The $6 port tweak for Sunosub I.

Discussion in 'Archived Threads 2001-2004' started by Patrick Sun, Feb 10, 2001.

  1. Patrick Sun

    Patrick Sun Moderator
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    My 1st DIY sub was dubbed Sunosub I. It consisted of:
    2 Shiva 12" drivers
    8" wide port 24" long
    Fb (box tuned frequency) tuned to 19Hz.
    24" wide sonotube and MDF endcaps.
    12 ft^3 of internal volume
    F3 was around 18Hz
    On paper, it looked good, but for those problematic DVDs/LDs, I would get some popping from the source LFE going too low and loud and for my enclosure's tuning. (Note: on 98% of other LDs and DVDs, this sub performs wonderfully) I was wondering, besides resorting to some rumble filtering to get rid of the sub-20Hz material, what if I re-tuned the enclosure somewhat lower?
    So with the use of Adire version of nifty LsdCAD program, I did some more modelling with lower tuning for Sunosub I. To put in a new port easily with very little re-work of the port (and the hassle of taking the driver out to get to the existing port), I compromised. I initially used Quikrete tube for the 8" wide port. I noticed that there are various width of these "nominal" 8" wide ports from Quikrete. I went to Home Depot with a used piece of tube in hand and tried to find a tube that was slightly less then 8" wide so that I could just slip in the new tube and seal it up once I got the right tube.
    The compromise was that this new tube is less than 8" in width, the usable width I got with the new tube was about 7.25" (outer diameter was around 7.85"). This is good and bad. The good being I could have less length to achieve a certain Fb, but the bad is that I would cut down a little on the port/power handling capabilities of the system, but I thought the trade-off was worth it if I could get the sub to stop popping.
    Now, what's kinda of cool is that this allowed me to experiment with different tuning frequencies by shoving in the right amount of port to get to the next tuned frequency. I settle on 17Hz for the new Fb as a starting point. This meant a length around 28" after subtracting the new enclosure volume due to the loss from the additional porting volume displacement. I took my new piece of Quikrete tube and cut it down to 28" in length.
    Well, my torture material is the Pod Race from The Phantom Menace AC-3 LD. At Fb=17Hz, I still got the pops, but only at a certain segment, I had eliminated pops elsewhere in the race (the footage that pops that remained were at the 9:00-9:18 range on the 2nd side of the LD). At my primo seating position over 2.5 meters from the sub, I'm getting 100-102dB at the pops measured with my SPL meter (without corrections, so throw in another 4-6dB if you'd like). This is about as loud as I listen to my films (75-80dB is the normal dialogue volume level at the master volume I was at during the Pod Race scene).
    So I figured out how much more port length to get to 16Hz, and that required another 4" of port, so I just massaged the port upwards 4" inside the port to get the port extension and made sure the port was basically airtight once I got the right extended lenght of port. Well, I still got the pops, but that were less intrusive and less loud. This was a good step in the right direction.
    Then I go down to Fb=15Hz, and this means another 4" of port, so I massage the port upwards again for a total port length of 36" and get the seal as tight as I can for the interim and do the test again. Well, finally survied the Pod Race with no pops. I did measure a 1-2dB decrease at the problematic spots where the popping occurs, but as the movie is playing you won't notice a slight dip in SPL output, believe me. This was to be expected just be looking at the SPL graphs with the new Fb in place.
    At Fb=19Hz, the F3 was around 18Hz on paper (I think I measure around 19Hz or 20Hz earlier last year).
    Now, at Fb=15Hz, the F3 bumps up to around 25-28hz (haven't measured it yet, will do when I get some more time). But the sub is less obstrusive in its operation for HT, so the trade-off for the peace of mind over a bit more SPL was worth it.
    The side benefit is that it tightened up the musicality of the side, it's a little less boomy (meaning the transient response was a bit less, making it a little tighter). So I think I'll keep this tune for now. I'll give the dts Haunting a whirl and see if Sunosub I survives the ghosts as well.
    Anyhow, the new port cost me about $6. Just thought I'd share my fun with ports.
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    PatCave ; HT Pix ; Gear ; Sunosub I + III ; DVDs ; LDs
     
