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Testing a cab - driver combo

Discussion in 'Amps and Cabs [BG]' started by Groove*Rocket, Apr 20, 2001.


  1. I have a JBL 15" driver in an unported cabinet that was built for me 20 years ago. I suspect that he cabinet is not optimally tuned for the driver. What sorts of testing can I perform to determine if I am getting the most from my driver given this cabinet? I would consider a new cabinet is there was evidence that it would make a substantial difference, and I could find a cabinet better suited to this driver.
     
  2. MikeyD

    MikeyD

    Sep 9, 2000
    You are asking exactly the right question. Unfortunately, I don't have the answer, but perhaps someone like Joris could help (he does cabinet design). Probably the first and most important step is to see if you can identify the driver. Get a model number off of it if you can. Its age might also be a factor to consider. Armed with a model number, someone might be able to search around and find the Thiele-Small (T-S) parameters for it. Then if you measure (or estimate) the internal dimensions of the cabinet, one could do the calculations. There are also tests that could be performed on the driver to estimate its T-S parameters, but that requires access to test equipment. This is all I can offer on this subject now, but if Joris reads this, I'd bet he could add a lot of wisdom to the thread. My guess is, at the very least, the cabinet would perform better with the right porting arrangement, but all of the aforementioned would have to be factored in.
    - Mike
     
  3. Hey Groove Rocket,

    Hi Mike,

    Tuning of a closed cabinet is very non-critical. There's not much that can go wrong, so I don't think you can actually speak of "tuning". Also, there's no real optimum for a closed box. If you aim at a deep low-bass response, it'll be big, but may sound flabby. If you want tightness, you'll lose depth, but the cab can be smaller.

    Ported is another question. I really doubt whether you can lay your hands on the T/S specs for that driver, after all, it's 20 years old. Because of the age, the T/S numbers may have been shifting around, because of a loosened-up surround or a slightly deformed basket. And back in those days, measurements weren't exactly precise to begin with.

    There are some safe ways to go, however. For a tuned cab, the tuning frequency could be slightly higher than the lowest note your bass can reproduce. For instance low E: tune 45 Hz. Any cab will sound "nice" that way. It could be better, but it will have to be experimentally tuned.

    So, which way do you wanna go?
     
  4. MikeyD

    MikeyD

    Sep 9, 2000
    Hi, Joris! Thanks for joining in.
    My understanding was that a closed (unported) box's volume could have a very big impact on the driver's frequency response, especially toward the small end (i.e., box volume < Vas). I have an old Univox closed enclosure - quite small, and loaded with a 15" woofer. Its bottom end is very lame. I'm certain it is primarily because of the small size of the box.

    My take on Groove's post was the cabinet itself was 20 years old, but that the driver might be newer. I would expect T-S parameters might be obtainable for it if its model number could be determined. I agree with you about the effects of aging, but maybe it throws the parameters (e.g., Vas) off by maybe 10-20% if not too old. Just a guess, but it might be enough to design a decent box for it. If Groove can find a model number and give us the cabinet dimensions, I'll try to do a search to find the T-S parameters. At the moment, I wonder if it could turn out be one of those situations where the cabinet is WAY too small and/or if simple addition of the right port might make a dramatic difference. Please let us know what you think.

    -Mike
     
  5. Hi Mikey and Joris,

    Thanks for your interest. I will try to pull the driver and get a serial number in the next 12 hours. The dimensions of course will be easy. The driver is 20 years old, but it has seen little action as my main rig for gigs has been an 810 SVT. Hopefully is the specs can be found, thsy haven't floated too much. Joris, Your comments were particularly to the point when you contrasted the ported vs non ported, as the cabinet was orignially twice the current size and had two ports. An engineering buddy of mine in college had designed the cabinet around the driver at the time, but the cab he built weight 90 lbs and was huge!!! Iy was totally impractical to move it. Moreover, the speaker was rather flappy sounding. I had the sense of good bass extension, but it was not fast. I bisected the cabinet in a very unscientific way to make it smaller and in so doing, eliminated the ports. Although the flabbiness is gone, I wonder if there is a happy medium, particularly since I intend to piair this cab with a 210 or 410, and may want a bit more bass extension without too much flab. I will get the specs to you guys soon for your input.

