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DIY cabinet volume question...

Discussion in 'Amps and Cabs [BG]' started by curiousfunk, Apr 26, 2005.


  1. curiousfunk

    curiousfunk

    Apr 24, 2005
    Hello all DIY cabinet makers,

    I'm a new TB-er (as of a few days ago) and am building a bass cabinet for practice at home, using a single Eminence Omega Pro 12 and Alpha 6 midrange. When calculating physical internal volume, does that include the volume of the port(s)? For instance, using a 3 cubic foot internal cabinet with a 0.15 cubic ft (approx 14" x 2" x 9.3") rectangular port, do I really need to make the cab 3.15 cu ft internal overall?

    Thanks!
     
  2. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    You do NOT include the port area in the internal volume computation. You also must deduct the internal space used up by any internal bracing, as well as the space taken up by the speaker/basket/magnet assemblies. Recessed handles also should figure into the computation. ANYTHING that takes away from the overall internal space.
     
  3. curiousfunk

    curiousfunk

    Apr 24, 2005
    Thanks. My local library was out of their loudpeaker cookbooks. I'm going to document the build - I'll post some pics and comments when it's finished. I'm going for "living room" friendly cabinetry here, which calls for veneers and all.
     
  4. The Omega Pro 12 should be enclosed in a net volume of 1.23 cubic feet and tuned to 39 Hz. This will give you an outstanding 12" cab that balances perfectly with the Alpha 6.

    The Omega Pro 12 driver displaces about 0.1 cubic feet, which you will have to account for when calculating the cabinet volume. You will also have to add the volume displaced by the port, and the sub chamber that holds the Alpha 6.

    Put the Alpha 6 into a sealed chamber just big enough to hold it. The ideal volume is 0.28 cubic feet, sealed. Smaller won't hurt any response, because the Alpha 6 works high up. Cross over around 800 Hz, or as low as 500 Hz, if not pushing it hard.
     
  5. curiousfunk

    curiousfunk

    Apr 24, 2005
    Thanks bgavin, this design is based almost exclusively on your posts in these forums. Until you mentioned the Omega Pro 12, I had just about given up on Eminence for bass drivers in (relatively) small enclosures.

    I play a 5 sting pedulla thunderbass, although I'm rarely hitting open b.

    In WinISD, a 1.23 cu ft 39 Hz cabinet will be 3dB down @ about 64 Hz and 10dB down at about 39 Hz. I consider the -10 mark about the limit of useable range. Granted, the 1.23 cu ft cab drops off very smoothly and the group delay looks fantastic. But that's a TINY cabinet, and fitting a port in there with decent mach values is going to get tricky. Not impossible - I'm a better capenter than a speaker designer... AND, I have to fit the 6" in there somwhere too. But using the generic plans on the parts express web site for the Omega Pro 12 (3.04 cu ft, 40 Hz tuning) results in 3 dB down @ about 41 Hz and 10 dB down @ about 31 Hz when modeled in WinISD. The group delay however is not as nice, peaking at around 17-18 ms @ 38-40 Hz.

    How "muddy" is 17-18 ms? And what kind of cone excursion can I expect between the tuning freq and the 31 Hz limit? I don't think my version of WinISD calculates this.
     
  6. Here is a link to my generic 1x12 Omega Pro 12 design sheet:

    http://www.ofgb.org/reference/music/eminence/plans/omega%20pro%2012%20sbb4.jpg

    These plans do not reflect a subchamber for the Alpha 6. Mine are in a separate cabinet because I bi-amp, and I have an array of (4) Alphas in a vertical cab at 8 ohms. They are intended to mate with (2) 2x12 pair of Omega Pro 12 or a single Magnum 18LF. A 2x12 makes for a better slot port design, easier to wire, but is twice the size and weight. Tradeoffs, tradeoffs.

    The slot port is the most efficient use of space for a front vent, but is the most pain in the butt to build and tune. The above plan will get awfully close to accurate tuning. This port is slightly wider than the 12:1 maximum I like to maintain for slot aspect ratios. Note the slot length will actually turn 90 degrees down at the rear panel for an inch or two. A larger diameter with longer slot would lower the ratio, but increases the size of the box. The 2x12 does not have this problem.

    You can hear 25ms delay. Note the SBB4 alignment shown here has much better delay numbers. The SBB4 is the closest one gets to a D2 sealed alignment, which is "critically damped". The SBB4 has a gentle roll-off from fairly high up. This is correctable with EQ (remember to cut the highs, not boost the lows). A significant portion of the bass energy is the 2nd harmonic, 62 Hz for the open low B, and higher.

