chuckears Posted September 10, 2007 Share Posted September 10, 2007 Wasn't sure where to post this one; it's a technical question about a DIY sub mainly for home theater use... most of the builders seem to hit this section, so here goes: I'm planning a traditional rectangular box (probably mostly birch plywood, with some MDF for motorboard and support pieces) with a 15-Hz tune to match my existing DIY Sonosub (aesthetics and logistics do not allow for another Sonosub). I have two 12-inch drivers to work with, but do not have their complete T\S parameters - just efficiency, power handling, and XMax. I know from using them in another project that each driver individually is not quite capable of the 15-Hz tuning I am planning (they will bottom out at the SPL I require somewhere under 16 or 17 Hz), but the big question is: Can both drivers, working together in the same enclosure, handle the lower tuning; in other words, will a dual-driver system provide not only the obvious increase in SPL, but at lower frequencies beyond the drivers' individual capabilities? I would hate to find out after-the-build, and hear that dreaded fapping sound... Quote Link to comment Share on other sites More sharing options...
STL Posted September 10, 2007 Share Posted September 10, 2007 That depends how you set them up. If you do an isobarik setup then I believe that can help with extension issues (but then you don't gain the SPL increase), but if you just run two of them in an enlcosure that's twice as large (but the same design as the box that had issues) then you really won't gain anything but SPL. What drivers are they? What kind of enlcosure were they in before and what do you plan to put them in now? Quote Link to comment Share on other sites More sharing options...
masterxela Posted September 10, 2007 Share Posted September 10, 2007 ISOBARIC: (isobarik, compound loading) - a method of using two drivers working in tandem in order to achieve a smaller box size for a given design. Theoretically, the Vas of the complete system will be half that of a single driver, which results in a net box size that's also reduced by half. The sensitivity of the isobaric system will be the same as that of a single driver, but you've got to spring for that extra driver! Quote Link to comment Share on other sites More sharing options...
DrWho Posted September 10, 2007 Share Posted September 10, 2007 Short answer = no The only advantage to isobarik is that you can make the enclosure half the size. It does not increase maxSPL, which is to say that it is not a solution that should be considered in this situation. At best, doubling up on drivers is going to yield 6dB more output at every frequency. The overall frequency response of the system is pretty much going to remain the same. Why are you striving for 15Hz extension? Especially with drivers not suited for it? It has been my experience that sacrificing transient response through the audible passband (frequencies above 20-25Hz) does not out weight the benefits of lower frequency extension - especially into the inaudible passband. Do you have any kind of measurement equipment? Do you have any kind of EQ or "bass boost" available to work with? There are some tricks you might consider if your drivers can handle more power and not more excursion... I know it's not what you want to hear, but in all honesty I think you should sell the drivers and approach the situation with a driver intended for your purpose. Figuring out the displacement you need for the bandwidth and SPL's you want at the listening position is an easy mathematical exercise. I'd love to tell you that your Geo Metro could do an 8 second quarter mile, but that's just not gonna happen.... Quote Link to comment Share on other sites More sharing options...
chuckears Posted September 10, 2007 Author Share Posted September 10, 2007 Thanks for the replies and input... The reason I am looking for 15 Hz tuning is because the Sonosub I recently completed is tuned that low, and a problem I had solved a few years ago has once again become an issue; that problem being the ability to locate my right corner-positioned subwoofer with a lot of bass material (this is not a good thing, when the sound you are hearing is supposed to be straight ahead, or to one side, or behind you...). I solved it (and greatly helped my room response curve, as well) by adding a second sub (between my display and left channel speaker). (Yes, I know that the prevailing "wisdom" is that bass content is non-locatable, especially with the lowest material, but every room is different... my 12' high cathedral ceiling, the peak of which runs from my display to the back wall, does not provide for a kind of cut-and-dried solution you read from the set-up instructions. I have come to the opinion, like many, and through much experimenting, that the room is one of the most important factors in setting up an audio or HT system). Anyway... the new Sonosub has resurrected the problem of locatable bass; my second sub (which used to match the tune of, and was the same brand as, my previous corner sub) is not able to dig down as deeply as the Sonosub, so is not "doing its part" to equalize the sound I am looking for. ...and the reason I am\was looking to use drivers not suited to the task is simply because I have two on hand, and was looking for the most economical solution. I have a notion of the physics behind why this is not going to be feasible; I am relatively inexperienced with subwoofer design (I have built two, but only with the help of emulating others, and a couple of nifty little programs called Sonosub.exe and WinIsd), but had the hope that enough air could be moved by using two drivers in tandem to reproduce these lower-lows. I appreciate receiving good, honest, experienced advice... I will continue analyzing and see what else I can come up with... Quote Link to comment Share on other sites More sharing options...
STL Posted September 10, 2007 Share Posted September 10, 2007 I mentioned the iso config because I thought that a larger enclosure (effectively not literally) might allow the drivers to play lower than they could in the current enclosure size -- and it might allow an enclosure configuration change. That said, you would have to compensate for the lower efficiency by driving the speakers with more power which might not be possible physically (if you were already driving them at or near their power limit) or financially (meaning you don't want to buy another amplifier). So are you really saying you can locate the source of a 15Hz output? If so, I believe your problem might very well be due the resonances (octaves higher) caused by the 15Hz signal rather than that very low frequency itself. Have you considered that? Again what brand/model are the drivers? And tell us more about the enclosure they are currently in (and/or the one you want to put them in). Quote Link to comment Share on other sites More sharing options...
chuckears Posted September 10, 2007 Author Share Posted September 10, 2007 Both drivers came from Hsu subs; one from a TN1220 cylinder, the other from a VTF3; the space where the new enclosure would be located is best suited by a traditional rectangular box-type sub (I have more Sonotube from previous builds, but it would not work in this location). I cannot say I am hearing\feeling a 15 Hz output from the new corner Sonosub... that is merely the "tune" of the unit, which is lower than my second sub (the one that helped to "dis-locate" my old corner sub)... that is why I assume that a sub tuned to match the new one would once again help to balance out this sound. ...and it's not an issue that is ruining my audio\HT experience... I am just a chronic tweaker, and since these drivers are just lying around... Quote Link to comment Share on other sites More sharing options...
