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  1. #121
    Senior Member Ian Mackenzie's Avatar
    Join Date
    Apr 2003
    The Tardis
    Below is a link that took about 25 seconds to find on port design and flare design.

    What l like a about this link is someone has actually done some experiments (the work) recorded the results and concluded with a port-flare calculator. The article is well written in and easy to follow.

  2. #122
    Senior Member RMC's Avatar
    Join Date
    Feb 2017
    RE POSTS # 116 & 117

    Following the path traced by faithful followers, the reply along the same lines.

    The first one's noise is certainly a pretty good description of who he is and what he does (the hat truly fits him), grabbing/copying other's stuff off the Net, TONS of pictures, graphs, charts, response curves, sketches, etc. and re-posting them with little or no value added, nor mention of source. His most significant audio achievement is copy and paste buttons. King of "Picture audio". He hasn't realized yet he was writing about himself too busy trying projecting it elsewhere. Its also revealing to see he's not annoyed by his buddy's own ramblings! Btw those who don't care aren't annoyed, they're elsewhere...

    As for the other's ramblings, his "reality" is nothing more than his virtual reality show he stars in. "Really?" From savior buddy to the rescue, as expected no surprise there. "Prick"? He sounds pretty familiar with that, he taught me, learned from him the master. It takes one to know one, right? Furthermore It shows how desperate he is, being out of feasible or valid arguments, to "debate" as he says...

    Seeking validation by peers and contact of authors? Pursuant to that logic, same rule would apply to those who criticize JBL's new products should seek peer validation and contact the Co. before to clarify the matter? Those criticizing JBL's direction change, and in particular the ones attacking Paliwal should do the same peer validation plus contact Paliwal, right? Did he comply with his own rule? Seems he's "forgotten" to mention the "negativity" of quite a few. Logic that simply doesn't fly...

    Plus he certainly disregarded his own "ignore button" suggestion to others, LOL, as he just loves coming back, FOUR times after his own advice! to the thread he's faithful to. The first time to edit his post, adding more of what he knows well: made up stories and fake news. Seems that "normal people" he uses don't keep making a fool of one's self...

    First he was against step-down and the thread (i.e. relevance), then became favorable to it, later switches back to being against it (i.e. thread closure), and now in post # 118 is backing it again with the same "Keele news" (though 25 years ago that idea was already in EV's 1993 TL 3512 data sheet that HE posted earlier!), as being the only post (his) that matters in the thread! Talking about large ego? Changing direction like the wind, showing the level of seriousness. Depleting his own credibility...

    When caught up by their own things they get all frustrated, bullying with the customary low ball and name calling, like teenagers do. No doubt its frustrating when one plays the "experience" card and "seniority status" but in reality knows a lot less. They still haven't tried competing the thread with stuff THEY raised. As "experienced" as they claim, with two of them it should have been a piece of cake. Nothing though...

    Bottom line, they rarely have real meat to deliver. They prefer teaming up to play their usual game of seeking "status" recognition and bogus "experience" attention. Buddy helping/saving buddy from the corner he paints himself in frequently. They hang around together, filling threads and complimenting each other, then save each other's rear end, regardless of merit, earning low credibility. Bailing out the other when the low value stuff they post catches up with them. Audio's entertainers...

    I treat people the same way as they treat me, often playing by their own rules, along with their own pompous language. These sure don't like whenever what they serve is being served back to them, as large egos don't. When they throw punches, I punch back. When one throws mud, he has to expect receiving some too. If not, well they know what to do...

  3. #123
    Senior Member RMC's Avatar
    Join Date
    Feb 2017

    Jbl 12 cu.ft. Box addendum

    RE my: "Even though at least 3/4" material is indicated, the plan was in fact made with that material thickness, and no mention of Baltic Birch use for example."

    The note below Fig. 10 on P.6 of the article indicates "Exterior dimensions shown produce correct interior volume when 3/4-inch board is used for walls; ..." Looks pretty clear what is the intended material to use.

