First ported sub advice

Discussion in 'DIY Speaker & Subwoofer Building' started by 494930, Jul 22, 2018.

  1. 494930

    494930
    Well-known Member

    Joined:
    Dec 15, 2016
    Messages:
    1,556
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    116
    Location:
    Lisburn
    Ratings:
    +1,401
    Hi all. I'm looking to build a single large ported sub to replace the pair of very small sealed ones I'm currently using. Main use is for movies with the occasional bit of music, and the loudest i listen at is -10dB (semi-detached ;)). I am reusing the JBL drivers (GT5-12 14.2Xmax circ,24mmXmech)and have a maximum usable size of 240x75x28cm, I estimate the volume to be about 300L after bracing etc. Is building to such a shallow depth compared to the length a cause for concern?

    I've had a play with WinISD and think I'm pretty happy. I've modeled three different port tunes 19,17 and 15Hz. All three variations will hit 110dB with 150W and [email protected] from 50Hz upwards. Port velocity is below 17m/s and 30m/s respectively.

    I'm not sure which tune to go for and would appreciate any input. The 19Hz tune is -3dB at 17Hz. The 15Hz tune obviously digs the deepest (-6 @14dB) but is -5dB @20hz. How much should room gain affect my decision making?

    Excursion 500W.JPG Excursion.JPG Port 500W.JPG Port.JPG SPL 500W.JPG SPL.JPG

    Thanks, Colin.
     
  2. 494930

    494930
    Well-known Member

    Joined:
    Dec 15, 2016
    Messages:
    1,556
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    116
    Location:
    Lisburn
    Ratings:
    +1,401
    Heres the link to the full WinISD files including the driver. Please let me know if i've done something wrong. Thanks again, Colin.
    WinISD - Google Drive
     
  3. mattkhan

    mattkhan
    Distinguished Member

    Joined:
    Nov 14, 2005
    Messages:
    9,561
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    166
    Location:
    Beckenham
    Ratings:
    +4,527
    2.4m is big enough for modal resonance to be a concern as the first mode is at 71Hz for that length, appropriately placed stuffing (i.e. not blocking airflow to the port) should mitigate this but it is something to keep an eye on. Bracing will be v important for such a large panel as well. It also sounds like a bear to actually build :)

    30m/s is high however add an appropriate high pass filter to each design and you'll probably find the peak velocity comes down a bit so it may not be a real issue in practice.

    lots, building a sub that specifically blends with your room is one of the advantages of DIY.
     
  4. 494930

    494930
    Well-known Member

    Joined:
    Dec 15, 2016
    Messages:
    1,556
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    116
    Location:
    Lisburn
    Ratings:
    +1,401
    I hadn't thought about modal resonance within the enclosure and my ceiling height is 2.4m potentially compounding the problem. Definitely gave me something to think about there. I know what you mean about being a bear to build, it's going to have to be done in the room due to the size/weight.

    The models were with a 10Hz highpass added. The 30m/s was for 115dB but at my maximum expected volume of 110dB port velocity was down below 17m/s. Do you still think this could be an issue?

    My room is small at 3.3x4.1m. I believe room gain at 20Hz to be 6+dB in an average sized room? So given that, would you opt for a lower tune that keeps you flat down to c.14Hz or a higher tune that gives a +6dB boost @20 as a natural house curve?

    Also what about port compression? I know it exists but thats as far as my knowledge goes on the subject. Is it likely to be an issue with say a 5x25cm port 80cm long?

    Sorry for the mass of questions, but i've only got one chance to build this as my screen wall will be going up infront of it. Thanks again, Colin.
     
  5. mattkhan

    mattkhan
    Distinguished Member

    Joined:
    Nov 14, 2005
    Messages:
    9,561
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    166
    Location:
    Beckenham
    Ratings:
    +4,527
    should be fine

    varies by room, do you have a measurement mic?

    this is driven by port velocity rather than size (assuming you don't use some extremely thin port which you're not), remember to check the first port resonance though. I guess it should be ok at 80cm though.
     
  6. 494930

    494930
    Well-known Member

    Joined:
    Dec 15, 2016
    Messages:
    1,556
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    116
    Location:
    Lisburn
    Ratings:
    +1,401
    My first port resonance is above 200Hz for either option, so should be ok there.

    I do have a umik-1, how do I use REW to measure room gain? A quick Google suggests to measure the response at mlp then again right at the driver and compare the difference. Have I got that right?
     
  7. 494930

    494930
    Well-known Member

    Joined:
    Dec 15, 2016
    Messages:
    1,556
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    116
    Location:
    Lisburn
    Ratings:
    +1,401
    Had a stab at measuring my room gain. I disconnected one of the subs and disabled Audyssey. Took a measurement approx 1cm away from the driver of the still connected sub and then another measurement at MLP.

    I'm not 100% on what frequency I'm supposed to match the measurements(or even if I've done the right thing at all). If I match the measurements so that the 60-200Hz region follow the same trajectory I get 7.5dB gain at 20Hz and 19dB gain at 10Hz.
    Room gain.JPG

    If i match the measurements at 40Hz i get 4dB gain at 20Hz and 16dB gain at 10Hz.
    Room gain 2.JPG
     
  8. mattkhan

    mattkhan
    Distinguished Member

    Joined:
    Nov 14, 2005
    Messages:
    9,561
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    166
    Location:
    Beckenham
    Ratings:
    +4,527
    So the difficulty is knowing where to match them up which is hard to do without measuring the sub outside. My guess would be at about 30Hz is where room gain is kicking in.

    This lends some support for the idea of a lower tune.
     
  9. 494930

    494930
    Well-known Member

    Joined:
    Dec 15, 2016
    Messages:
    1,556
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    116
    Location:
    Lisburn
    Ratings:
    +1,401
    Thanks again for your help Matt, it's been really appreciated.

    I think I'm going to go with the 17Hz tune. After room gain that should see me [email protected] and [email protected] Just need to buy lots of MDF now :smashin:
     

Share This Page

Loading...