SVS B4+ and porting issues.

Discussion in 'Speakers & Subwoofers' started by Craig Chase, Jun 13, 2004.

  1. Craig Chase

    Craig Chase Gear Guru
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    There is a Velodyne thread on AVS now titled "Which one should I get, The Velodyne DD-15 or the DD-18"

    There were some statements made about the B4+ and its performance in John E Johnson's test of both the DD-18 and the B4+.

    As this unfolds, please understand any flaming remarks toward either product will be immediately reported... this is for feedback and learning... not war.

    The poster asserted that since the B4+ could "only" hit 100 dB @ one meter @ 16 Hz with 31 % distortion in the Four ports open, 25 Hz setting THAT...

    With the 16 Hz setting, the 100 dB (@16 Hz and one meter) maximum would be even lower with two ports blocked... and the 16 Hz setting selected.

    Is this true??... and why or why not?
     
  2. Edward J M

    Edward J M Cinematographer

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    Here's a great article on the subject.

    http://www.hometheaterhifi.com/volum...ayporting.html

    15 Hz is FAR below the 25 Hz tune point (with all ports open). Driver excursion is very high at 15 Hz, and excessive excursion is THE primary reason for increased THD.

    Plug two ports and lower the tune point to 16 Hz. Cone motion will be heavily damped because the drivers are well coupled to the resonators (which will also shoulder most of the output at 15 Hz). Less cone motion = lower THD.

    If JJ plugged two ports and ran a 15 Hz sine and again pushed it to 100 dB, the THD reading would be much, much less than 31%.
     
  3. Marcelo T

    Marcelo T Stunt Coordinator

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    If I'm not mistaken, the B4 was tested outside while the DD18 was tested in room, therefore better SPL.
     
  4. Edward J M

    Edward J M Cinematographer

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    The B4+ was the 1st subwoofer that Doc JJ tested outside. It's very hard to compare the his data sets between the B4+ and the DD18.

    If anyone wants to see what both are capable of across the spectrum, it's best to test them under the same conditions. 2 meters GP is the gold standard, but not all reviewers can do GP all the time (I sure can't due to inclement weather).

    The B4+ should be tested in the 16, 20, and 25 Hz tunes to see which turns in the best overall performance. That subwoofer seems to have an overabundance of clean output in the mid bass regions, so plugging a port (or two) should really shore up the deepest octave while not diminishing mid bass performance too much.

    Also, the DD18 will limit itself at the lowest frequencies in order to keep THD around 1-3% (IIRC depending on the control settings). For example, the DD12 was just tested (I think by Nousaine) and only pulled 78 dB at 20 Hz at 2 meters in a 7500 ft3 room due to the distortion limiter. The DD12 could play louder at 20 Hz and still stay clean. 10% THD or less seems to be the industry standard for defining clean output in the bass regions. That would make the Velo digital servo limiter a bit conservative by most standards.

    The balancing act between output and THD needs to be kept in perspective; if the distortion limit is set well below audible threshold, you're leaving unused output on the table. If Velo wicked up the limiter to say 8-10%, THD would still be commendably low, and overall output in the bottom octave would be considerably higher.

    It would be cool if Bruce could shed some light on this topic and why Velo opts to keep THD in the 1-3% region for the DD series of subs. There is assuredly more to it that what I've described above; I've probably just scratched the surface on what is a complicated subject.
     
  5. Tom Vodhanel

    Tom Vodhanel Cinematographer

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    I think the new DD series allows the user the flexibility to let a little more THD thru for the sake of maximum output. I'm not sure on that though. The DD-12 did 4dB more than the FSR12...which is a significant increase. I haven't seen the review yet...so I don't know what its FR was(?). There is only so much you can do with a 15" cube, and 120v AC..[​IMG]


    On the B4+...

    Under the tuning point of the enclosure, the porting and the driver won't be in phase anymore...the enclosure will just act like it has a big air leak in it(the port). with very careful electronic work(combination of EQing and highpass/subsonic filtering) and quality drivers...you can extend the clean output capabilities of a porting unit SLIGHTLY below the actual tune of the enclosure. But the very best you can hope for is usually 1/6th of an octave.

    The next time we drag out a B4+ for ground plane testing we can try finding 31% THD at 16hz with all ports open....and then plugging two ports and seeing how the THD is affected. John Johnson uses the same THD software as I do (Soundtechnology) so the results should be fairly representative.

    Also, trying to compare in room measurements to ground plane (quasi-anechoic) is very tough. You can make reasonable guesses...but unless the two models were measured by the same person(with the same equipment used for both)...reasonable guesses is all they are..[​IMG]

    Tom V.
    SVS
     

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