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Please analyze/critique my 1st DIY bass cab...

Discussion in 'Amps and Cabs [BG]' started by Crockettnj, Oct 31, 2005.


  1. Crockettnj

    Crockettnj

    Sep 2, 2005
    North NJ
    I woudl love to hear input from anyone on here who has box building experience- below are my plans for my first completed box . this weekend I built a rough box out of an old sterao speaker to jsut test things out a little... i dont count that.

    The box is for 4 string bass guitar at quiet to moderate practice volumes in a 20 x 15 x 7 foot sheetrocked & carpeted room. I used a SPL meter 3 feet in front of our amps and found that 100 to 110 db covered our level range over several practices. I will be driving the cab with an eden wt300 or hartke 5000, both pretty much 250 to 300 watts at 4 ohms. i tend to run things fairly flat, without excessive EQ-ing.

    Box dimensions: 19"wide x 13"tall x 13" deep EXTERNAL. Front baffle inset 1".
    front and back are 5/8" MDF, the rest is 1/2" plywood. 1/2" quarter round strips along each interior seam, carpenter glued and screwed with 3/4" or 1" wood screws. ( is gorilla glue better?)

    a shelf brace of 5/8 MDF with numerous large holes drilled in it running front to back roughly bisecting the baffle board, glued and screwed with 1/2" quarter round as described above. The woofer will be left of center, the mid centered along the same axis right of center.

    rear port of 3" drainpipe mounted behind and below the woofer in the lower left corner. an elbow first, with the straight portion extending along the inside of the cab for a total of 17".

    I estimated the speaker displacements at 90 cubic inches, and subtraced all bracing and walls to get an Overall net internal volume of 1.2 cubic feet without stuffing. I am on the fence in terms of acoustic stuffing- thoughts?

    drivers- peerless 831759 (len's special) 10 " : fs either 21.7 or 22.4, depending upon which publication you look at. vas=121.9L qts = .27 or 28, depending. max power = 220W Z = 4 ohm
    91.4 SPL sens Re 3.6 Xmax .295"

    Midrange- no t ordered yet- still shopping and ...
    Crossover- possibly the eminence 2 way 800hz xover for 35 bucks.
    not purchased yet, and open to suggestions.

    I ran this on win pro and it seems that:
    1.2 ft3 @ 29Hz (3" round port 17" long)
    f3 = 40hz

    group delay is 12.5 ms @30hz, 9.5 ms @ 40hz, and 6.5 ms @ 50 hz. I read on a speaker builder site that less is better, but that 13 ms is probably the limit of human detection. Thoughts? I dont want "mud".

    Cone excursion gets bad &hits max @ 200 watts @ 23 hz . (I should be running below that wattage in practice though). PLus, I shouldnt be sending a 23 hz signal into it with a 4 stringer... That said, I am on the fence about a low pass at about 30 hz to protect the speaker from "bumps".

    Port velocity- @ 200 watts the port velocity exceeds 54 feet/second (approx 17m/s, the value some have suggested to stay below) at frequencies below 35 Hz. (Since i am a 4stringer, I shouldnt hear too much chuffing since i have little below 40hz. ??)

    Fudge factor!-
    If i am off alittle on my volume calc's, if the box is really closer to 1.3ft3, or if i add stuffing and that effectively increases apparant volume, the vent needs adjustment a little to 15.5", f3 drops to 38.5, group delay is 13.5 ms @ 30hz, and essentially unchanged @ 40 and 50 hz, and cone excursion is exceed at 23.5 hz , and port vel is unacceptably fast at 36.5hz- All of this seem acceptable for my needs (?)

    If the box is really closer to 1.1 ft3, Port vel goes south at 33 hz, port length needs adjsutment up to 18.6", f3 goes up to 41.3hz, and group delay slightly better at the lowest freqs and essentialy unchanged above 40 hz. Still fine, right?

    SO, in summary, I went with a modified qb3 because the box size got too big (for me) to use a SBB4. the box is a hair smaller and should be noticeably lighter than the (superior) ACME low b-1... which costs comfortably more than twice as much at 400 bucks. My cab should also be more to my liking in terms of dimensions. I hope to cover it in Tweed, so it looks vintage fender-esque. Maybe not.

    so long as i do not push the cab hard under full wattage (250 - 300) , and stay in the practice volume range of 110 db, i think i am in the 150 to 200 watt range, and as a result cone excursion and port noise (farting, chuffing, whistling!) should be contained. (?)

