JBL S38 II sounds hollow when I knock on it...

Discussion in 'Speakers' started by Nick Laho, May 13, 2005.

  1. Nick Laho

    Nick Laho Extra

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    Okay, so I was wondering. I have a pair of JBL S38 II's. Now, having read a bit about speakers, Im assuming that when you knock on a speaker, it should sound like its relatively solid. Well..knocking on my speakers makes a really hollow sound. I was wondering if there was something I could do to fix this problem, and would it even make any difference?
     
  2. John S

    John S Producer

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    I use S38's on every channel. Cabinet resonances are the biggest knock on them. I like the way they sound so much, I think I may like the effect as crazy as that sounds.

    Not real sure what one would do to help them really.
     
  3. Nick Laho

    Nick Laho Extra

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    Well, if the problem isn't easily fixable, then I guess I just want to know a little bit about cabinet resonance. What effect does that actually have on the sound of a speaker?
     
  4. Jack Gilvey

    Jack Gilvey Producer

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    Does it sound good when you're not knocking on it?
     
  5. VinhT

    VinhT Second Unit

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    Nick,
    You may want to examine this part of a paper submitted to the Audio Engineering Society by John Atkinson of Stereophile. About halfway down the page, he discusses the issue of cabinet resonance and what audible effects it may have on loudspeaker performance.
     
  6. Tim Stumpf

    Tim Stumpf Stunt Coordinator

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    The long and short of it though is that loudspeakers really are not that complicated. You've got your drivers, your crossover and your cabinet. These components are, if done correctly, designed to work as a unit. One of those, "the whole is more than its parts" things. If you like the way your speakers sound, then there is not an issue. If you don't there could be a number of factors that could cause you to not like their sound, one of which could be cabinet resonance.
     
  7. Marcelo T

    Marcelo T Stunt Coordinator

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    If you look to the cumulative spectral delay plot of the s38

    you can see that this speaker is not well braced. You can see that it has a strong ressonance mode at 385hz, more at 250hz, which can most certanly degrade the sound. Just for kicks, look at the same graph from this dynaudio speaker.
    That's the difference of a well braced against a lousy braced speaker [​IMG].
    As a former owner of the s38 (good ridance) I can atest that this JBL is more like a mid-fi speaker, it's tweeter sux, there's lack of presence and detail in the mids and the bass is kinda lean.
    At that price, you can't expect much anyway..
     
  8. Allen Longcor

    Allen Longcor Supporting Actor

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    LOL, tell us how you really feel.
     
  9. John S

    John S Producer

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    Marcelo, in a 1 meter test with a sweep. The speaker comes out flat though. Interesting.
     
  10. Chu Gai

    Chu Gai Lead Actor

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    Can you post that?
     
  11. John Garcia

    John Garcia Executive Producer

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    Yes and no. In the price range of the S38s, that may very well be true, but it is not a simple matter of slapping anything in a box. Yes, the components are relatively simple, but the key comment there is "done correctly". To get that harmony where it all works together and sounds good is NOT as simple as it would seem.

    Cabinets are one of the places where the manufacturer is going to keep costs down, so it doesn't surprise me when a sub $500/pr speaker has a less than well braced cabinet. Speaker cabinets function similarly to wooden instruments, and they can definitely affect the resulting sound.
     
  12. Chu Gai

    Chu Gai Lead Actor

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    Indeed! In fact, some manufacturers, for example Cain & Cain, rely upon the speaker functioning almost like a guitar and choose wood based on its resonant properties. A bad idea IMO.

    BTW Vinh, that was a good link.
     
  13. John S

    John S Producer

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    Chu, I did this like three years ago. But I was pretty sure I posted them on here once. I can see if I can dig them up.

    I was getting borderline +/- 4 DB to nearly 30hz from my sometimes convoluted memory. I was pretty darn surprised.
    I was using a Denon 4802 AVR to drive them.

    I should be able to find it. May take a week or so.
     
  14. John S

    John S Producer

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    PS: I will post up my new results instead.

    Denon AVR using power amps
    Using Denon AVR with external power amps
    Using Denon AVR to power mids and highs and external amps to power my low end drivers in a true bi-amp situation.

    I have decided to do these tests in two channel as I only have a mono tone sweep generator anyways.
     
  15. SteveLeach

    SteveLeach Stunt Coordinator

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    NicK,
    I also have S38ii and I did a few non destructive mods to them over the years.

    I bought a queen size poly pillow, and stuffed one half of it in to each speaker. Try it, if you like it great, if you don't pull the stuffing back out. You can check on the web, for how much stuffing should go into each speaker.

    Then you can add a brace from the back of the cabinet to the front of the cabinet. Go to your local hardware store buy a 1/2inch to 3/4inch dowel. Cut it to length and wedge it between the front and back cabinets between the woofer and the mid-range/tweeter location. It's really the only place where you can add a brace. If you like it great, if you don't pull it back out. If you like it, I'd glue it in place so it stays where you want it.

    You can buy the rubber damping material from Parts Express that is generally installed in car doors, install it as described, if you like it great, if you don't pull it back out.

    Then there is the extreme step of replacing the components of the cross-over with higher quality components. All available on the web. Not for the feint of heart. This one is more difficult to undue if you don't like the change. I've worked with electronics for more than a decade or two, so this was a no-brainer for me, your mileage and experiance may vary. I think there was a thread here on doing this as well. At least I think that's where I got the idea.

    Like I've said, I've done all of these changes. Then did some blind listening tests with 1 pair of un-modified speakers on the A channel and the modified speakers on the B channel. Then I had friends over and did A/B comparisons. They didn't know which was which, but generally picked the modified pair over the unmodified pair.
     
  16. Tim Stumpf

    Tim Stumpf Stunt Coordinator

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    I think we basically agree, but that's a good point. That's why people talk about "tuning a cabinet" with ports etc. much like tuning a piano. I think my point was the same as yours. What are apparently simple parts combine in not so simple ways. Some designers take months or years to achive the desired balance and synergy between these relativey simple components.
     

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