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Building a subwoofer

Discussion in 'Amps and Cabs [BG]' started by Gabu, Jan 11, 2002.

  1. Gabu


    Jan 2, 2001
    Lake Elsinore, CA
    I have a cabinet that I am going to convert into a subwoofer. I have used Bgavin's charts, and port calculation website in order to design it. But Now I am wondering about the location on the speaker and ports.

    is there any advantage to have the speaker closer to the ground?
    | o o |
    | .O. |

    as opposed to higher:

    | .O. |
    | o o |

  2. downstairs


    May 13, 2001
    Pasadena, MD
    supposing the holes are ports, i think that if you have the speaker above it'll give you some mids, and if you have it below it will give more bass.

    EDIT: if you design it right though you can have it so you can flip it for different sounds.
  3. Gabu, what driver are you using?


    I've not seen measurements taken either way, so comments from me would be just opinion. And I have no opinion on this, because you can turn the box over if necessary.

    My design is behind your Door Number One, where the woofer is on the bottom, and my single port is in the upper corner. My second sub is the same, but with the port in the opposite corner so both ports are close when the two cabs are side by side. Frankly, I don't think it makes a whit of difference, other than visual. My woofer is on the bottom cuz I don't like heavy stuff up high and causing out-of-balance issues.

    Which Programs to Use

    Use either Perfect Box or Robert Bullock's DOS Box Modeler instead of WinISD for your volume and tuning design. Accordingly to a couple of commercial subwoofer design engineers I correspond with, there are some aspects of WinISD that are just plain wrong.

    Use WinISD to graph the Group Delay plot for your driver in a vented box. The higher the number, the more muddy the sound, so note that Group Delay gets worse around the vent tuning frequency. The accepted rule of thumb is 24ms or less at the tuning frequency. Go for less, and as low as you can get. Group Delay is what makes audiophiles love sealed boxes and hate vented boxes. GD is the cause behind why bassists dislike "slow" or "muddy" cabs. Note that lowering the tuning frequency shifts the GD downward and away from 31 Hz.

    The Bullock DOSBM program is clunky, but dead accurate when graphing a single driver. Note that the SPL graphs of DOSBM are wrong when comparing two drivers with different sensitivities. He fixed this in his later Windows ($) version. The DOS interface isn't all that intuitive, but it does work, and Bullock's Vent Velocity and Cone Excursion graphing features are worth using the program. Both are real eye openers. You can grab all this from my site. Get my various *.SES files which are prefilled in with various driver types. The files have to be copied to "BOXMODEL.SES" for the program to use them. Yeah, clunky, but it does work. The ever popular Carvin PS10 driver is one. Plug it in and understand why the RL210 performs the way it does.

    Thoughts on Tuning

    If your driver will allow it, there are a number of advantages to tuning the box below the lowest usable frequency. The sluggish Group Delay will be below your operating range, and this will give you a "tighter" or "faster" sounding speaker. You can also get away with a smaller vent cross section area because the maximum port velocity is also outside your operating range. Using DOSBM, if you can keep your port velocity below 0.10 MACH at 31 Hz, even though it goes much higher at the tuning frequency, you will still avoid port noise. A lower velocity is always better, but requires larger ports and longer ducts.

    Understand the impedance charts. The two impedance peaks are the resonance points below and above the tuning frequency. The trough (low point) is the tuning frequency. Port radiation is 180-degrees anti-phase at the lower resonance (Fol), about 90-degrees anti-phase at the tuning frequency of the box (Fob), and in-phase at the upper resonance frequency (Foh). Cone motion is minimum at Fob and typically near maximum about 1/2 octave or so up the scale from Fob. This is awfully close to the Foh, and can be graphed with DOSBM. Foh is where most vented boxes "boom" due to the combined in-phase output from the cone and port.

    Equalizing and Cone Movement

    Cone movement increases 4x for every octave lower at the same loudness level. A cone moving only 0.125" at 62 Hz will need an Xmax of 0.500" at 31 Hz for the same loudness. This is why vented boxes tuned at 31 Hz really crank, and sealed boxes wimp out at the same frequency. The sealed box cone movement is maximum and the vented box movement is minimum due to the coinciding port frequency. But... the sealed box is MUCH tighter. There is your trade off.