    NOW- could anyone analyze this design and my thoughts/comments, and offer suggestions based on your real life and/or designing experiences?. Mind you, i put roughly 1,100 hours of planning into this so far, so be kind ;-)

    (Also, if anyone has a xover/midrange suggestion, i am open and have SOME paypal money still available. I know i will need to have an impedance compensation since most xovers are designed for 8 ohm, and the driver is 4 ohm. (I am looking at a 4 ohm mid as well on ebay, does this simplifiy things or is that irrelevant?)
    thank you VERY MUCH in advance.
     
  2. I'm questioning your vent and tuning frequency. Since you said you only play 4-string, it's lower than you need.

    I'd go with 35Hz or so (vent frequency), giving you a 10" port instead of 17". More manageable, leaves more internal volume. It also raises the cabinet's response in the 35-80 Hz range, at the expense of sub-35Hz response. It would also help a bit with the port velocity.
     
  3. Thor

    Thor Moderator Staff Member Gold Supporting Member

    I think I will lurk here a bit. As you know, I am building
    the Len M project with no design changes. I am curious about the modifications and comments as well.
     
  4. The Clap

    The Clap

    Jan 5, 2004
    Scottsdale, AZ
    Ok, I've got some suggestions and practical notes from the cabs I've built.
    I wouldn't use MDF, stick with plywood all around. If you're never going to move it, then weight and durability aren't a concern, but everything gets moved. I use MDF for home speakers only.
    I wouldn't use MDF for the brace either, and instead of cutting holes in a large shelf of bracing, just make it more of a strip, without holes, may 4 inches wide.
    You don't need a driver with so low of a resonant frequency. Especially with a 10" driver, this represents a significant tradeoff in the system's sensitivity. I don't know too much about how you intend to use it, but I've used a single 10" with similar specs and it didn't hold up with a drummer. Tell us more about what you're specifically going for with this build; I think you're looking for a more traditional pro sound driver than this though.
    Since you're into the more technical aspects and unafraid of numbers, try building your own crossover, you'll be able to do one that specifically fits your project.

    Overall, it's hard to know what factors most affect the final sound before you've gone and built some speakers, but there are some knowledgable guys around here that will comment further though, I'm sure. Basically, tuning so low, and using a 10" driver to do so is going to leave you disappointed unless you play rather quietly.
     
  5. Crockettnj

    Crockettnj

    Sep 2, 2005
    North NJ

    AHH. i was really looking in that direction around hour 400 on the speaker builder program i was using! No, seriously, i agree with that. HOWEVER, i noticed that group delay numbers go to hell as i raise the tuning frequency for a box in this size range- I get a huge spike in the 40-ish hz range, if i recall, and I fuigured that spelled more mud right around the low e/f/g. I opted for the HOPE of tighter bass . HOWEVER, I may have made a poor descision- the trade off would be better useable freq (35-80 as you say) and smaler port therefore more volume. Perhaps I made the wrong descision... The questions that beg's answering is, WILL I PERCIEVE a difference in tightness/accuracy (group delay) from my alignment compared to the one you propose?> If the answer is no, then I think you have the better idea. If yes, then i choose tightness. I dont know the answer... and noone seems to have numbers as a reference for where Group delay is "bad" vs "not noticeable" except a passing reference on one sight mentioning 13ms.
     
  6. Thor

    Thor Moderator Staff Member Gold Supporting Member

    Oh, I forgot to add this. You don't need a 4 ohm midrange
    driver. A 8 ohm midrange is fine. Note Len's use of the Peerless 1646 in ghis project midrange which is 8 ohms.

    I asked the same question last year to Larry Hitch at Madisound and he replied:

    Also, why not just use 3/4 good quality plywood, leaving your
    ID's the same and forget the braces. This is a rather small
    box, I can't see a lot of flex in a 19x13x13 vented box to be
    quite honest with you. The extra weight of the bracing
    likely offsets any weight advantage the thinner wood gives you in this size box IMO.
     