STL Posted September 11, 2007 Share Posted September 11, 2007 I guess I incorrectly assumed the two drivers you have were the same. Now that I know they are not, I cannot recommend even considering and iso configuration. If you really want to attempt to reuse one or more of those drivers then you'll need to buy something that'll give you the specs on the drivers (like this: http://www.partsexpress.com/pe/pshowdetl.cfm?&Partnumber=390-804 ). But even then you still might find that you cannot reuse either driver -- and you'll just have to buy one that'll do what you need. That said, even if you have two subwoofers in like (as in both ported or both sealed) enclosures that are both tuned to 15Hz they can still sound VERY different overall (even if driven with identical amps). Maybe your new sonotube -- that you are trying to match -- is tighter or even looser than your current sub and that is what is leading to this perceived imbalance. This difference could be caused by the box's alignment, the driver's characteristics, or the amplifiers characteristics (including any signal processing it might do internally) -- so you're got a lot of variables. Is the subwoofer you are wanting to match one you bought or built? Quote Link to comment Share on other sites More sharing options...
michael hurd Posted September 11, 2007 Share Posted September 11, 2007 It takes an awful lot of cone area and excursion to reproduce the lowest octave with authority. What is your target sound pressure level, budget and maximum footprint in length, width and height? Quote Link to comment Share on other sites More sharing options...
masterxela Posted September 11, 2007 Share Posted September 11, 2007 I'd stay away from 2 different drivers in the same box for sure. I know it might be a fun inexpensive experiment to build this box, but afterwards, are you going to happy with something satisfactory? Just trying to save you some time and money. I'd sell 'em 3 different drivers in what would have to be 3 different boxes.... I am having extreme trouble getting a much simpler "setup" sounding how I wish Quote Link to comment Share on other sites More sharing options...
chuckears Posted September 11, 2007 Author Share Posted September 11, 2007 I guess I incorrectly assumed the two drivers you have were the same. Now that I know they are not, I cannot recommend even considering and iso configuration. If you really want to attempt to reuse one or more of those drivers then you'll need to buy something that'll give you the specs on the drivers (like this: http://www.partsexpress.com/pe/pshowdetl.cfm?&Partnumber=390-804 ). But even then you still might find that you cannot reuse either driver -- and you'll just have to buy one that'll do what you need. That said, even if you have two subwoofers in like (as in both ported or both sealed) enclosures that are both tuned to 15Hz they can still sound VERY different overall (even if driven with identical amps). Maybe your new sonotube -- that you are trying to match -- is tighter or even looser than your current sub and that is what is leading to this perceived imbalance. This difference could be caused by the box's alignment, the driver's characteristics, or the amplifiers characteristics (including any signal processing it might do internally) -- so you're got a lot of variables. Is the subwoofer you are wanting to match one you bought or built? I'm trying to match the Sonotube sub I built... Quote Link to comment Share on other sites More sharing options...
chuckears Posted September 11, 2007 Author Share Posted September 11, 2007 It takes an awful lot of cone area and excursion to reproduce the lowest octave with authority. What is your target sound pressure level, budget and maximum footprint in length, width and height? MIke, I haven't performed reference-level SPL test. The rest of the system is calibrated at a level that doesn't make my ears bleed when I run the test tones; usually setting my HT receiver\processor at -15dB (per the manufacturer recommendation), and setting all speakers at 75 Hz on the SPL meter from the listening position. Target budget is so far as low as possible, since my original intent was to use drivers and materials I already have on hand (except for maybe a piece of Birch plywood)... I am starting with down-firing driver)s) in a rectangle approx. 24D" x 26H" x 18W" (but correct tuning may require a couple of inches more). I actually planned to raid the driver in my second sub (a Hsu VTF3) since it is a close match to the other driver I pulled from a Hsu TN1220 - I have yet another driver to take its (the VTF3's) place, that will work well for what I intend to use it... this is all just theory and planning at this point, and it looks like I am not headed in quite the right direction... Quote Link to comment Share on other sites More sharing options...
STL Posted September 11, 2007 Share Posted September 11, 2007 I'm trying to match the Sonotube sub I built...Okay, so tell me all about it then. Getting info out of you is like pulling teeth. [] Quote Link to comment Share on other sites More sharing options...
chuckears Posted September 11, 2007 Author Share Posted September 11, 2007 Sorry... I'll elaborate later today... I have to go buy a car right now. Be back when I'm poorer... Quote Link to comment Share on other sites More sharing options...
michael hurd Posted September 11, 2007 Share Posted September 11, 2007 Assuming 3/4 construction with no bracing or driver displacement you have 5.86 cu ft. to work with. Unless you are willing to live with vent noise, this would be a small box that is tuned low. Low tuning requires a long port relative to a higher tuning, and making a small box tuned low is pretty tough. The port must be longer yet, eating up precious volume inside the enclosure. One way you can alleviate the problem is via use of passive radiators. Quote Link to comment Share on other sites More sharing options...
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