Help and Advice needed by newbie

Discussion in 'DIY Discussion' started by merlin, Jul 25, 2003.

  1. merlin

    merlin

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    Today I took a bold step into the unknown and could use any input and help that the forum might provide :eek:

    Been looking for some bass units for my Tact setup for months, nothing seemed available from the manufacturers. I needed a bass cab suitable for covering the range from 20hz up to 325hz, all subwoofers seem to be designed to work to 120hz max. I tried the Tact subs, but for my musical tastes, they were a little dry, lightening fast but anaemic.

    I wanted JBL sound:D . So today I forked out for a couple of JBL 1500AL bass units, the ones found in the new 9800K2 speaker.

    The TS parameters mean little to an ignoramous like me but are as follows:

    Qts = 0.27
    Qes = 0.28
    Qms = 12.84
    Fs = 27.3hz
    Vas = 264.55 l
    Sd = 880 sq cm
    Re = 5.4 ohms
    Mms = 139.59g
    Bl = 21.64 T*m
    SPL = 94.7db

    The intention is to build them into individual ported enclosures, no need for crossovers as the Tact will handle that.

    Any suggestions for cabinet dimensions, materials, and construction tips would really be apreciated.

    Regards

    Michael
     
    merlin, Jul 25, 2003
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  2. merlin

    themadhippy seen it done it smokin it

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    how much do you want to understand the theory behind the actualy desing process merlin?
    theres sevral good books in avalible like this or this that give a good insight.Theres also a few links at http://zerogain.com/forum/showthread.php?s=&threadid=30.
    software wise i like winisd ,cant find that particulre driver in there data base so youll have to enter the values yerself(might have a play myself).
    Material wise,depends on the cabinet size,but id guess 18mm mdf at as a minimun.
     
    themadhippy, Jul 26, 2003
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  3. merlin

    michaelab desafinado

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    Michael - what's the system in your avatar? Looks impressive :)

    Michael.
     
    michaelab, Jul 26, 2003
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  4. merlin

    merlin

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    Thanks for the links mate, my pc is sadly playing up and I'm having trouble downloading :(

    Looking at the K2 crossection, the bass enclosure seems to have an internal volume of roughly 770mm x 460mm x 260mm, although the cabinet has an irregular shape and tapers to the rear. Guess I'd be better running the figures:rolleyes:

    Materials wise, I was planning on copying the K2, using 1" MDF plus an additional 0.75" MDF panel for the front baffle. Plan on buying the MDF ready veneered. Unsure about how to brace the cabinet though! It seems that they use 1" fibreglass wadding as filling internally, so I need to try to souce some of that.

    Did you manage to run the figures through the software yourself?

    Michael it belongs to a guy in Scandinavia, uses 2 x 18" subs, 1 x 15" bass cab and a 3" bi radial horn per channel, all JBL:D looks like he has it in his sauna too;)
     
    merlin, Jul 26, 2003
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  5. merlin

    themadhippy seen it done it smokin it

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    managed to run the figures in win isd,seems a bit disapointing,merlins driver is the pink one,the green trace is my own 8" driver in its optimum cabinet size:D
    [​IMG],
    The F3 point is around 48 hz (mines around 31hz:p ), but its a small box(mine maybe a bit big for some people) .i havent played around with it yet,been playing with animation software for a forthcoming dance show:D :MILD: .however when i get 5 minutes i'll do some manual calculations :bookworm: see what they give me.
    Depending on the final cabinet size depends on how much bracing is required ,i used something like this
    [​IMG],
    i olny used 2 and it apears fine.
    The fiberglass wadding is most proberly the standard accoustic wadding,use enough of it and it can "increase" the box volume the driver sees. something to do with absorbing heat and changing air pressures :confused: i just lined 3 walls with the stuff,seems to work:D
     
    themadhippy, Jul 26, 2003
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  6. merlin

    zanash

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    I'd keep clear of glass fibre on health grounds, you can get dacron from decent haberdashery stores that possibly accustically better. Even getting the proper stuff wont break the bank.
     
    zanash, Jul 26, 2003
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  7. merlin

    test tone

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    merlin,

    Very nice drive units.

    When I first saw what you wanted to do, especially the required bandwith (up to 325Hz), I was a little concerned. Not least that you bring into play standing waves (at frequencies as high as that required 1/4 waves will fit within the likely enclosure boundaries). Further you've also got to contest with cone breakup modes as the frequency rises - this often being increaingly relevant to large area (sub)woofers. However, i've seen the scant information on the 1500AL, that the web has to offer, and it transpires that the unit is desiged for wide (in relative terms) bandwidth use.