    Thanks again,

    Groove
     
  6. The driver is a JBL K-140 and the cabinet's inner dimensions are 17 x 18 x 19 inches and is composed of 1" thick high density particle board. I have visitied JBL's web site and found a PDF document on the K series. The link is http://www.jblpro.com/pub/obsolete/k_series.pdf. It's a 1.4 MB download. In short, the specs are
    Nominal diameter 380 mm (15 in)
    Nominal Impedence 8 Ohm
    Power capacity 300 W continuous program; 150 W continuous sine wave
    Sensitivity 98 DB spl
    Freq range 40-2500
    Voice coil diameter 102 mm (4 in); copper
    Magnet assemby weight 5.4 Kg (12 lb)
    Flux density 1.20 Tesla (12,000 Gauss)

    I suspect that the critical information may be missing. I am curious how these specs compare to current drivers. Is it worth messing woth the cab to try to tweak any improvements? I am very interested in your perspectives. Thanks again for your help.
     
  7. Mike, no speaker thread gets overlooked by me :D:D:D. However, I won't respond to the really hopeless cases, like that kid that put 12" glow-in-the-dark 87 dB car subwoofers in his 2x10!

    Back to the topic. I agree with you that the internal volume of a closed box has great impact on the frequency response. A small box gives the speaker less freedom of movement, and that leads to less low end (simply put). What I meant was, you can't mis-align a closed box, like you could with a ported one. If you choose the port resonant freq too high, you end up with a very boomy box, and maybe even a loose cone.

    Groove, 17x18x19 inch is not that small for a 15. But why the h*ll did your friend build it with 1 inch thick HDF board? He must be a hifi freak, and never moves around his own cabs. Also, if that cab was originally much bigger, he must have chosen a hifi alignment. Hifi rules don't apply to bass guitar cabs. Anyway (enough criticism), I always use 3/4 inch high-grade 7-ply wood which weighs less than half of that.

    I should have kept my mouth shut about obtaining the T/S parameters, I had them in my own personal database. And if the speaker hasn't had much action, the only problem I can see is that the surround may be a little dry. Anyway, the cab can hugely be improved by putting two 4" ports in it (preferrable in the front panel). Use thick PVC drain pipe, and cut them at 6.75" length. If you can't find room for two 4"ers, you could use one 6"er 6.5" long. But 6" piping is hard to get, especially when you only need half a foot.

    It will difinetely NOT sound flappy, tuned this way, I guarantee you.

    I attached a nice graph of it, just to show how much I was surprised by the difference:

    - The red graph is a simulated response for the closed cab
    - The blue graph is the same cab but ported
    - The black line is what you gain (left axis scale, over 0 dB is gain, under 0 dB is loss)

    Remember, these are simulations. The real world is a bit less perfect.

    Cheers!
    Joris.
     
  8. MikeyD

    MikeyD

    Sep 9, 2000
    Groove and Joris - nice work, both of you! Groove, I didn't bother to download the .pdf from JBL, but the specs you did show makest me think it's a very nice driver. My guess is you have a driver that way outclasses its automobile - I mean cabinet! I agree with Joris about the material in that cabinet - it must be really heavy. Joris - cool graph, and good advice! If I were in Groove's groovy shoes, I'd think seriously about getting/building a better cabinet for that good driver instead of messing with it. Then again, Joris's recommendation would be a good, low-cost, and major improvement.
    - Mike
     
