WinISD and perfectbox problems

Discussion in 'Amps and Cabs [BG]' started by Mottlefeeder, Jan 28, 2004.

  1. I have a Fender BXR100, which was fine with a J type 4 string, but sounds uncontrolled with a soapbar-equipped 5 string. Putting the speaker TS details, box and vent dimensions into WinISD gives me a -3db of 75 Hz and a vent tuned to 53 Hz, which suggests that I have found the problem.

    My cheapest option seems to be to retune the vent down to 30 Hz, and WinISD and Perfect box both give the same vent sizes, but one says the air speed is OK and the other says that it is way to high.

    Is there a volunteer out there who can check what I have done, and tell me where I am going wrong?

  2. notanaggie

    notanaggie Guest

    Sep 30, 2003
    The air speed is variable depending on what was assumed on the vent. Air speed normally is mostly concerned with vent noise. Too fast and you get wind noise from it.

    If the edges are rounded or "faired" for good flow,that changes the effective length, but also causes the air to flow well enough and quietly enough that a higher "air speed" is acceptable.

    Does one of those assume square vent ends and the other not?
  3. Retuning the box lower will only make it sound crappy over a wider range. The lower tuning might keep it from farting on low B, but the whole bottom end will droop.

    Don't expect a Toyota Tacoma to haul the same payload as a Dodge Ram, ain't gonna happen. Same with cabs.

    Software programs will give you a general idea, but only a signal generator and digital volt meter will tell the real story. Until you know how your cab is actually tuned, you are just guessing, and will most likely ruin the resale value of the BXR.

    Sell it as is, and get something more to your liking.
  4. There is something that I am missing here...

    WinISD shows that GD reduces above 30 Hz, but increases below 30 Hz, and that the phase angle reduces above 30 Hz but increases below 30 Hz, when you retune the cab down from 50 to 30 hz. I would have thought that both of those would lead to better sound.

    I appreciate that I am not going to get 30 Hz fundamentals out of a cheap combo, and I know that they sound muddy anyway, but I do not have a mental MP3 of a drooping bottom.

    What is it that I am going to hear/not hear?
  5. A Carvin RL210 is a good example of no bottom.

    There is more to it than the tuning frequency. Otherwise, anybody could stick an el-cheapo driver into any size box, and have a great sound. Doesn't work that way...

    There is an optimum tuning for a given driver. This means a specific internal cabinet volume and tuning frequency. There is an optimum minimum port diameter for a given driver. Deviating from these changes the sound produced by the driver. It always gets worse, never better than optimum.

    Vented boxes give you a lot of flexibility in the tuning.

    The SBB4 alignment (Fb = Fs) offers the best transient performance, lowest group delay and least amount of mud. The penalty is a steeper rolloff curve, and from higher up the scale. The SBB4 is my personal favorite because of the lowest tuning frequency. This is closest to the lowest bass guitar note, and offers the maximum acoustic loading at the bottom end.

    The QB3 alignment is widely used, because it offers a smaller cabinet in many cases, and good performance. In some cases, the QB3 offers superior power handling and better excursion than SBB4. This is a case-by-case basis.

    The Optimal Flat provides the lowest *flat* frequency response, but at a cost of higher GD.