    With regard to the enclosure - I don't personally like winISD - it tends to lapse into inaccuracy to easily. However, running the specs through BassBox (Harris Tech), there isn't a great deal of difference in the results. Based on that i'd suggest an internal volume of 2.35cuft (this would be the net internal volume, don't forget to take into account displacement of the motor assembly of the drivers, port, and any bracing).

    With regard to the port, I modelled the box using a 4" diameter port of 7.5" length. This tunes the box to around 36Hz. Ports tend to stick to integer lengths, and 5" length is a common off-the-shelf dimension - win ISD opts for 5", which (for the same enclosure volume) would raise the tuning by a few Hertz. So, it may be worth experimenting with port length, using something like drain pipe, and final tune by ear. When so done i'd recommend a proper port, preferably with a minor rounding or flare of both ends.

    Personally, 36Hz is fine, and with room gain will have useful output an octave below that (be careful running very high power into it though, as the (sub)woofer will unload very quickly below tuning). I don't know if your TACT system has subsonic (infrasonic) filtering available - it may be worthwhile taking the ultra-low stuff out. I did have a look at increasing the enclosure volume (and keeping port lengths between 5" and 7.5"), and you can drop tuning by around 8 - 10Hz, giving 26, 28Hz. However, vent air displacement goes skyward - so it'd whistle and chuff - increasing the diamteter solves that, but you'd have to increase length to silly proportions. Also group delay exhibits a nasty peak, and in general is well above that which is acceptable for sound quality. Classic slow and low bass. Whereas the 36Hz design should be fast, and much better at handling transients.

    Just a last word about stuffing. I'd only recommend lining the walls of the enclosure. And then with an 'egg-shell' type foam to disperse standing waves. There's no need for wadding, and in fact, it may well be detrimental to the operation of a reflex enclosure. When used; particularly, in a sealed enclosure it can 'increase' the apparent volume (by a process known as Isothermal Propagation), but in subwoofer enclosures i've yet to hear it make an audible difference.
     
    test tone, Jul 27, 2003
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  8. merlin

    merlin

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    Thanks for that TT,

    We are slowly getting there! I am happy to go with the higher tuning, the cabs are really quite small.

    So excuse my ignorance, but this means we should subtract the displacement of the driver, port and bracing from the total internal dimensions to end up with a figure of 2.35cuft:confused: How does one calculate the displacement of the port?

    The cabinets will be built from 1" MDF, with an extra 1/2" for the front baffle. Thanks for the advice on stuffing, but all JBL's use non shearing fibreglass 1" thick which is covered with a thin layer of laquer. I plan on using hairspray to seal the fibres in.

    The standing waves issue is a concern, maybe I should consider building a hexagonal cabinet design? How difficult will this be, I am building the cabinet with the help of a good friend who is a furniture maker.

    The Tact enables me to cut the bass wherever I like. I plan on measuring the subs in room via the Tact software, then following the natural roll off of the drivers. With room gain, I am hoping this will be around 25hz F3. By doing this, I am not artificially boosting the bass and so should not damage the 1500's. The 9800K2 itself is listed as 45hz-25khz, but has measured F3 of 20hz in room farfield. Amplifier will probably be a Rotel 991, rated at 200wpc into 8ohms.

    Once again thanks for the help

    Thanks again
     
    merlin, Jul 28, 2003
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  9. merlin

    themadhippy seen it done it smokin it

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    easy use the black art of "guestamation":D ,add 10 % to your calculated volume ,then work the cabint sides out from there
     
    themadhippy, Jul 28, 2003
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  10. merlin

    test tone

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    Merlin,

    Guesstimation will probably do the trick, and with driver displacement, that may be the only option. Better manufacturers often supply the displacement figure alongside the Thiele/Small specs. Otherwise 3.5 litres would account for the displacement of an 'average' 15" (sub)woofer. Venture into multi-magnet-large-voice-coil-underhung-motors, hanging off cast alloy frames, and the displacement will increase considerably.

    Port displacement: assuming the ends aren't wildly flared then treat it as a cylinder: pi*r^2*h (r being the radius of the port, and h the height).

    One point: it can become confusing working in different volume measurements. I prefer the final figure in cubic feet because I can 'visualise' it. However, during the calculation process I will often use litres - simply because they are often used in enclosure design software, Thiele/Small specs, and for quoted driver and port displacements. A good conversion program is a definite plus, and I use Convert, which is freeware.

    http://www.joshmadison.com/software/convert/

    With regard to standing waves and box shape. Being as you are lining the sides - I wouldn't be overly concerned. Strategically placed types of bracing will also help. I'd still look at using quite steep filtering at the upper end though. If standing waves are left unchecked at 325Hz, and certainly in the octave beyond (as a result of less steep filtering), then it will likely have a detrimental effect on, particularly, vocals. Re the hexagonal enclosure - it would effectively negate the issue of standing waves, but would be considerbly more complex to build. I've never attempted a hexagonal design (and I know my carpentry is well below the level needed :rolleyes: ) so you really need to speak to your friend on that one.

