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Thread: JBL 2235 with 2450SL and 2332 horn?

  1. #16
    Senior Member 1audiohack's Avatar
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    Like this?

    Barry.
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    If we knew what the hell we were doing, we wouldn't call it research would we.

  2. #17
    Senior Member RMC's Avatar
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    Slot type vents




    I certainly don't intend to de-rail the thread but I do note no one has answered here since the last picture was posted days ago...

    I'm of the opinion that slot side wall vents give a low-frequency cabinet better appearance than tube ones as shown here. I'm mostly interested by the very good looking LF enclosure having narrow vertical side wall vents on the baffle. They seem convenient to make efficient use of front panel space available. They also tend to minimize the perceived appearance of vent size (less gross) VS the big hole(s) in the box...

    I see these vents from time to time on commercial and DIY cabinets. For example, the small Genelec studio monitors 1030A (6.5", 80W) and 1032B (10", 180W) both have slot vent on each side of tweeter, not wall sided though, and their look isn't as nice as here in my view. Plus those small speakers are not as large air movers as the woofers in the post.

    I estimate the vents seen on post # 16 to be about 1" wide and 15" high, so about 15 sq.in. each, or the equivalent of a 4.4" diameter port. Not far from JBL's minimum recommended for a 15" driver (i.e. one-third of the 15" woofer's diameter). Also power input related. Their latest advice is to use the largest port area that will practically fit in the box, makes sense with the ever increasing output of more modern woofers.

    As a form of rectangular vent, slot ones are ok in principle. Vent shapes like square, rectangle, round or triangle usually don't present a problem. However, narrow ones seen on some enclosures have been a question mark in my mind for a while, certainly not regarding the nice look they give to a box!

    I've always wondered about their good or less so? acoustical performance, more so when the narrow slot includes one or two cabinet walls. That concern I've had about them is probably why I never dared using such. Prudence has made me keep vents away from walls as often as I could. They could work as well as others or not, I wasn't sure about this.

    In normal home use sound levels these may not present an issue at all, but what happens as power goes up markedly? In the example seen two relatively high power VLF drivers are used, possibly in parallel, and these drivers can move a sizeable amount of air. Will that port's width cope adequately with the large air flow? Is the vent located to avoid turbulence or are slots prone to port noise or whistling at high level?

    Since I haven't used such ports up to now, I looked into my speaker building stuff to see what JBL and others had to say about such vents. As expected, location of a vent is GENERALLY not critical, but some manufacturers seem to have a preference.

    Fane Acoustics (note 1) says ports should be located near the drivers, and between these when multiples are used. Based on their image shown a slot vent would be horizontally oriented and located between both woofers (in the present case it would be impossible re cab baffle size). In E-V's doc, there's no mention about vent location, however out of five box plans with double woofers, four of them have the vent(s) somehow placed between the woofers (note 2). On JBL's HLA Series 4897/A, very high output LF cabs the large aerodynamic vent is also placed between the two woofers.

    Fortunately, JBL provides a little more insight with regards to vents including 1-2 walls. After stating that if required "insulation should be removed from the immediate vicinity of the port end", it goes on to say that "... as long as extreme length-to-width ratios are avoided." What is an extreme ratio? No say. I have not modeled scenarios to try to get an idea. Though making such vents much wider (height shorter or length longer to keep Fb) would reduce their good looks and efficient use of space, though improving their ratio.

    JBL adds " One or two sides of the box may be used as sides of such a port, but this will cause an alteration in the expected tuning. Common wall ducts should therefore be designed to allow for some length adjustment after the box is completed." (note 3)

    Also, the presence of an unobstructed air path between the driver and the vent is mentioned. In practice, this would depend on how deep such a vent goes inside the box, as well as how high a vertical slot vent is. On the issue of where to locate the port, JBL says "Overall, it's safest to locate the port somewhere on the baffle with the woofer(s), far enough away from side walls to avoid interaction between port and enclosure wall or the fiberglass insulation on the wall." (note 4)

    "safest" doesn't mean slots are plain bad or must be avoided, but rather more like can be used with an eye kept on common wall interaction/tuning aspects, length-to-width ratio, distance and air path with driver.

