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Reverse engineering a subwoofer?

Discussion in 'Amps and Cabs [BG]' started by JACink, Mar 9, 2013.

  1. JACink


    Mar 9, 2011
    A while ago, I posted a thread concerning 36 (identical) spare subwoofer drivers that I have laying around.

    So far I haven't actually done anything with these, although I got a lot of interesting ideas from you all!

    After some research, these drivers are supposedly 8", 120W and designed to go in a cabinet that is 295x362x175mm. That is all the details I can find!!

    If I want to make a cabinet that contains more than one of these, can I just multiply the original dimensions of the cabinet, to get a rough measurement for how big the new cabinet should be?

    For example...
    4x8" = 590x724x175mm?

    Also, can I use the overall area, and change the shape etc?

    For example...
    590x724x175 = 74753000mm3 = 590x633x200mm?

    I am not looking for perfect, just a good starting point!!
  2. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Rules of thumb, going from 1 driver to 2:

    Double the internal volume of the cab

    Same port length

    Double the frontal area of the port, or equivalently, double the number of ports
  3. Hi.

    IME a small subwoofer driver is almost without an exeption designed to be used in a vented enclosure, so You'll need the port dimensions as well.

    If You can't get the dimensions, find out the resonance frequency with rice, and go with that.

    Multiplying the volume of the enclosure for additional speakers should work fine.

  4. JACink


    Mar 9, 2011
    Thanks for the input, that makes life a lot easier!!

    @T-Bird: How does the "resonancy with rice" thing work??

    I think I can find out the approximate dimensions of the port that the original cabinets have, but does it need to be exact?

    Does the port need to be in the same or specific locatiom, or will whatever is easiest work?

    Thanks again!!!
  5. JACink


    Mar 9, 2011
  6. Just provided as it would eliminate the guess work ;) :cool:
  7. Cab seems very small, even for an 8, maybe sealed is the way to go?
  8. JACink


    Mar 9, 2011
    These were actually replacement drivers for 2.1 subs. I remember that the subs they are supposed to fit in did have ports, but I could only find an approximate dimension for these (about the size of the cardboard roll inside a kitchen roll.)
  9. Hi.

    When a speaker reaches its resonance frequency the cone movement is at its minimum amplitude.
    You can find out that frequency by driving the speaker with some uncooked rice on the cone and varying the frequency. When the rice dances the least, that's your free air resonance frequency.

    With a cab that small, it has to be pretty exact, but using the rice test and the approximate dimensions as a starting point, You can get a decent result.

  10. JACink


    Mar 9, 2011
    Thanks again Sam!

    What would be the negative side to putting these in a sealed enclosure? I mean, would I just loose overall volume (which is not a problem) or would I loose other things?

    If going sealed, would the same internal dimensions apply?
  11. Hi.

    You're welcome.

    The amount of volume and frequency response below 120Hz or so You're going to lose if the driver is designed to be primarily used in a vented enclosure is significant and will become a problem if You're going to actually use the cab for rehearsing or playing live.

    The (assumed) HiFi-like sensitivity of ~90dB alone will make the experiment academic at best, but if You shave an additional 10dB or so by using the speakers in the least efficient way, well... ;).

    Difficult to say without measuring the response, but since the Helmholz resonator (which a regular vented enclosure is) and a sealed box load or dampen, whichever way people like to call it, the driver in a completely different way, I would double the internal volume for the trial run(s).

    If those drivers or the same amount of other drivers were in my posession, I'd probably build a tubular test-enclosure out of a 10" PVC drainage tube with an airtight piston so the internal volume can be adjusted, and start measuring.

    A PC, cheap measuring microphone, WinISD signal generator and Audacity for checking the amplitude would most likely do all right.