    Good luck with the design.
     
    test tone, Jul 28, 2003
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  11. merlin

    merlin

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    Thanks to you both for the help, it has been invaluable. Without your help, I would never have been able to grasp the basics of cabinet requirements, with it at least I have a start.

    I guess everything is in the tuning and with this in mind, I decided to mail JBL over the weekend. I must admit that the help desk at JBL have been stunning, today I received a mail from their chief engineer offering assistance, what great customer service. Will keep you posted.
     
    merlin, Jul 29, 2003
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  12. merlin

    merlin

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    To continue the story, this is the reply I received from Greg Timbers, Chief Engineer Harman Group.

    Talk about helpful:) It transpires that Greg himself is using a similar arrangement, so I think we can assume it's a good shout.

    Now to my problems:rolleyes:

    I don't seem to be able to source the required port (100mm x 300mm flared), any ideas for a supplier:confused:

    Also looking for a source of the 1" acoustic fibreglass which does not seem to be available on the forums links.

    As ever any help apreciated
     
    merlin, Aug 1, 2003
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  13. merlin

    test tone

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    Merlin,

    Practical application by a skilled engineer is indeed a good shout. One point of interest is that regarding group delay. When he says 'much better', i'd have to ask 'than what'. I've modelled the suggestions and they follow, exactly - slight under-damping, minor improvement in the LF response (prior to room gain estimation), absolutely no knee in the response. However, the transient response is still degraded compared to the smaller enclosure/higher tuning example.

    The one thing that is difficult to model (accurately) is room interaction. The fellow said that his design suits the room gain, so inputting an estimation into BassBox I modelled the potential effect. He's very right, the extension is marked, and well into infrasonics. It's whether you trade this extension (and indeed question it's necessity) against group delay - I wouldn't, and certainly not for a hifi system.

    Port placement is a good call.

    On the subject of the port - 300mm is long, especially for a domestic (sub)woofer. Falcon Acoustics do one of 110mm by a max of 280mm (using this in the given design would only raise the tuning by a couple of Hertz). Another option is to find a supplier of JL car audio and enquire about their 'FlexPort' (although i'm not certain of current availability). You could also consider a slot port - ie built as part of the enclosure.

    http://www.falcon-acoustics.co.uk/

    ps: Falcon should also be able to help with box linings.
     
    test tone, Aug 2, 2003
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  14. merlin

    themadhippy seen it done it smokin it

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    B+Q,wickes,plumbers supplier should stock 4 inch pvc pipe,ok its not flared but adjust the length to suit.if 300mm is to long for the cab just put add a 90 degree bend
     
    themadhippy, Aug 2, 2003
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  15. merlin

    merlin

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    Thanks for that TT:)

    Room gain is less of a factor in my Tact system, as I can contour the LF response to suit digitally. The only thing I cannot do is boost the low frequencies past the natural roll off of the speaker, as this would sap far too much power from the amp, and probably cause clipping.

    For my music, a lot of dub, trance, electronica, I really need the extra extension. With my current setup, I am running flat to 20hz. When altering the curve to roll off at even 30hz, there is a very noticeable increase in sharpness, and a loss of ambience. Whilst I would accept that transient response is improved slightly, the trade off is too great elsewhere. The real surprise is how much the midrange and vocals in particular, benefit from the extra extension. The Tact RCS has tremendous success in controlling bass, the Dynaudio theatre sub I currently use is totally out of the question for hifi use in most systems.


    Thanks for the suggestions. I have actually been able to find the port today over at Parts Express in the States. Flared both ends and adjustable up to 14" in length. The cabinet is 24" deep so fitting should be fine.

    One last thought. Should I apply a damping compound to the interior walls, or just apply the fibreglass direct to the MDF?

    So now for the amplification. I was thinking of trying to get hold of a Crown K series amp s/h if I can find one. 380wpc and a damping factor of 4000 to 1 should control the boys :D
     
    merlin, Aug 2, 2003
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  16. merlin

    kermit still dreaming.......

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    i don,t know anything about this sort of thing , but i,ve always assumed that when making boxes that manufacturers have to follow cost restraints .
    the diy,ers will know whether theres any point doing this , but as your only making two subs , can,t you get a bit exotic (and is it worth getting a bit exotic?)
    ie adding a skin of ply to the inside of the sub enclosure , or lining with lead , or one i,ve thought about is applying fibreglass to the inside of the enclosure (although you,d want to brush up on your fibreglassing technique)
    or how about lead and fibreglass?
    and on and on .......:D

    so any point doing this?
     
    kermit, Aug 3, 2003
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