    Each box designer (DIY or Pro) makes his own choices/decisions and has his reasons for doing things a certain way. These are most often as good as anyone else's, whether it be budget, space/box size, material available, complexity, look, sound perceived, etc.

    Richard

    Note 1: Fane Acoustics, Loudspeaker Enclosure Design And Construction, P.5 ( Now called Fane International, Acoustics is now their Musical Instruments Speaker Division)

    Note 2: E-V, Pro Sound facts, No. 7, P. 16-18

    Note 3: JBL, Vented Loudspeaker Enclosure Construction And Operation, P. 3

    Note 4: JBL, The Most Commonly Asked Questions About Building Enclosures, P. 3, 4

  3. #18
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    Have you compared your JBL 2234 with your TAD 1601 yet?
    Im wondering which woofer sounds better to your 1khz crossover?

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    In regards to the dual woofers... Have you considered trying them on their sides a la the 4350 and your TAD monitors? Maybe there is something to horizontal vs vertical configuration. Then both woofers are equidistant from the floor. May get too much reinforcement from that, but maybe the ticket.

  5. #20
    Senior Member 1audiohack's Avatar
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    Slots

    Hello Richard;

    Esthetically I like slots.

    In air flow round is king and a slot with a high aspect ratio is the poorest choice.

    Over a decade ago I did a ton of modeling in LinearX Encloseure Shop and found that if you stick to what I found is a thumb rule (at least for me) that is driver Sd times Xmax for port area, depending on aspect ratio and port location you can move things around several Hz is about all. I did a six box experiment on very different port placments and shapes to verify the models were correct and called it good.

    On slots I err just a tic big on area since they dont flow as well but have yet to hear them. I should do a port compression test on round vz slot sometime.

    As far as front or rear it seems like nearly all of JBLs speakers designed for domestic use have the ports on the rear to minimize midband radiation I assume. Virtually all the pro boxes including real (large format) studio monitors are ported out the front.

    All the best.
    Barry.
    If we knew what the hell we were doing, we wouldn't call it research would we.

  6. #21
    Senior Member RMC's Avatar
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    Hi Barry,

    Thanks for the input on slot vents.

    The LF cabinet shown not only has great looks, but with the slot type vents its a definite homerun, for appearance and efficient use of space available.

    I don't know if round vents are always King. They probably are in terms of number of uses for practical reasons (easy to find and install, many dimensions, low cost, etc.).

    All vent shapes considered, THE King for air flow performance could be the huge type of vent on JBL's HLA 4897 and 4897A. Its not round, though it has rounded corners, and could also be perceived as a large slot with flared ends... Developed through extensive aerodynamic studies of air velocities for extreme LF outputs.

    With regards to vent shape (round, rectangular, square or triangular) I don't recall seeing info naming one type better than others for general use (as long as vent proportions are acceptable). Round flared ones have their fans, having some merit concept wise, but their execution details/specs are often unknown, and possibly plagued with some issues that are not necessarily known to all (note 1).

    The use of Vd (Sd x Xmax) plus a little margin for port area is clever. When that does the trick, great then.

    Both Small (1973) and Engebretson (1984) use the Vd number, as well as Fb, in their equations to determine vent dia. The latter's results however are usually larger dia. than the former's, which in practice makes sense being 11 years more recent, and considering there were already some higher power drivers with increased LF output capability in 1984 (e.g. 2245H). Today with 2 KW or so drivers the vent diameters should increase even more.

    Regards,

    Richard

    Note 1: A. Salvatti, A. Devantier and D. J. Button, Maximizing Performance from Loudspeaker Ports, JAES, Vol. 50, No. 1/2, Jan./Feb. 2002, P. 19-45.

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