    You can play What-If with the DOSBM program to simulate various EQ. For example, if you are adding +3dB EQ at 31 Hz, this is the same as doubling the input power or reducing the power above 31 Hz by -3dB. You cannot just turn up the EQ forever. There is only so much EQ that a driver can stand before over excursion or exceeding the power rating occurs. This is easier to see and play with, using a sealed box plot, but it applies to vented also.
  4. Gabu


    Jan 2, 2001
    Lake Elsinore, CA
    I have not yet picked out the driver. I am going to use a 15" driver. I just need it to go up to a maximum of 80hz or so. I have a box that is 3.9' on the inside. The inside is also padded. I have heard that when the inside is padded it simulates a larger box. Is this right? If so is there any rule of thumb to approximate that?

    Thanks for the earlier info too. I will put my speaker low for better balancing.
  5. Only with sealed boxes. Not with ported boxes.

    Download my spread sheet and check the DRIVERS tab. Sort in ascending order on the DBK CU FT column. This will give you a big list of everything that is optimum in 3.9 cubic feet and smaller.

    Look at the RCF L18P300 driver.

    [ edit ]

    Joris has experience with RCF, and with this driver in particular if I remember correctly.
  6. leper


    Jun 21, 2001
    hey bruce, just an fyi, i think the link to perfbox on your webpage is broken...i get 404 when i follow it
  7. Thanks!

    I added another 60gb to the server and broke a few things. Fixed now.
  8. The speakers I used in the PARA subs are RCF 18P200N in a 240 liter enclosure with 4 11 cm bass tubes. The box is tuned to 35 Hz.

    Those are large, very heavy boxes.

    Check my site for more info.
  9. Gabu


    Jan 2, 2001
    Lake Elsinore, CA
    I can't use the 18. It looks like the bracing is in the way.

    I realise that I have not really given any details on what I am doing so here it is! We are trying to setup our gear so that it's more manageable. We are playing small rooms. This means playing at stage volume only and running everything through our PA. But the PA right now is not really good to play bass through, not to mention a bass drum!

    Right now I have 2 112 mains on stands. and a humungous 115 on the floor. The 115 is loud, but not a sub so I am building a sub to replace it (and selling the 15 to help me buy the sub speaker). My PA is a 300w amp, so I guess I am also going to need a power amp for the sub.
  10. This is just opinion on my part, so take it as such. I don't understand the "run it through the PA" line of thinking, because most PA systems are not up to the task.

    You mentioned two 1x12 with a 300w amp as the PA. IMO this is way under powered for a bass amp, let alone as a full spectrum sound reinforcement system. The 12's are going to reproduce the full brunt of bass frequencies plus attempt to modulate voice over the same cone... it ain't gonna happen with any clarity. Our band uses a pair of Peavey 2x15 mains with mid-range horns and compression tweeters, driven by a QSC RMX 850, for voice and harp only. The powered mixer console drives the monitors only. We still have problems with vocals being drowned out by loud guitarists and drummers, and we don't run any instruments into the PA.

    If your band was willing to go to a completely controlled environment, it would be feasible to build a solid PA for complete sound reproduction. All the guitar and keyboard cabs would go into isolation boxes and mic'ed into the PA. The bass would go DI, and the drums and vocals are mic'ed. Drive it all with a high power amp for lots of head room. Two true subwoofers, a pair of 2x15 with horn mids and horn tweeter, plus monitors would do it for most venues.

    The isolation boxes will let your sound man control the most obnoxious guitar players. He can crank until his tubes turn into Kryptonite and he won't drown out anybody.

    But... you are going to be hauling LOADS of cabs. The mains, the monitors, the isolation boxes, plus the guitar/key cabinets and amps. It would be helpful to have a means of balancing and EQ'ing the PA for quality reproduction in a given venue. This means a rack EQ and perhaps a full spectrum analyzer to balance all those difference components and different amps.

    And when the band breaks up, who gets stuck owning/carrying/storing all that stuff? IMO, build yourself a nice bass amp and a sub if needed, and stick to a small PA for voice. Strictly opinion on my part, but I know what a drag it is carting my rig + a big PA.
  11. Gabu


    Jan 2, 2001
    Lake Elsinore, CA

    Thanks for your thoughts on this. My band has been together for maybe 6 months and we have not played out as a full band yet.

    In some sound experiments it seemed that our 12s were fine for the vocals, against my Carvin RC210-18, Two 150w Boogie guitar rigs (each has a 212 cab) and acoustic drums.

    We have not yet tried to play our instruments through the PA, but I knew for sure it would suck for the bass and drums. That is why I have been working on this sub... To help in the low range. I am thinking of using the 300w for the 12s, and maybe a 600w power amp for the sub.