  9. Joris and Mike,

    You guys are amazing, especially Joris. I really appreciate the analysis. You are right-on that my buddy was a Hi-Fi freak and never moved cabs. The crazy part is that the cab used to be twice as big and weighted a ton. It still weighs way too much, but I have huge casters on it that make moving it pretty easy. Nevertheless, it looks pretty tired. It is covered with old industrial carparting that is fraying and never had proper handles for lifting. I think that I could cut the ports that you recommended and seal it back up, but I know wonder if I should get a new box. I don't have the tools or time to make one myself. Joris, I see that you live in the Netherlands, which is a bit far away from me in California (insert smiley face icon, that I don't know how to paste in). What do you guys suggest? Is there a good source for a new box? Am I going to end up paying too much to get someone to custom build it? And, starting from scratch Joris, what dimensions would you recommend for sound quality and portability?

    In the meantime, I think that I will modify the box as Joris has suggested as that is easy and low cost, and I am really intrigued to experience the improvement that Joris' simulations suggest. I have lived with the weight of the cabinet for a long time, so I probably don't need to rush into a new cabinet right away. I would like to recover and regrill this one, though. What material do you guys like and do you know sources? I wonder if you have seen this link, which describes a fairly detailed approach to ground up speaker design:

    http://www.core-sound.com/bottom-article.html

    I found it interesting reading. What do you guys think?

    Thanks again,

    Geoff (aka Groove)
     
  10. MikeyD

    MikeyD

    Sep 9, 2000
    Hey, Groove,

    For empty speaker boxes, the only sources I know off hand are Parts Express (http://www.parts-express.com/) and perhaps Layne Speaker Supply (http://www.speakersupply.com/). If anyone can suggest other good sources, please add them. Parts Express sells all kinds of speaker and cabinet components, including grilles and corners, etc.

    You might also contact some of the major music outlets (e.g. Used Gear By Mail). Sometimes they stock used cabinets whose drivers have been damaged or removed.

    A custom-built box might be going overboard, unless your driver is truly exceptional. I'm sure there must be a 15" box out there on the market that could be tuned to it.

    BTW, I don't know nearly all the "emoticons", but a few seem to magically appear when you type a standard text smiley - e.g., the ":" followed by a ")". If I had put them together here, they'd be replaced by a :).

    - Mike
     
  11. MikeyD

    MikeyD

    Sep 9, 2000
    Interesting you should ask! Revered Talkbass member Rickbass1 just sent me the link this past week via e-mail, so I read it before. I critiqued it quite a bit. Here it is (excerpt from my reply to Rick):

    "[The article] is interesting (I had not seen it before, but I do have an old copy of Speaker Builder - a really cool magazine). It still doesn't quite change my overall perception of the physics of the situation with speakers.

    "The author gives rather short shrift to the sensitivity rating, because while he does correctly discuss the 2.83 volt vs. 1 watt discrepancy, he then neglects to discuss that fact that when one measures sound pressure level, one is getting a representation NOT of sound pressure but of sound power. (It's actually a pressure-squared measurement.) So to determine "efficiency" of a device, one must compare power in vs. power out. Anyway, as I suspected, the upshot is that the cabinet is really not that efficient over its operating range. The author states it should generate 112 dB SPL at 1 meter with 220 watts input. This translates to 88.6 dB @1 watt/1 meter. This is about 10 dB lower than a modestly efficient bass cabinet - and probably close to 20 (!) dB lower than your old Fender folded horn beasts! It takes serious power to overcome those differences. 112 dB isn't really that loud (at low frequencies), IMO. (Yes, by implication, I guess I'm saying the author doesn't really know what loud is!) I bet your Fender PS 400 with even a single folded horn probably easily did 127-130 dB at 1 meter with only 135 watts input. My new Eden 215 (103 dB @1w 1m) will probably do 129 dB at its rated 400 watt input.

    "The other thing is, he mentions rubber surrounds for the cone suspension. My hunch is that rubber does not hold up to the elements (ozone, aging, etc.) as well as the paper-type accordion surrounds on speakers such as Carvin's or Eden's. You've probably heard of "foam rot", and I wonder if that is a similar problem with these drivers.

