Pencil Tuning Tips
I have recently completed a pair of Pencil 7.3s and Pencil 10.2s. ALL of my previous builds were sealed cabinet designs. Stuffing and tuning those cabinets was easy. Stuffing the Pencils was, and remains, a mystery. I'm just not sure how to achieve the best performance by tuning the type and amount of stuffing in a Pencil.
Which begs the questions.
> What is the stuffing used for in a Pencil?
> What tips can experienced builders provide for those of us building Pencils or similar designs?
Glad you posted. If its Ok for you, please give room retails and info on your set up (amps, source etc). Also if you time, can you take photos of the inside of the Pensils (assuming yours have removable backs).
More info will likely help.
My room is 26 feet x 14 feet by 8.5 feet. Speakers are "firing down the barrel" positioned along the mid line of the room (13 feet from the wall behind them) about 3.5 feet from the side walls. Listening position is midway between the speakers and the back wall about 7 feet from the speakers in an equilateral triangle.
Electronics are all Herron Audio ( Herron Audio ), latest products. Tube preamp and phono stage, 150 watt mono ss amps. Primary front end is a VPI Aries table with a JMW-12 arm and AT OC9ML/II. Second arm tube with a Lyra Clavis da Capo. Digital is a limited production CD player from Herron Audio. Interconnects from Herron, speaker cable is something I found.
Backs of my pencils are removable.
I may be somewhat computer literate, but don't do too well with pictures. Will probably play around with that in the next few days.
Beyond just getting this information for my own use, it might help those who are interested in building, or have built, Pencils.
Bill (Silver) and me have been exchanging info about how best to help him tune the boxes and "tweak" his set-up. At the moment, there's some sibilance coming out of the drivers. Bill's described it mostly happening on Trumpet and similar instruments. Given to docile character of Alp 10 Gen.2's, its a more a case of adjustment.
Bill's room is quite large, if I've understood correctly, the Pensils are sort of half way along the length of the room, some way off the walls. I've suggested to Bill he takes an SPL reading. If no SPL meter, maybe his mobile phone has an SPL application that will suffice.
From my experience, these areas are the ones to check first but others may have better suggestions:
1 - Damping immediately behind the driver. If its close and tight packed, reflection onto the cone rear could be a problem.
2 - Stuffing adjustment. Might to too tight packed, or too much material volume to suit the room condition. The resonant frequency of the driver could be pushed up due to excessive compression behind the cone. This will effect the cone's emittance.
3 - Amp and source. Herron look excellent, I've no experience with them. Not sure if they're warm or hard? They are powerful, so volume control will require care, easy to push the drivers a bit hard, the mid range will be first to show the effect. Maybe Bill can play with the stuffing, room positioning, them borrow a less powerful amp from members if near?
Bill, try to do some pics soonest, include room and equipment. Pics speak a 1000 words.
Hoping members will add to this thread and given Bill some help.
One thing that wasn't mentioned was what was the perceived problem?
I also recall a thread where the pencils were completely stuffed and described as "congested". Leaving the lower third of the line unstuffed seems to be a reasonable starting point.
As Mark has suggested, stuffing to close / tightly around the cone has been reported to negatively affect the sound. Cabinet leaks can also be an issue.
I personally have no experience with drivers being that far from the back wall. Given the built in BSC of the drivers, I would imagine the placement has more impact than might ordinarily be assumed.
Bill has mentioned this issue on I think at least one other thread as well, and your points 1 &2 above were addressed by the following comment:
I've built designs by both Bob (M10-A10) and Scott (too many to count) , and while they may use similar tools ( MJK among them), their targeted performance goals and construction details differ - not the least of these differences is in regards to stuffing / fill.
While the 12P is certainly a different driver than the 10.2(3), if Jeff is lurking, his observations on tuning of fill / stuffing to his SuperPensils might helpful as well. I've thoroughly enjoy listening to them several times, but don't remember any conversations regarding his own tuning of fill levels.
I built the Pencil 10.2s from MDF with an off-center brace about one-third of the way up from the bottom of the cabinet. (This is a "test box" and was never particularly intended to be the last word for these 10.2 drivers.) I deeply beveled the edges of the driver opening in the baffle as recommended. The box was stuffed with teased polyester above a "sling" made from polyester batting (like for a quilt) to hold the stuffing in place. The sling is about half way up from the bottom of the cabinet. I thought the stuffing was pretty sparse, but maybe not (this stuff REALLY EXPANDS A LOT!!). There is a 1 inch or so layer of recycled denim on the back panel top to bottom.
The results from this arrangement was decent bass but a very sibilant treble (as in "running out of the room with hair on fire and ears bleeding" sibilant).
I removed about 60% of the polyester fibre fill and experienced a serious improvement in the treble / reduction of the sibilance but an unexpected chestiness and minor congestion in the mids.
I'm wondering if a different TYPE of fill may be better.
Also interested in understanding whether it's necessary to line the box with some sort of absorbent material.
chrisb, that earlier thread was associated with the Pencil 7.3 which was experiencing a shouting characteristic. That has been corrected but with some recommendation for a different type of stuffing rather than the denim. I will experiment with that as time permits. (BTW, some additional breakin made a big difference as well as moving some of the stuffing a bit further back from the driver.)
This thread is about more general questions about what stuffing does inside the Pencils. Also about some of the 10.2 issues I've experienced, but my original intent was to gain a better understanding of the goal and use of the stuffing in the Pencil design.
IMO, a better understanding of why stuffing is used (beyond controlling reflections) in Pencils would go a long way in helping understand how to tune Pencils and what type of material would be appropriate to different room applications and sound characteristics.
OK, lets give this a try....
I went back to something chrisb said about removing a lot of stuffing and using dacron fill. These are pictures of the CURRENT Pencil A-10.2 boxes. I have removed about 1.5 cu ft of stuffing from each box just leaving a little above the sling. The current sound is reasonable. Bass is OK, treble fairly smooth, but a bit congested. The sibilance is pretty much gone, just a slight edge. Maybe some more breaking will help overcome that anomaly.
Only one speaker shown due to what seems to upload size limitations..
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