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Designing an open back midrange speaker

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  • #16
    Originally posted by rpb View Post

    Do you think that there is much perceived quality to be gained OB compared to a relatively large, and well damped mid chamber? By well damped, I mean roughly a .2 cu-ft chamber that is chocked full, or close to full, with dense fiberglass like that used in ceiling tiles?

    I have a mid-bass that I like, but it seems to have a thin / transparent cone. I damp excessively, but if OB will be even better, I might give it a try again. (Last OB I built was close to 40 years ago, and the x-over was a WAG.)
    0.2 ft^3 would be the minimum I’d use for a sealed mid enclosure. I generally suggest a bit larger at 0.25 ft^3 but the truth is most any reasonable midrange enclosure volume will exhibit distortion artifacts regardless of stuffing mass.

    Speaking of stuffing, years ago I designed a 3 way with a stuffed sealed mid in a 7 ltr. enclosure. Because of the enclosure inner dimensions, I had audible standing waves at roughly 300 Hz and 500 Hz. Vowing to remove these unwanted frequencies, I worked late into the night trying various stuffing material combinations and stuffed the enclosure until they were eliminated. The next day I listened with fresh ears. While the audible standing waves were indeed missing, much to my chagrin so was the life of the recording. All the vivacity and energy had been eliminated along with the standing waves. Pulling out a great percentage of the stuffing brought the life back into the recordings. – And well, the standing waves too. Stuffing can be a fine balance and was a good lesson that anything in excess is generally not a good thing.

    Reducing or eliminating the unwanted sonic aberrations in a midrange enclosure is a Herculean challenge of compromises. Make the enclosure large enough to attenuate some of the issues and standing wave frequencies fall squarely within the mid’s passband. Make the enclosure small enough to push the standing waves out the passband, and there is not enough volume to sufficiently attenuate the acoustic energy reflected back to the rear of the cone. The incorporation of non-parallel walls into the design can be challenging as well. One novel solution was demonstrated in 1993 with B&W’s Nautilus design. – Now if I only had a very large 3D printer…

    Although cloning a Nautilus style enclosure may be impractical for many of us DIYers, with a bit of forethought, one may be able to improve the performance of the basic 3 dimensional cuboid shape. One method would be to add angled internal panels that reflect the sonic energy in non-orthogonal directions. The clever builder could use these panels to ameliorate standing wave issues by shortening or lengthening path lengths, create folded lines for attenuation, and even have these panels double as internal bracing. Other possibilities include a spherical enclosure which virtually eliminates all standing waves, while a cylindrical enclosure only exhibits one path for standing wave generation.

    Originally posted by ChrisLudz View Post

    Curt - Your post is much appreciated.

    I am planning to build the Statement center channel. If I use the unshielded version of RS180-08 and reduce the depth of the enclosure by 2 inches, the length of the open transmission tunnel also will be reduced by 2 inches. Will this have a negative effect on the mid range acoustics?

    CL
    Chris, It would be best if you kept the tunnel length the same to make sure there will not be a dip in the midrange response. Remember the Center channel also needs about a foot of room behind the enclosure to sound its best.

    C


    Curt's Speaker Design Works

    "It is the mark of an educated mind to be able to entertain a thought without accepting it."
    - Aristotle

    Comment


    • ChrisLudz
      ChrisLudz commented
      Editing a comment
      Curt, Thank you for your expertise and insight. I will maintain the original Statements CC enclosure dimensions and use the shielded RS180-08 drivers which are still available from PE.

  • #17
    Originally posted by curt_c View Post
    0.2 ft^3 would be the minimum I’d use for a sealed mid enclosure. I generally suggest a bit larger at 0.25 ft^3 but the truth is most any reasonable midrange enclosure volume will exhibit distortion artifacts regardless of stuffing mass.

    Speaking of stuffing, years ago I designed a 3 way with a stuffed sealed mid in a 7 ltr. enclosure. Because of the enclosure inner dimensions, I had audible standing waves at roughly 300 Hz and 500 Hz. Vowing to remove these unwanted frequencies, I worked late into the night trying various stuffing material combinations and stuffed the enclosure until they were eliminated. The next day I listened with fresh ears. While the audible standing waves were indeed missing, much to my chagrin so was the life of the recording. All the vivacity and energy had been eliminated along with the standing waves. Pulling out a great percentage of the stuffing brought the life back into the recordings. – And well, the standing waves too. Stuffing can be a fine balance and was a good lesson that anything in excess is generally not a good thing.