    "So, in my view, the author basically has a hi-fi speaker with high power handling. (If you've ever plugged your bass into a good stereo system, you'll know it's a fabulous sound - my old AR 2ax 10" bookshelf speakers give a heavenly low end when I used them as studio monitors for my basses.) My understanding of Euphonic Audio speakers is similar: people keep saying you have to dump huge amounts of power into them for a loud output. So once again we are "victims" of physics. I think good manufacturers (e.g., Eden) have done a good job with the tradeoffs between frequency response and efficiency. The opposite approach to the author's is to simply take a speaker whose sensitivity is really good (say above 105 dB) but whose low frequency extension isn't great - and then just EQ the bottom up and feed it plenty of power. The outcome is then similar to what you have with the author's speaker - lots of solid lows, but not a huge volume output....

    "Anyway - I could be wrong!! As usual! Most of the above is based on "paper" knowledge. It would be great for me to actually hear the author's speaker in person. Maybe then I might change my mind."

    It would be great to get others' viewpoints on this.
    - Mike
     
  12. I appreciate being appreciated.

    Sorry guys, I don't have time for a very lengthy reply right now. But I'll say this: I read that article before (he's talking about a bottom, I guess he's not English) and I totally agree with you Mike, the lil' bugga must sound impressive, but will be completely drowned out by a band any louder than a jazz trio.

    My "Yellowstone" cabs (see link in my signature) have even superiour (sorry to sound proud) frequency response, but will do somewhat over 120 dB (in theory ;) ), and they keep up with a really loud metal band and ditto rock band, both with 2 roaring guitarists and a tough drummer. My amp is only 350W. Which is IMO not much for the type of music I play, especially with those low-efficient cabs (my total setup has 95.8 dB theoretical eff., which is LOW).

    Joris.
     
  13. MikeyD

    MikeyD

    Sep 9, 2000
    I remember seeing pictures of them. But your comment confused me. Either your speakers are louder than you think, or your amp is putting out more watts than you think, or your band is actually quieter than you think. But you did say "theoretical" sensitivity - so now I'm thinking maybe they're doing a lot better than the calculations suggest! Regardless, it's great when something you designed and created yourself works really well. There's a lot of satisfaction in that!
    - Mike
     
  14. Hi,

    I'm really lovin' all the great advice. I visited Parts Express, and I like what I see. Looks like a great resource. Joris mentioned that my surrounds might be a bit dry. Is there some treatment that I might apply to make them supple again? Cosmetically they are in good shape. I also note that at parts express they affer a product called "WET LOOK FOR ONE BLACK " (The Wet LookTM is a new generation of high gloss polymers formulated especially for the speaker industry. This superior coating provides a protective "coat of armor" for your paper cone speakers). What do you think of this. Is it a good idea to try, or might it change the propoerties of the drivers performance?? Also what do you think of the carpet they sell. Do you think that it will be tough enough? Are there better alternatives?

    Thanks,

    Groove
     
  15. MikeyD

    MikeyD

    Sep 9, 2000
    I wouldn't worry about that right now. No idea about that goo, either. It sounds like a marketer's dream - snake oil - but maybe I'm wrong. Put it this way: I have a paper cone 15" instrument speaker that I bought in 1975 from Radio Shack (!), and others I bought used (were probably manufactured in 1969-70), and they still work just FINE!. I recommend deciding how you want to proceed with the driver/cabinet, then go ahead. If you decide to add ports, do that first and see if you like the results. Then you can think about carpeting and other stuff. You might wind up finding a new box to put the driver in anyway. I have not bought any cabinet carpet, so I can't comment on it. If it's like the stuff they put on my Carvin and Eden cabinets, it would be fine for most applications.
    - Mike
     
  16. Okay,

    I'll leave the cosmetics for later and just cut the ports. This may take a few days or a week before I get the time, but I'll let you guys know how it all turns out.

    Thanks again for the advice,

    Groove :)