  2. Greg Monfort

    Greg Monfort Supporting Actor

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    >The side benefit is that it tightened up the musicality of the side, it's a little less boomy (meaning the transient response was a bit less, making it a little tighter).
    ====
    I believe you mean the transient response increased, as the box Q dropped down to ~0.5. Group delay was also increased, but shifted lower down where it's impact on transient response was less audible. [​IMG] With few exceptions, 16Hz was my preferred Fb, and adjusted Vb to maximize FR.
    It's interesting that someone would mix in such a low signal that they know the typical consumer sub can't handle, and falls below the DD standard 20Hz lower limit.
    GM
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    Loud is beautiful, if it's clean
     
  3. Neil Joseph

    Neil Joseph Lead Actor

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  4. Patrick Sun

    Patrick Sun Moderator
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    GregM, yes, that's what I meant (it's all plain when you look at the graphs that I didn't show).
    Neil, I don't have Titan A.E. (though I know about Chap 16 - the Ice debris field). What chapter in Toy Story 2 would you like me to listen to?
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    PatCave ; HT Pix ; Gear ; Sunosub I + III ; DVDs ; LDs
     
  5. Julian Data

    Julian Data Second Unit

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    Patrick
    for TS2.. Chapter 1-2.. Those seem to be the most low LFE content ..
    Julian
     
  6. Patrick Sun

    Patrick Sun Moderator
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    Neil, well, let me lay out some groundwork:
    I used Avia to calibrate my 5 speakers to 75dB, and then got my sub's SPL to read around 78dB (the Avia DVD tells you to match the speaker SPL with the sub's SPL, so it's a little goosed). This required my Sub output setting on the receiver to be -5dB (variable from -10db to +10dB), so I have plenty of headroom if I wanted more sub juice going to the subwoofer.
    Listening at the 75db reference level, at the 51 second mark of Chapter 1 of Toy Story 2, I was able to get a medium pop from the sub (before I calibrated, I got a loud pop! with my sub setting around -1dB). My SPL meter measured 103dB without any low end correction apply to that value. That transient is a monster! But I would never listen to 75dB reference level. My comfortable "loud" listening level is around 70-72dB, and my sub survived that peak at 51 seconds with the speakers calibrated in my first paragraph (and the peak SPL was 101dB).
    Just for curiousity's sake, I may take the tune down to 14Hz and see if I'm hitting the limit of diminishing returns.
    So, I think if you were to either ease up for the 1st minute of Toy Story 2, and then turn up the volume a little louder afterwards, you'd get a nice loud DVD viewing experience.
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    PatCave ; HT Pix ; Gear ; Sunosub I + III ; DVDs ; LDs
     
  7. Patrick Sun

    Patrick Sun Moderator
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    Here's some photos to explain what I did:
    Photo 1 - this is a shot of the 29" section of 7.85" wide (outer diameter) Quikrete tube that I used for the new port.
    Photo 2 - this is a shot of my sticking the new port into the existing port hole, and applying some weatherstripping to help with the seal on the port.
    Photo 3 - this is a shot of the new port fully inserted into the old port hole. This got me to a tune of 17Hz.
    Photo 4 - this is shot of the new port moved up in the original port hole about 3-4" to get to an effective port length of 32" to get the tuning to 16Hz.
    And finally, I moved the port up another 4" for an effective port length of 36" to get the tuning to 15Hz. I forgot to take a photo at this point in time, but it's fairly easy to remedy that oversight soon
    If I wanted to so, I can use my leftover new Quikrete tube and truly extend the new port to 36" by taping the new port to another 7" section of the tube and then truly seal it up. At this point, there's no reason to do so, as the new port seems to be as air-tight was it needs to be. I will leave it alone unless I want to play around with a tune of 14Hz later.
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    PatCave ; HT Pix ; Gear ; Sunosub I + III ; DVDs ; LDs
     

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