    Reducing or eliminating the unwanted sonic aberrations in a midrange enclosure is a Herculean challenge of compromises. Make the enclosure large enough to attenuate some of the issues and standing wave frequencies fall squarely within the mid’s passband. Make the enclosure small enough to push the standing waves out the passband, and there is not enough volume to sufficiently attenuate the acoustic energy reflected back to the rear of the cone. The incorporation of non-parallel walls into the design can be challenging as well. One novel solution was demonstrated in 1993 with B&W’s Nautilus design. – Now if I only had a very large 3D printer…

    Although cloning a Nautilus style enclosure may be impractical for many of us DIYers, with a bit of forethought, one may be able to improve the performance of the basic 3 dimensional cuboid shape. One method would be to add angled internal panels that reflect the sonic energy in non-orthogonal directions. The clever builder could use these panels to ameliorate standing wave issues by shortening or lengthening path lengths, create folded lines for attenuation, and even have these panels double as internal bracing. Other possibilities include a spherical enclosure which virtually eliminates all standing waves, while a cylindrical enclosure only exhibits one path for standing wave generation.


    Chris, It would be best if you kept the tunnel length the same to make sure there will not be a dip in the midrange response. Remember the Center channel also needs about a foot of room behind the enclosure to sound its best.

    C

    I'm trying to get a feel for what part of the music range could improve. Issues at 300hz to 500hz, would be in the vocal range mostly, wouldn't they? What instruments would be effected? Most of my 2-way speakers do vocals well, if it's one singer. When it's three, then all bets are off. A choir, while not music that I like anyway, sounds distorted. Complex orchestra suffers the same fate.I always thought these issues were in the tweeter x-over range, ie around 2k or so, or maybe the room is too lively, and decay for the treble is longer than ideal, or maybe I'm hearing IM distortion. I also assumed that I might be getting a slight grittiness from the internal reflections. Damping seems to helps a lot. A lower x-over point helps sometimes, but has trade offs at higher spl. I'm getting the urge to try OB again, but it's so much easier to just build little 2-way boxes. I'm also looking to build a "final" pair of speakers, for use with a sub. I could go with a 3-way, but the depth needed for the mid tunnel might look a little odd, unless I use shorter stands. I guess I just really want an idea of what to expect before I decide.

    Comment


    • #18
      One could always incorporate the mid tunnel into the stand. Elbows and folds are certainly acceptable.

      Frequency range of various instruments/vocals:
      http://1.bp.blogspot.com/-Dy6Yfqz6se...600/RangeE.jpg

      My example of frequencies of 300 Hz and 500 Hz are roughly contained in the frequency range of C4 to C5 (C4 being middle C on the piano) and would include both male and female vocals, as well as most instruments. These example frequencies are for an enclosure with internal dimensions of roughly 7” x 7” x 12”. Note first of all that two of the dimensions are the same, which will generate standing waves of the same frequency in two planes. Not the best choice, of course.
      Standing wave resonances are very high Q and will not be excited except over a very small frequency range. This means one musical selection might not excite the standing waves at all as those exact frequencies were not reproduced in the recording. Another may use those frequencies throughout the recording and be quite offensive. But they are not the only issue we contend with suppressing the back wave energy from the cone.

      Another significant enclosure-induced distortion is the energy reflected back to the speaker cone after bouncing off the internal panels. This frequency spectrum encompasses whatever frequencies the driver reproduces but due to the path length is delayed in time from the original signal. The delay can vary depending on how many times it reflects in the enclosure before reentering the cone. The audible result is smearing of the sound. Here tunnels perform well as a significant percentage of the energy does not internally reflect back to the cone.

      Of course the driver system itself is more than capable of producing audible distortion. Simpler, or poorly designed or implemented motor structures will likely have higher harmonic and IM distortion levels. These will both congest the resultant sound although IMD will sound more disharmonious as the resultant frequencies will not necessarily be harmonics of the fundamental. Instead being the products of the sum and difference frequencies of multiple frequencies. Distortion issues of this nature can often are described as ‘veiled’ or imposing a ‘glare’ and a loss of micro-detail in the ‘black spaces’ in between the notes. Simply put, the noise floor is higher and the IMD will increase with the complexity of the music. . Spending a few dollars more for high quality vs. high value drivers can potentially pay big benefits once the enclosure issues are resolved.

      And the 2 way / 3 way quandary: I always fall back on what I call the ‘intelligence band’. - Those frequencies where the ear is most sensitive. This varies somewhat with SPL, but generally around 300 Hz to 3000 Hz. In my experience, it is far more important to minimize response and phase aberrations as well as limit distortion and energy storage in this area than in the audible bands above and below it. I find it is not surprising that so many well respected designs utilize the same driver for the bulk of this frequency range. – Not that a well turned out 2 way cannot perform well, but since the average 2 way Xo is smack in the middle of the intelligence band, it is less forgiving of crossover induced issues than a 3 way where the crossover frequencies are pushed to the limits of this frequency band.

      C
      Curt's Speaker Design Works

      "It is the mark of an educated mind to be able to entertain a thought without accepting it."
      - Aristotle

      Comment

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