" friendly impedance, (Scott's plots post #29 SuperPencils not... point to that http://www.diyaudio.com/forums/full-range/233461-superpensils-not-doing-3.html#post3443578 ), etc. etc.
With that box size and tuning, there will be an exaggerated bass response around the mid to upper 30's, which will cause the driver to reach it's excursion limit very quickly. You're better off with powered subs if this is the type of bass you desire.
I don't buy that, as a simple 15L vented box has a modeled f3 of 45Hz.
jeff
I've looked at the cone excursion plot, and it doesn't get scary until you start pushing sub-30Hz content through it at 20 watts. I will never run my drivers that hard, but I am considering implementing a simple RC high-pass at 30Hz to rule out the dangerous frequencies (in case any teenagers are at my house while I am not there). Not really a "need" for me per se, because I am basically always pushing less than 6 watts, at which point the cone excursion is well within safe limits all the way to 15Hz.
Scott pretty-much said mid-fifties was the goal. I find this difficult to believe, too, but whatever. Horses for courses and all that.
I've heard the 10.2 tuned to 37Hz or so, in a simple BR, and it sounds fantastic (in my room, but not just to my ears). It spoiled me.
I bet they would satisfy me without the extra compromises of the ungraceful (but impressive) BR that I slapped together.
I'm kicking myself for not building your A10-M10, Bob. As I said. Live and Learn.
I'd be trying that right now, if I could, but I am locked out of my house.
Long story... Maybe tomorrow.
(Plan is to make the port tunable by sliding the shelf in or out and running sweeps till I get a nice balance with the room response. After all, why is a 3dB hump centred at 38Hz (or whatever) such a big deal when we all know our rooms are doing way "worse" things to the bottom end?)
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Cogitech,
IIRC you had put a black cloth grill over the port of your Pensil 10.2 - that might be interfering with the LF?
In my experience room gain tends to make the 3 dB hump around 38-40 Hz sound worse. Lot of FR designs tend to run out of steam in the 60-80 Hz area and things like floor-bounce (and/or room suck-out?) tends to affect that region negatively too.
IIRC you had put a black cloth grill over the port of your Pensil 10.2 - that might be interfering with the LF?
In my experience room gain tends to make the 3 dB hump around 38-40 Hz sound worse. Lot of FR designs tend to run out of steam in the 60-80 Hz area and things like floor-bounce (and/or room suck-out?) tends to affect that region negatively too.
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I actually only did it to one port, as a proof of concept. The "finish" hasn't progressed at all since then, as I am struggling to love the Pensils. I can detect no discernible difference between the two speakers, though, so I do doubt the grille cloth has anything to do with it. It's brown, after all, it should sound warmer, no?
All the more reason to make a shelf port that can be tuned in real-time, starting at 28 Hz and opening up all the way to whatever the default is (54Hz as far as I can tell). If I have a room mode or null somewhere, it will become evident with multiple HolmImpulse sweeps.
Sacrilege, I know. What can I say? I'm a non-conformist.
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Ha, ha, there can't be too much harm in trying out?
I am not sure if Pensil 10.2 is also tuned to ~55 Hz... that is for the A12 variant, and the A10.2 can go lower than the A12. Scott can confirm that.
Even with the A12 in Pensil cabs I had no problems getting below 50 Hz...
You know, I don't know why no one has made a speaker that has a tube or shelf that telescopes in and out with a rack-and-pinion style knob - so you can "Tweak-the-Tuning" in real time. The mechanism can be found from a surplus store that sells telescope focuser tubes. Although gears really are not needed - just friction drive from a rubber roller bar on the flat of the shelf or a tube will work.
Some simple wooden sliders, forming uniform channels on the inside of each side panel, will do just fine. Slide the "shelf" in for lower tuning, out for higher tuning. A centimeter scale on each one will help acheive balance and as a reference when making notes during sweeps.
Along those lines, my audio buddy, Dave Rosgaard built a couple of expanding boxes with registration marks to vary the displacement or volume of the box. They have switchable panels for the drivers and different vents. It's just several airtight sliding boxes within boxes. It's harder to describe than it is to figure out when you see it. Trust a Boeing Engineer to come up with something like this.
Best Regards,
TerryO
Best Regards,
TerryO
Hmm. 10.2 in a 2.3ft^3 'standard bass-reflex box' tuned to ~Fs? Anechoic attached from UniBox & WinISD. To each their own.
Fb on the pensils varies depending on the design. Note: Fb is not a brick-wall filter. They should provide decent output to the low 40Hz regions or lower under most practical conditions, depending on the box. They will not go as low as Bob's MLTL or an equivalent. They are not designed to. If that's what you want, you need to build something else.
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Indeed. To each their own room and preferences. The simulation is not the full story, and you know this better than most here. Put that design (or close to it, see below) in a particular room and it will sound boomy and graceless. In my room, it sounds fantastic. I haven't done enough sweeps to figure out why yet, but I plan to. Also, at moderate listening levels I appreciate a little "loudness" enhancement. Not bone-crushing, window rattling "thump" but at least the nice, deep, rich bass tones that are so essential to so much music. Without it, it doesn't matter how good the mid-range and treble are; something is missing. The thing with the Alpair 10.2 is that the mid-range and treble are incredibly strong. I wouldn't call them "shouty", but when the bass is lacking they certainly approach "shoutiness". They deserve to be in a cabinet that takes as much advantage of their excellent bass as possible, IMO. They have an Fs of 35Hz, after all.
Perhaps the Pensil is rolling off right where I have a room suck-out. I don't know (yet). All I know is that I stuck the 10.2s in an old console (enclosures are more precisely 2.148 cu.ft.) that I tuned to ~35Hz and I could not believe how good the bass was. Very rich and smooth down in the deepest notes but maintaining speed and detail in the upper bass, and it all extended right into the dining room and kitchen, with zeroed EQ. I was amazed.
Having said all this, I can coax similar response from the Pensils after changing the lining/stuffing extensively and if I get relatively aggressive with the EQ. Of course, aggressive EQing of bass response is not doing the drivers any favours. I'd rather expect the box do that heavy lifting (especially a box this huge). Hence, the plan to play with an adjustable port shelf. Forgive me for taking the liberty of attempting to tune this enclosure to my particular room and preferences. I do not intend it as a personal attack on you.
Someone mentioned that I should use distributed subs. Apparently they get the impression that I want sub-30Hz waves at goofy SPL. I don't. I simply think that the 36-46Hz range is critical for truly "full range" audio, and asking that of a driver such as the Alpair 10.2 is not a ridiculous proposition. In fact, I know it isn't because I have heard it with my own ears.
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