  7. Taking your net 1.2 cubic feet as accurate, I get a tuning frequency of 29 Hz with 17" of 3.0" ID pipe.

    The difference between QB3 and Optimal Flat tuning in this instance is about 1 Hz, same cabinet volume. Even with a digital VOM, you aren't going to get it that close. The tuning frequency will be a wide trough, with a number of frequencies reading the same voltage, on either side of the tuning frequency.

    Port air velocity is a non-issue here, because your tuning frequency is below your lowest note. Velocity rises and falls very rapidly at/away from the tuning frequency.
     
  8. Crockettnj

    Crockettnj

    Sep 2, 2005
    North NJ
    I may very well do this- I chose mdf for the baffle since I had some comments (either here or another site?) that 1/2 ply will resonate too much as a baffle. I DONT want a heavy cab, therefore i figured 1/2 ply all around EXCEPT the baffle. As for the back, i would need to buy more 1/2 ply, so i figured i'd use the mdf i have sitting here. ( i know, lame). However, your suggestion is well noted, and I may take it.

    As for the shelf brace, i have some 1 " by 2.5" poplar strips i could use... jsut the outline of what the shelf would have been.



    I got it on EBAY a year or so ago after I read the len moskowitz article and was enamored by the concept. I was ignorant on a million other aspects at that point, but it's the driver i have, It was inexpensive, and i am going with her!

    Myintended use is practice and learning songs at home. I want something very REAL sounding, no coloration, and sort of the "hifi" sound. full range, deep through crystle clear highs. ALSO , something small and light to play along with acoustic guitars at social gatherings. In hindsight, I thikn it is safe to say i am trying to reinvent the acme low b1... I am not going to achieve that, but i am close (?) and for way less $.

    This is NOT a realistic gigging cab at all. I am about 75% sure I can get away with it at practices with the band, so long as i keep on keeping my guitar player under control. We arent very loud in practice, and as i said, 110 db was max out of the guitar and bass cabs during practices over a couple weeks of testing.


    [/QUOTE]Since you're into the more technical aspects and unafraid of numbers, try building your own crossover, you'll be able to do one that specifically fits your project.[/QUOTE]

    I would actually prefer this, however at this stage I need to do some more learnin... Mostly I think as to phase shifting and the CONS as well as pro's of the various slopes/ types of crossovers. Also, i dont know a cheap cap form an expensive one, and I am not sure why i cant use a cap in parallell instead of a coil in series... do you NEED inductors? Like i said, i still have mucho to learn, but i enjoy it.

    thanks for your replies, and i look forward to more.
     
  9. Crockettnj

    Crockettnj

    Sep 2, 2005
    North NJ

    I dont have a frame of reference, having never built one nor seen the inside of a good bass cab. Is 13" fine without bracing? I dunno. 19"? i dunno either. I know that my braces plus the 1/2 ply is WAY lilghter than the sheet of 3/4 ply. mobility portability is key here.

    Some of the experienced guys here said 1/2 is jsut fine ( and lighter) . I also noticed that some criticisms of heavy cabs with thick wood was that it was to offset lack of adequate bracing (which drives up cost and production time.

    I would very much like to sit our completed cabs side by side and do an A/B test! it should be very interesting. ( i knew i would see you lurking around on this thread... but i was hoping you'd be DONE so i could piggyback off of your "wish i had done it THIS way..." list!

    ;-)
     
  10. Thor

    Thor Moderator Staff Member Gold Supporting Member

    Well, the one thing I can tell you is what the stuff weighs, as
    it is all cut out of a sheet of maple finish plywood I saw on
    sale. I will weigh it tonight and report back.

    I haven't finished it as summer demands on my time put it
    on hold, and after finishing the last cab, I went on to some
    different projects. I won't be done for a while, some
    carpenter will be making a mess in my house so meh,
    I won't get back to it till early spring the earliest.

    Mine is basically directly from Len's specs and components.
    I had a need for a very compact home cab and low level
    practice amp. I also plan to use an Eden Wt-300 to drive it.

    I haven't run WinISD on the project yet, I am curious to see
    what that reveals. That program was not available when
    Moskowitz published the article.

    The sensitivity of the driver is noted in the article, but he
    does not expand on that much.

    I emailed him and asked him about what kind of rig he was
    using and he mentioned that he used a AMP 400 iirc and
    2 of these cabs 'presented at 8 ohms', i.e. wired in series,
    I have to presume. He mentioned in the article, if it wasn't loud enough, run 2.