    The 3.04 cubic feet at 40 Hz is classic EBS tuning and results in a flabby bottom end. Typically EBS is 150% of nominal cabinet volume, but this is 240% of the SBBS size. Yes, the bass extends down lower, so does a fat lady's gut. Neither is pretty. The worst mud is at the tuning frequency, which is right in the middle of your useful 5-string range.
     
  7. curiousfunk

    curiousfunk

    Apr 24, 2005
    Yeah, the SBB4 alignment exhibits many benefits over the EBS. Designing/building ported cabinets is rather new to me. Every cabinet I've built to date has been sealed - which has been fine for home and car audio, but my aray of sealed 2x10 and 4x10 bass cabinets have never quite sounded/felt satisfying.

    I probably have enough MDF laying around to prototype close to 3 cabinets, so I'll build a 1.23, 3.04 and something in between. Actually, given the projected flabbyness of the 3.04, I may skip it and go directly to the something-in-between cab.

    Hypothetical in-between design: begin with a group delay limit of 16ms. With that in mind, adjust Fb and volume until we reach the same F10 achieved with the 1.23@39Hz cab. A 2 cu ft cabinet tuned to 49 Hz works (F3: 48Hz, F10: 37Hz). Group delay peaks at a little over 13ms @ 49 Hz. AND, we have reasonable port dimensions - no right angles or other assorted funkyness.

    The 39Hz tuning of the 1.23 cu ft box requires a pretty lengthy port - and maybe I shouldn't be freaking out about it.

    Is it a waste of time to build the 2 cu ft cab? Well, will I notice the 3.5dB (ish) difference between the 2 cabinets between 50-60 Hz? probably not. Will I really miss whatever's going on below F10 because the 2 cu ft box drops off steeper? Hopefully not. Will I appreciate the easier construction of the 2 cu ft cab? Probably.

    Does anyone know if any consumer level (cheap) cabinet modeling sofware can model room/cabin gain? I assume WinISD, Bassbox and the like calculate estimated anechoic response? It would be pretty cool to model loudspeakers in a user-definable environment.

    In other news, my Avatar B212 will be ariving tomorrow. I'm pretty excited to try it out.
     
  8. rpatter

    rpatter Supporting Member

    Sep 18, 2004
    Round Rock, TX

    I've used that driver in a 2 cu ft box. It's not optimal, but it does work. I think you would be happy with a box around 1.5 - 1.75 cu ft. I would also tune it more to around 45 hz. Don't forget to put some kind of high pass filter on the midrange.

    Ralf
     
  9. IMO, yes.

    The Omega Pro 12 varies between 1.1 and 1.23 cubic feet for the popular vented alignments. 2.0 cubic feet is an EBS version of the SBB4, and will be tubby. Why bother?

    The SBB4 cab is musical sounding and blends well with the Alpha 6. And the cabinet is small. This is a good combination all the way around.
     
  10. curiousfunk

    curiousfunk

    Apr 24, 2005
    Thanks for the input everyone.

    bgavin, in your second post, you linked to your generic 1x12 Omega Pro 12 design sheet. I come up with massively different port dimensions for respectable mach values.

    For your mach .045 case, you list dimensions: 0.95 x 13.0 x 14.883.

    For your mach .100 case, you list dimensions: 0.701 x 13.0 x 10.636.

    When I run these numbers in WinISD (1.23 box size & 39 Hz tuning), the port dimensions roughly agree. But my Mach values are much higher - 0.24 for the 1st case, and 0.32 for the 2nd case.

    For me to get mach values at or below 0.15 I need an approx 22" long port. By the way, I'm not using the Omega Pro 12 specs that come preloaded in WinISD - I checked the eminence datasheet and inputted the current values. I've double checked them a couple of times because of these port length disagreements.

    What am I doing wrong?
     
  11. The origin of this relationship is Dick Small's excellent paper on vented boxes:
    "Vented-Box Loudspeaker Systems Part II: Large Signal Analysis"
    Journal of the Audio Engineering Society, Volume 21, Number 6
    July/August 1973

    Small came up with an approximate formula which limits the velocity of sound in the port to 4.5% of the speed of sound for a wide range of vented box alignments.

    The formula he gave is as follows:
    Metric:
    Av >= 0.8 * Fb * Vd
    area of the vent, (Av), is in square meters
    peak volume displacement of the cone, (Vd), is in cubic meters.

    Imperial:
    Av >= 0.02032 * Fb * Vd
    area of the vent, (Av), is in square inches
    peak volume displacement of the cone, (Vd), is cubic inches

    In my design sheets, I use the Imperial scheme, as all my data is in Imperial units. I add a multiplier for the number of drivers to the above formulas.
     