    We don't play Metal, and the guitarists are cooperative when it comes to the bands overall sound. So I am not worried about us playing super loud. But, do you think we still need more than what I am suggesting? I know for sure that you know a lot better than me, so there is no need to be diplomatic or anything. :D
  12. IMO you need more. At least 2x600 watts for the lows/subs and 2x300 watts for the mids/highs. Last thursday I played a gig with the above mentioned power, and then some for monitors and an extra pair of mid/high cabs. Everything is always cranked on this system (I listen to other bands play through it a lot). It was loud, but not too loud, and anything less would have hurt the sound.

    Drums were mic'ed, I was DI'ed & mic'ed.

    Was a very nice gig altogether.
  13. It's not a know-better thing at all, it's a been-there-done-that experience from lugging around sh*tloads of gear for years. In the younger days, I carried my full sized Hammond C3, the full size Leslie, a bench and foot pedal rack. I also owned the PA, which was a pair of JBL 375 drivers attached to the Hartsfield-style wavy lenses and horns. Each bottom cab was a 2x15 set of JBL 130Bs, and a big fat N-500 crossover in each cab. Then mics, stands, cables, PA amp, power cables.

    Load it into the van.
    Unload at the gig and set up.
    Tear down and pack up after the gig
    Load it all into the van.
    Unload it when we got home

    It took all four of us young bucks just to move that Hammond. I did this for years, and finally bought a banjo in 1970 and said screw it to moving equipment.

    Years passed, and now I'm into electric bass, and back into moving gear again. My subs are 150 pounds each, and the 2x10 JBL box ain't light either. The rack is probably 135 pounds or more, and I cannot move it by myself. I carry a hand truck in my pickup truck to move this stuff around. Haven't hit stairs yet... I think I'd stay home first. And the drummer owns the PA system and moves it along with his kit.

    Moving too much junk takes all the fun out of the music. Give lots of consideration to having less, rather than more. Those two 150w Boogies would severely overshadow the bass, drums, and PA, if they decide to unleash them all the way.
  14. Gabu


    Jan 2, 2001
    Lake Elsinore, CA
    Correction! One guitar rig is 50w, and the other is 100. Doh, I always get confused on guitar watts. But in any case, your points are well taken. We are probably going to have to stick to just playing our vocals through the pa for now.

    I am going to watch for Carvin's DCM 1000 (or greater) power amp on ebay.

    Forgetting about what I have already, is there any semi portable pa gear I could get for 2 to 3 grand? Although I can't do that much yet... I like to have goals and I would definately pay more for portability.
  15. bizzaro


    Aug 21, 2000
    Gabu, the DCM 1000's are going for $350 at Carvin now. I can't imagine anyone unloading one on ebay for much less and you won't get the warrenty etc. from an ebay sale. Bruce did I understand you correctly? "only in sealed boxes" Does that mean a cab that is ported doesn't need anything in it for padding? And if it doesn't will a padded cab with a port, in effect, reduce the size of the box?
  16. Yeah, the DCM 1000s are a real deal. A QSC PLX 1202 from Lord Valve is $509.


    Stuffing a small sealed box completely full makes the box appear acoustically larger that its physical dimensions anywhere from 10 ~ 35%. The fibers absorb and release heat from the sound energy in the box to slow the effective speed of sound and decrease the wave lengths. The maximum theoretical volume increase is 40%.

    This effect diminishes with increasing cabinet volumes and also diminishes with too much stuffing. A 1.5 cubic foot box is more sensitive to this technique than a 6 cubic foot box. 1.5 pounds of stuffing per cubic foot is a good guideline for a guesstimate of a 30% volume increase. My wife works at a Hancock Fabrics store, and says you can get Fiber Fill for less than $2 per pound.

    Link to Sealed Box Volume Calculator

    A 35% too-large error in sealed box volume have little real effect on the reproduced sound. Run the cabinet volume numbers for your favorite driver here, and notice changes in volume don't make much change in response.

    Vented boxes are more tricky. The inside of this box also operates with compression and rarefaction from the driver, but this is compounded by the air mass in the port. Care must be taken to not stuff the box to the point of obstructing the port. A rule of thumb is leave 1 port diameter distance beyond the port unobstructed (inside and out). This means a restraining mesh of some sort to secure the fiber fill away from the port. The box volume without fill operates "normal" (adiabatic) and the box volume with fill operates "modified" (isothermal).

    Errors in vented box volume have more significant effect on the sound. The only means I know for sizing an isothermal vented box is trial and error.

    Stuffing is different than padding, which is used to control reflections and reduce higher frequency standing waves. Typical padding is the rear wall of the cabinet that faces the driver and port. My subs never see frequencies above 200 Hz so the 5.6 foot and longer wave lengths aren't an issue. I run no padding or stuffing in either of my subs.

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