  12. BogeyBass


    Sep 14, 2010
    there just home stereo 8" speakers
    sensitivity is going to be really low around 82 to 85 dB
    so your gonna wanna use alot of them to get anywhere
    like a 8x8 or 8x10

    the current enclosure they are in is around .38 to .5 cubic feet
    or 9 to 14 liters.

    if its just a standard 8" woofer the resonant frequency is going to be around 45 to 55 hz

    if its a actual sub type woofer then it will be on the lower side of
    39 to 45

    in the range of 39 to 55hz just go smack in the middle at 45hz which will work well for bass guitar.
    or measure the current port opening and length and we could figure out where its at.

    just build a 8x8 or 8x10, wont have to be very deep since they dont need much volume just wide and tall enough to have that many 8" speakers

    something like a 2x15 size cabinet and just load it with 8" instead maybe even do a crossfire style cabinet to rid the combing of horizontal speakers since your gonna have to go horizontal to even fit that many speakers in a decent sized cab

    possibly something around 24" x 38" x 12" would fit 8x8
    you would have to go 40 to 42 high to squeeze 10 of them in there
    you could just use pine shelf boards and build a older fender style cabinet.

    something i would not pursue for the most part
  13. Helluva breakdown there.

    I assumed OP wants a sub cab, which cuts out the crossfiring requirement. He could put them all around the box,

    mayby even go isobaric.
  14. BogeyBass


    Sep 14, 2010
    Isobaric is a interesting idea.

    the benefit of isobaric is that you get the same amount of bass response in a smaller box. meaning you can roughly half the size of the box.

    the downfall to isobaric is it also cuts the sensitivity in half.

    But since he does have 36 or so of these speakers he could do an isobaric just to drink up all the speakers.

    basically to get the same sensitivity of 10 speakers you would have to use 20. Since its isobaric you actually need 40 speakers. think thats right 40 speakers to equal the sensitivity of 10 .......but the box would be smaller.

    since he does have 36 speakers, split that in half that is 18

    so now try to figure out something ridiculous on how to fit 18 speakers into a relative small sized cabinet. and use the benefit of the isobaric to really stuff them in there. because now we dont even need 14 liters you can use 7 liters per speaker.

    so figure out some weird contraption to stuff 8x18 into with small compartments for each. And simply load them back to back,or front to front isobaric. Which would actual be a total of 36 speakers. But you only need cutouts, of course for 18.

    18 speakers would have the sensitivity of 9 speakers, so at 82 to 85 dB per speaker on the high side would hit 94/95 dB low side 91/92 dB
  15. Scratch that one!

    Horn loading?
  16. BogeyBass


    Sep 14, 2010
    well horn loading can increase sensitivity about 5 to 10dB

    better suited for a single driver. hopefully a driver with known TS parameters.

    Best option would be to stuff as many as you can into a friendly box size. like 8x8 or 8x10

    otherwise much better to make a high sensitivity 2 way home stereo speaker system. If they are 8" with decent midrange you could cross them over high. So just a Low Frequency 8" and a good tweeter that will cross over low enough to match the 8" cutoff point. Or for better off axis you would crossover about one half or one quarter under the beaming frequency of the 8"

    since in a usual 2 way speaker Tweeters usually have higher sensitivity at least with a horn type tweeter. So you would have to add inline resistor into the crossover design to bring down the level of the tweeter.

    In his case since he has so many 8" drivers you could simply use 4 or more woofers to bring up the sensitivity to the same level as the tweeter. And use a much simpler crossover design and have good power handling for the LF.

    something similar to this diy project The Stentorians
    using the same idea multiple LF speakers to match sensitivity of one tweeter. more ideas here
  17. JACink


    Mar 9, 2011
    Thanks for all the ideas and (complex) info!

    I do not plan to use these for any real practicing or jamming, but I may make a few subs for around the house (maybe 2x8 or 4x8) and then play with the rest.

    So, to try a 4x8 sealed sub, I would aproximately double the original enclosure volume, and shrink it from there?
  18. JACink


    Mar 9, 2011

    If I build a 4x8 that is rated at 4 ohms (so 2x parallel lines to 2x in series), is it possible to add a 5th driver just to change the ohm rating?

    In other words, could I have a 5th driver that is either disconnected (leaving the rating at 4Ohms), connected in parallel (making a 5x8 at 2Ohms), or in series (making a 5x8 at 8Ohms)??

    Is it a bad idea to do this?

    Could I just do this with a switch (maybe under a cover to stop accidents!)??

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