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Sealed cab resonance???

Discussion in 'Amps and Cabs [BG]' started by friedtransistor, Feb 6, 2014.

  1. I have no experience with thiele small, and I don't have any parameters for the speaker I have. I only know the xmax, which is about 1cm p-p. it's an 8" speaker, which gives me this: suface area= ~50.25 sq in., displacement= ~19.78 cu in., and the cab has a volume of about 1440 cu in.. Knowing this information, how do I calculate the resonant frequency at xmax? I haven't been able to find info on google because a) every equation uses data I don't have or can't get since I don't have the equipment to measure the speaker, and b) the ones that might work use constants that I can't calculate since I don't know the required parameters...

    There has to be a way to calculate resonance with just volume and displacement, right? Or is there more black magic involved than I thought?
  2. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    You don't have enough information, as you found out.

    I suggest you find two or three apparently similar 8" speakers and look at their parameters and from there make an educated ballpark guess about yours. Plug your estimates into the equations and see what you get. It won't be precise, but will be better than nothing.
  3. Pretty high xmax, might be a car sub, notoriously lacking for bass guitar.
  4. This value of surface area is a bit high because it does not account for the surround of the speaker which is subtracted from the radius. I am going to take a guess that a typical 8" speaker will have a surround of 0.75 inches so its effective radius is 3.25 inches. This works out to an area of 33.2 in[SUP]2[/SUP].

    I had a look at the data for one of Eminence's speakers and its area is 0.021m[SUP]2[/SUP] which works out to 32.6in[SUP]2[/SUP] so it appears that 0.75 was pretty close - the actual value is 0.78.

    FWIW, the recommended sealed cabinet volume for that speaker is 0.18 to 0.25ft[SUP]3[/SUP] which is 311 to 432in[SUP]3[/SUP]. This page has the specs for several 8" woofers so you can see the values for others.
  5. I'm sorry, the xmax is actually lower than I thought. It's more like 0.5cm, not 1cm. It's not a sub, just a cheap no-name speaker with a paper surround. Plus, my cab volume sizes are just an estimate, I'd have to measure that at some point. My cab is about (roughly) 12"x12"x10", that is, if you subtract the area taken up by the head.

    Ok, so the xmax is closer to 0.25cm, as I didn't realize xmax meant from center to fully extended, I thought it meant P-P.
  6. Ok, well, idk where to start on finding a similar speaker. But I think I have something to practice finding. First off, I'm using this site to get speaker data: http://www.thielesmall.com/
    Second, idk if it has all speakers possible, as I have a three inch sony speaker I got out of a 5.1 center channel cab (if it can be called such), and it has no info for 3" Sony speakers. It has a number on the back, uh, something-505-312-11. Any info on that?
    Also, I scored myself a couple Magnavox stereo speakers yesterday at goodwill, for $2.99 apiece. :D They are a little woofy with my radio, but putting them in series and hooking them to my bass amp makes that Fender 15 watter sound like a Peavey 1x15. Before I go designing a cab, I don't want to risk tearing them open to find the speaker type and possibly damage them in the process. Here is what I know about them:
    1)They are Magnavox
    2)They have a tweeter, idk what size, and a woofer/midrange 5"
    3)they are rated at 14 watts peak (damn you marketing crap), which comes out to about 9 watts rms if I calculated properly
    4)They are 8 ohms
    5)They are ported on the front bottom
    And 6)The sticker says "Type FW25"
    Anyone have any clue what these things use? Google has zilch on them, except for a couple ebay ads, which are blocked on my device. What I have deduced thus far from other Magnavox speakers is that they might have Peerless woofers, but that still doesn't narrow the searching range down far enough. Also, what little I have found hints at they being "2 Way Bass Reflex Ported Speakers", or something to that effect. Magnavox website also is crap, as they only show TV systems and home theater **** and stuff like that, but nothing radio related. Did I happen to snag a couple dinosaur eggs or something? I love the way they sound with my bass and want to make them into a bass cab, but don't want to risk destroying them to just make a half-assed cab because I don't have the data to calculate anything...