    "B.C. Fir" 3/4" 7-ply wood is said to be void-free by some. As I have two pieces on hand, I can say I see no voids on the perimeter, inside?, but that sample is too small to certify void-free. Moreover, "Douglas-Fir is very stiff and strong for its weight, and is also among the hardest and heaviest softwoods." (From the Sitka Forest products Web site). Still away from Baltic Birch plywood in terms of density and rigidity though.

    Since "those of us with real practical experience" sets the tone, we'll follow the pattern. Those who know, know that panel thickness is rightly adjusted to box size (e.g. E-V's mention that about 2 cu.ft. or less boxes can be successfully constructed of 5/8-inch material." Pro Sound facts, #7, P.8).

    Re post # 120, sounds less convincing and helpful going from "At “least” being the point.", on to "follow the article..." (i.e. 3/4" use), then "offers latitude for a choice of enclosure options". The only physical box options given in plans and article are board thickness (the "at least") and the vent length depending on tuning frequency.

    The relevant issue mentioned, the weight factor, being in practice that of a 15.5 cu.ft. exterior box volume. Whether one uses 3/4" or 1" particle board or Baltic Birch or equivalent, a cabinet of this size and material will remain pretty heavy when loaded. Space taken by such a large box is another item mentioned further. Box too large? Then one can take advantage of an assisted alignment and benefit from a notable 4 cu.ft. reduction in size as shown by Kramer/Timbers: assisted 8 cu.ft net Fb 26 hz VS unassisted 12 cu.ft. net Fb 25 hz.

    As for "bracing has been scaled to the panel thickness it is fastened to.", in the present context this would mean thicker panels go in hand with thicker bracing and thinner panels go in hand with thinner braces. 25 seconds of thinking plus real speaker building familiarity indicate the proper relation between bracing and panel thickness is more nuanced than indicated.

    JBL adjusts the amount of bracing this way: "Proper bracing ..., depending on the size of the box panels." (note 1 P. 4, note 3 P. 2) Therefore according to box size. Electro-Voice is no different but interestingly indicates from what point one needs to do MORE: "In the largest boxes - greater than about six cubic feet- extra internal bracing is usually required for the largest expanses of wood in order to prevent sympathetic vibrations from affecting overall system performance." (same EV doc. P. 8) Vibrations mean that "... wall vibration indicates that energy is being wasted to move enclosure panels rather than moving air."(note 2 P. 5). And both E-V and JBL recommend a front to back panels brace in large boxes (same EV doc. P. 8, JBL note 1, P. 4).

    Also, the relation can be the following too. Other things being equal (e.g. wood type, box size, LF/levels involved) thinner panels are more vibration prone than thicker ones, therefore should require more extensive bracing to avoid resonance. Panel thickness/density and bracing being complementary towards rigidity. Less on one would require more on the other to get adequate stiffness. Meaning, If one uses somewhat thinner or lesser density panels on a box, then he/she must be prepared to do a more extensive job on the bracing needed to achieve acoustical deadness. This tells In cases the relation can be inversely related instead of a direct one.

    More extensive bracing can compensate for less dense (or somewhat thinner) material: after stating that "particle board is the most cost-effective material for general enclosure construction", and that "Birch plywood is very expensive", JBL indicates an alternative: "... a carefully braced enclosure made of high grade void-free Fir plywood can do the job just as well in most cases." (note 2, P.5)

    Therefore, panel thickness/density and bracing size and/or extent of it all work together towards the same goal of a rigid enclosure, plus will vary according to the situation: material type, density/thickness, box size, LF/levels to be reproduced.

    Large box weight control at the expense of rigidity and acoustical deadness? Interesting question as I have 5 lengths of 1X3" softwood in stock (not for bracing). 4 of them recent from big box store and 1 older one. Measurements reality follows.

