PSB sub bottoms out easily - Modifying a commercial sub

Discussion in 'Archived Threads 2001-2004' started by Neil Joseph, Aug 26, 2002.

  1. Neil Joseph

    Neil Joseph Lead Actor

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    I know I know, it is not the best of subs out there but now the worst either. I have a subsonic 6 that seems to bottom out easily. I had the driver out about a couple of years ago. Anyway, when I was in there, I noticed stuffing around the lower part of the sub. It is dual ported around the top part of the sub....
    [​IMG]
    Also, here are some specifications. Naturally, I have it calibrated to the rest of my system. Is there anything I can do to help it out?
     
  2. Fred Seger

    Fred Seger Stunt Coordinator

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    Maybe use a subsonic filter. It will kill off the low frequencies that are causing the driver to bottom. it's the frequencies below your subs tuning that are probably the culprit..

    I'm sure somebody else will have a different/better idea.
     
  3. Ron D Core

    Ron D Core Stunt Coordinator

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    You could tuneit lower to reduce the excursion at the frequencies that seem to be bottoming it. You would just have to increase the port lengths if at all possible.
     
  4. Neil Joseph

    Neil Joseph Lead Actor

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    Would adding more stuffing help the cause? I may try removing the driver again to see what port lengthening possibilities I have.
     
  5. Dustin B

    Dustin B Producer

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    The problem is those are 2" ports. The rule of thumb is that a port should not be any more than 5 times its' diameter in length. So you shouldn't be going much past 10" in length which will really limit how low you can tune the sub.

    Adding polyfill will make the box appear larger to the driver and ports, which will lower the tuning point somewhat. An optimal stuffing job can increase the appearant volume of the enclosure by ~30%. Which will lower the tune of the enclosure ~3hz. Tom Nousaine found that somewhere between 1 and 1.5 lbs of polyfill for every cubic foot of internal volume gains the largest appearant volume increase. Just be careful not to impead air flow of the ports or pack the polyfill too tight. 1.5lbs/ft^3 is a lot of polyfil.
     
  6. Neil Joseph

    Neil Joseph Lead Actor

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    The specs say it is dual 2" ports both 2" long and the box is 1.65 ft^3. The stuffing in there now is almost like the pink insulation used in basements. If I remove that, what kind of insulation should I get and what is it called? I assume that the gross volume is 1.65 ft^3 not accounting for the amp, ports, bracing and driver. Let's say the net volume is around 1.4 ft^3 and I use 1.4X1.5=2.1 or say 2 ft^3 of stuffing, can I just cram it in there but leave space around the ports for airflow?
     
  7. Dustin B

    Dustin B Producer

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    Ouch, if we say the actual internal volume is 45L. Then two 2" ports that are 2" long will tune that enclosure to over 40hz.

    Head to Wallmart. You can buy bags of polyfill for like $2.50 there. Then yep, just try and stuff it in there evenly keeping the ends of the ports clear.

    You may also want to try and find some 2" PVC pipe and couplers. Then extend those ports and see how it sounds. Try adding maybe 4" to their length and see what happens (this should tune to ~30hz). Without more specifics on the driver and amp, you can't really know what will happen when you get the tune down a bunch from the original design. So don't glue the extensions in place, tape them in or something similar until you are sure you are happy with the result, then glue them on.

    If you get the appearant volume up to 55L and extend the ports to a full 10" then the enclosure will be tuned to 25hz.
     
  8. Neil Joseph

    Neil Joseph Lead Actor

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    I don't have driver specs but the sub specs say it has a -3dB point of 25Hz so it must be tuned to a point much lower than 40Hz to achieve those specs. Anyway, I will go to Walmart and pick up some of this Polyfill, assuming they know what that is there and I will remove the driver and stuff it to see what that buys me. I may try to extand the 2" ports as well afterwards as well.
     
  9. Brian Bunge

    Brian Bunge Producer

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    Neil,

    Polyfill is just pillow stuffing and is made of dacron I believe. It will be in the crafts section and there will probably be an entire aisle of the stuff.

    Brian
     
  10. Dustin B

    Dustin B Producer

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    But Neil, they don't tell you what level that signal was that was down 3dB at 25hz. If it was an 80dB or less signal, I'd believe it. Subs can still play well below their tuning frequency. Just not very loud or cleanly.

    Net 1.6ft^3 enclosure with two 2" ports that are 2" long tunes the enclosure to 43hz. This is plugging the numbers into LspCAD which is quite accurate.
     
  11. Neil Joseph

    Neil Joseph Lead Actor

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    How exactly would you know without knowing the specific specs for the driver (qts, vas, fs, xmax, qes etc), or do you also have that info in LSPCad. Curious.
     
  12. Dustin B

    Dustin B Producer

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    Enclosure tuning is independant of driver specs. It is only dependant on the appearant volume of the enclosure and the number/diameter/length of the ports.
     
  13. Neil Joseph

    Neil Joseph Lead Actor

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    Well I removed the driver. It turns out that the dual 2" ports are not 2" long but 9" long and they are flared at both ends. Can you calculate what the enclosure tuning is based on this new info? I am removing the stuffing it came with and trying as much of this new stuffing as I can.
     
  14. Dustin B

    Dustin B Producer

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    That's better. That tunes it to just under 30hz. I wouldn't try and make those ports any longer. But you can try increasing the amount of stuffing using polyfill. You might be able to get the tuning point down to almost 25hz.
     
  15. Neil Joseph

    Neil Joseph Lead Actor

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    Well I stuffed it and it fixed my problem. I had removed the driver some time ago (maybe 2 years) and since I tinker so much with my equipment, I honestly couldn't quite remember why I was in there until now. I was trying to tinker with the box to see if I could eliminate this rattling (bottoming out) and I re-arranged the stuffing a bit while I was in there., probably aggravating the condition. Anyway, I stuffed the upper chamber except the area around the ports and I completely stuffed the lower chamber (which was previously stuffed with 3 sections of insulation which is similar to home insulation.

    Now the thing holds its own with the Toy Story 2 intro and only begins to bottom out with Titan AE. My DIY sub handles the lower end much better.
     
  16. BrentPollard

    BrentPollard Second Unit

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    This is the thread Iv'e been looking for. I too have a PSB sub that I would like to add more fill to. My sub is split side to side with two ports on the right side. There is no filling at all on that side. So If I understand correctly I should be able to add poly to the enclosure providing I don't block the ports. Is that correct? My thought is to fill it from the back to front up to the length of the ports. Then can I fill from the bottom up to just below the ports? I guess what I'm asking is, does the cavity size have anything to do with the tuning? [​IMG]
     
  17. Neil Joseph

    Neil Joseph Lead Actor

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    Let me update this thread with a serious warning!! I overstuffed my sub, at least that is my theory. About a month after I did that, the sub would not power on due to a blown fuse. It would blow any fuse I put into it. Lucky for me I know a few things about electronics and traced the problem to the main power supply of the amp. Since it had 4 power transistors on it, tose were suspect above anything else so I bought and replaced all four transistors and that fixed it. Cost of parts was
     

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