    I wish I could be of more help, but as Heinlein said,
    'I am just an egg.'
     
  11. Crockettnj

    Crockettnj

    Sep 2, 2005
    North NJ
    wow. I just sold my hartke5000 and picked up a wt300 on ebay for 350. I was pleased with th eprice. but seriously, funny we are both building the same cab (sorta) and using teh same head. You dont play a KSD proto-J, do you?

    I usedboth ISD online and the Win Pro. the Pro version is really great. Len's cab was Optimum flat alignment. the pro version illustrates SBB4, QB3, sealed, and Optflat. obviously yo ucould modify it. very cool.

    the AMP is at the top of my GAS list. some day? I think that if this isnt loud enough, I would use one of my other cabs.

    Anyway, pretty ironic with teh Wt300 and all.
     
  12. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Forget about group delay, below 80 Hz at less than 20ms is undetectable by human beings. Elephants maybe. But they aren't saying.
     
  13. Crockettnj

    Crockettnj

    Sep 2, 2005
    North NJ
    Bill, why is it that there is so little info re group delay? the 80hz - 20ms figure you gave gives some perspective that I was lacking.

    this means I will "rejigger my figgers..."

    no elephants here to worry about- well, 'cept my prom date but thats a different story.

    anyway, thanks much.

    Greg
     
  14. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Group delay is very significant in the midrange, roughly 500 to 2kHz, where human hearing is most sensitive. In the bass it just doesn't matter. Detecting its presence is accomplished in very much the same manner as directional information. When the distance between the eardrums is sufficiently large compared to the wavelength of the frequency involved the brain can decipher two distinct arrival times. Since a wavelength at 80 Hz is some 14 feet long the ears are simply too close together for either direction or phase to be discerned by us, though an elephant with eardrums some two feet apart in theory would be able to note both direction and phase shift down that low. I've messed about with delay and have found that with the typical subwoofer that crosses over at 80Hz I can't detect any anomalies until there is at least a 20ms differential between subs output and that of the midbasses.
     
  15. Thor

    Thor Moderator Staff Member Gold Supporting Member

    Hey Croc,

    As you requested:

    I weighed the 3/4" maple ply for the original Len M design,
    (I didn't use MDF) and it weighs in at 20 lbs for the wood.
    Woofer, midrange, grill, x-over, wiring and hardware weight not included.

    Also, I rechecked the original design INTERIOR Dimensions
    and they ar 18 x 15 x 12 inches = 3240 sq inches = 1.875 cu ft. As the original article stated 1.6 cu ft, it would appear he
    calculated the basket and port volume displacement and deducted it from the 1.875 figure.

    Any progress update?
     
  16. Pneuma

    Pneuma

    Feb 14, 2004
    Arkansas
    Am I incorrect in assuming that a 17" long port will not fit so well in a box that is only 18" wide ID? With a 3" port, you're closer to the other wall than you need to be (or am I wrong here?).

    Or, checking my math, you've got a port cross area of 7.065in^2, and 9.42in^2 of area between the wall and port (calculated as a 1" long breathing section of 3" pipe). So, maybe it's fine, and I'm wrong.

    P.
     
  17. Crockettnj

    Crockettnj

    Sep 2, 2005
    North NJ
    work schedule put a virtual damper on the project, but i currently have 90% of what i need.

    I took many of your suggestions to heart, analyzed on winpro, and mad some mods. i will post alter.

    As for hte port, it has an elbow. about 5" straight, then turn right! as for being too close to teh wall, I assume that the wall will act olike an extension of the port. i dont know how closeis too close... suggestions?
     
  18. kraigo

    kraigo

    Jun 21, 2007
    Minneapolis, MN
    Reviving an old thread: Thor - I'm curious what you thought of the cab when you finished it. I've spent a lot more time with mine loaded with a Parts Express/Dayton eight ohm driver in mine than I did with the original speaker. Tomorrow I should get my Eminence BP102-4 speaker to replace the Dayton, which is actually not a bad driver and will get used in another project.

    I've used the cab a lot. It's been good to me, I'm not planning on getting rid of it, but I don't know that I'd build it again. There are a lot of other options available these days.

    KO