    Recent ones are "Select" and have 2 3/8" W X 5/8+" thick (+ = about 1/32" more than 5/8", the former being the precision limit of my Craftsman US made measuring tape). As for the older length, same width, but on thickness add 1/32"+ to the above. Sure looks like skinny bracing in the present context of box size, driver used and VLF/levels involved, whether laying flat or placed on edge. Moreover, pine named in the plans is among the softest of woods.

    As thin and soft as they are, 1X3" on edge (preferred method for rigidity) it would be difficult to screw these in place for strength with regular size screws, in addition to glueing them. Screwing them from outside the box they would probably still crack with # 8 or 10 wood screws (# 6 having low pulling force), not even sure drilling a small pilot hole would avoid brace cracking here.

    3/4"+ strips of more rigid material (good plywood or particle board) of 3" width glued and screwed would do a better stiffening job than soft pine, flat or preferably on edge, without adding a lot of weight, if that is a real concern. In the present situation, being cheap on bracing is certainly not the proper way to go.

    JBL's own trilogy of speaker builder papers mentioned below offer guidelines that sure don't support the cause of small bracing when mentioning 2x4" or 3" strips of Birch plywood on edge (not laying flat) (note 1 P. 2, note 3 P. 2, ). As for the supporters of 1X3", JBL indicates " 1X3" pine bracing fixed on edge with glue and screws to the enclosure walls will help provide the minimum necessary stiffening... If you are building large subwoofer enclosures, bracing with two-by-fours works better...". (note 2 P. 5)

    New lumber dimensions have a tendency to "shrink" over the years: actual measurements VS nominal given as lumber mills repeatedly reduce dimensions a bit over time for profit and/or resources conservation. Like many others, 1X3" are progressively getting further away from their nominal sizes...


    The rectangular cabinet's longest dimensions are on the floor/on top instead of standing upright like some others, and use one of the two smaller panels to house the woofer and vent, the other one being used for the input terminal connector. So a very deep but low profile cabinet occupying about 9 sq. ft. The large 15.5 cu.ft. exterior box used this way minimizes its appearance in a room (less obtrusive look) and preserves more sight lines, while looking also like a "table".

    Compared to the other boxes in the article, this one uses a different configuration, or makes use of the box shape differently. Though no specific explanations were given for this choice (other than enclosure proportions of the 8 cu.ft. being equally suited to lowboy or highboy orientation on p. 6), one can figure out that required vent lenghts of 20" (Fb 25hz) or 30" (Fb 20hz) dictated the cabinet's configuration. Had the same cabinet been used as a conventional upright box with woofer on largest panel, the present 21" height becoming 21" depth, would not have accomodated these vent lengths properly.

    As seen in the Kramer/Timbers article and in the post # 26 plans, the vent is pretty close to the woofer (about 6mm). This appears to be less for reasons of large vents having less disturbances in close proximity to woofer, mentioned before, than as a result of closeness dictated by evident box configuration to accomodate vent lengths. Vent placement/proximity to panels/braces in the box seems unconventional compared to some guidelines, however with the box configuration adopted and vent diameter/lengths, the port is pretty much destined to be located where it is in order to try to clear bracing shown.

    More to come.


    P.S. RE posts # 105/107 Large cardboard tubes for concrete footings, to be used as very large speaker vent. Went to Home Depot recently and at the same time had a quick look at these tubes, as I said I would in post 107. The nominal 8" dia. has 7 5/8" internal and material thickness of 3-4 mm (1/8+"). I squeezed the tube a little to assess its rigidity, I'd say its "average". May need a touch of "doctoring" to add some rigidity to it, in order to keep it as dead as possible, specially as length increases. To reduce risk of resonance at high power, wrapping the outside of the tube with overlapping layers of "thick" rubber tape might do the trick since rubber is a pretty dead material.

    Note 1: JBL, Vented Loudspeaker Enclosure Construction and Operation.
    Note 2: JBL, The Most Commonly Asked Questions About Building Enclosures.
    Note 3: JBL Pro, Enclosure Guide.

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