  12. curiousfunk

    curiousfunk

    Apr 24, 2005
    bgavin,

    I'm somewhat familiar with this concept. Per Small, we shouldn't hear the air rushing in and out of the port as long as it stays under 5% of the speed of sound, so Small's formula calculates the smallest port area that will keep the air velocity under 4.5%. Thus your 12.353 in^2 port area.

    But port area is not the problem I'm having. It's the mach values generated in WinISD with these port dimensions. Given the port area and lengths, the mach values generated by WinISD are much higher than the ones listed in your generic design sheet. I don't know what to make of this.
     
  13. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    YOu're using the word Mach which indicates you're using the old versio of WinISD (Beta). In the FAQ for the new alpha version, it mentions that the old version was calculating vent mach incorrectly. They've done away with that feature on the new version but instead use a graph to map port air velicity and uses 17ms as a suggested upper limit.
     
  14. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    You're using the word Mach which indicates you're using the old version of WinISD (Beta). In the FAQ for the new alpha version and on the WinISD forum, it mentions that the old version was calculating vent mach incorrectly. They've done away with that feature on the new version but instead use a graph to map port air velicity. 17ms is the suggested upper limit.
     
  15. curiousfunk

    curiousfunk

    Apr 24, 2005
    Hello Petebass, and thanks for the info. That would explain a lot.

    Can you or anyone clarify this for me: assuming I have to build a right angle into my shelf/slot port because it is too long for the cabinet, how do I measure the length around the bend? Take the average of the inner and outer distances around the bend? Or, is it not about that. I'm beginning to think that it's really a question of volume, and that as long as the volume of the L-shaped port is the same as the recommended straight port, I'm safe. This is about resonating a specific qty of air (the air in the port), right?

    If I make the baffle and the tail end of the shelf/slot removable, I could trim it to tune the port. And then glue & re-screw with final tuning. This sounds like a serious pain.
     
  16. Slot ports are indeed a pain to construct and tune. Especially so when a bend is involved.

    I built a tiny 0.54 cubic foot cab for a 10" subwoofer that required a bend... lots of work. This is required when a small cabinet requires a large diameter vent (and long port) for proper tuning. This is when a dual passive radiator system is more appropriate.

    Several years back, I wrote a spread sheet template that accepts the requisite T/S parameters and calculates the results. The vent sizing is done for both 4.5% and 10% MACH values, and is exactly dependent on the entered T/S values of the driver.

    I calculate the box volume by including the bracing displacement, and that of the slot port and driver. When I'm all done, I find the final result to be very close to the design ideal, including weight. As with all slot port designs, this is a close estimation, and final tuning will be required (maybe) by adjusting the slot length.
     
  17. curiousfunk

    curiousfunk

    Apr 24, 2005
    Right - I understand the box volume calculations. But with port calculations, is correct to work with the problem in terms of physical volume? For example, let's say you need a (internally) 12" x 2" x 20" long slot in your 15" deep cabinet, which requires a bend. Your internal port volume is 480 cubic inches. The equivalent volume slot port would require a 13" long slot, with a 5" long leg after the bend ((12x2x15)+(12*2*5)= 480 cubic inches). Thus the volume of air inside the port is the same.

    Is bent slot port length dimension estimation as simple as this?
     
  18. curiousfunk

    curiousfunk

    Apr 24, 2005
    Also, air velocity by itself seems irrelevant. It's the sound of the entry/exit turbulance ("chuffing") that's the problem right? This is what makes flared ports so attractive. Can I get away with using a slightly smaller slot if I quarter round the port entry and exit, making a sort of flare?

    Related to this, high flow automotive intake runners look like flared ports that extend into a large, open intake plenum - I wish I had a picture of this to help explain it - basically what a series of flared ports would look inside a speaker cabinet. Engineers found that this design reduced intake turbulance around the "mouth" of the intake runner & promoted more laminar airflow. This allows them to use smaller diameter runners, increasing the air velocity through them. This design flows more than a flared runner that opens flush with the intake plenum wall. It all has to do with the mouth opening into an open space. I wonder if this concept has ever been applied to loudspeaker port design. It would not look pretty - having the flared end of the port sticking out of the front of the enclosure.
     
  19. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    B&W do it, as do others.
     
  20. curiousfunk

    curiousfunk

    Apr 24, 2005
    Dharmabass, I'm referring to this:

    [​IMG]

    as opposed to the more common:

    [​IMG]

    Although, the dimpled ports are very interesting. As usually, B&W at the forefront of loudspeaker design. Beautifull stuff.