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techie box design question

Discussion in 'Amps and Cabs [BG]' started by Ian Hall, Oct 12, 2002.


  1. Ian Hall

    Ian Hall

    May 31, 2002
    Rialto,CA
    I've done a search on this already, and did get a good deal of information, but still have a couple of q's. I am going to be using an eminence Kilomax 18 as a sub, and have used multiple box design programs to come up with a basic design(kind of an average of all of the results from different programs, besides any that produce wildly strange numbers compared to the others). The eminence site recommends a maximum box size of 12.5ft3, but my best curves regarding power handling, max SPL and tuning suggest a 17ft3 box tuned to 30 hz. Is there any disadvantage to this larger size than portability? Most of the design programs when auto-calculating the ideal size for that driver seemed to like 22ft3, so I can't see that it would be that much too large. Also, the average number I came up with for port size at that box volume without port noise would be 4x5" ports at around 7"L each. Does this sound right compared to these specs?.....

    Resonant Frequency (fs) * 33hz
    Impedance (Re) * 5.84 ohms
    Coil Inductance (Le) * 2.38mh
    Electromagnetic Q (Qes) * .58
    Mechanical Q (Qms) * 14.24
    Total Q (Qts) * .56
    Compliance Equivalent Volume (Vas) * 8.47ft3
    Peak Diaphragm Displacement Volume (Vd) * 1141cc
    Mechanical Compliance of Suspension * .13mm/N
    BL Product (BL) * 19.7 T-M
    Diaphragm Mass inc. Airload (Mms) * 186 grams
    Equiv. Resistance of Mechanical
    Suspension Loss (Rms) * 2.69Nsec/M
    Efficiency Bandwidth Product (EBP) * 57
    Voice Coil Overhang (Xmax) * 9.8
    Surface Area of Cone (Sd) * 1159cm2
    Impedance at Resonance (Zmax) * 150 ohms
    Maximum Mechanical Limit (Xmech) * 38.1mm

    Also, I am figuring when this is finished, I will get one of my cheeseball car audio test CD's and use the 30hz test tone to tune the box by eye according to cone movement as I trim the ports(I am going to cut them a bit long). Any thing wrong with this?

    Many thanks for any help.
     
  2. The Eminence reference of 12.5 cubic feet is for the BB4 alignment. This alignment gives you the maximum transient performance (read: fast sounding), and the least amount of mud.

    Tuned at 33 Hz, it will take the full 400 watts of rated input power from 32 Hz and up. The BB4 alignment gives a +3dB hump at 40 Hz so it will have a massive bottom to it.

    This driver calculates an SPL at 93.7 which is noticeably less than the Eminence Magnum 18LF or Magnum 18HO at 96 and 97.7 SPL, respectively. Also, the Magnums require much smaller cabs. See my spread sheet for details.

    If your tuning frequency is below your lowest usable note, the port size can be significantly reduced. Maximum port velocity occurs at the tuning frequency and is much less higher up. You can use a smaller port and still avoid port noise.

    If you have never hauled a huge cabinet, I encourage you to think long and hard about it. My subs are 21 x 21 x 31 exterior (7.9 cubic feet), but my net internal volume is 5.65 cubic feet accounting for bracing, baffle set back, driver displacement, etc. I figure yours would be 18.8" x 30.5" x 49.25" exterior dimensions (16.4 cubic feet) to get a net internal volume of 12.5 cubic feet. This is awful big for a portable cabinet.
     
  3. Ian Hall

    Ian Hall

    May 31, 2002
    Rialto,CA
    The Size is rather large, however we are used to hauling around a 2x10+1x15 bass rig for my drummers trigger setup. He will now be going through the pa system(with this sub), and not bringing the rig, so it will be almost like nothing has changed(as far as amount of gear that we haul around).

    I haven't actually placed the order for the sub yet, but just found out I will be able to afford a second sub, so do you think I should go with the Magnums instead? I suppose I will have twice the sub in a box just a little bit bigger(or two smaller boxes). Have you heard the Magnum and Kilomax subs side by side? If so, which do you think is a superior speaker for this? If it claims better transient response, I'm sure it will be better for us, since we may all be mixed into the pa system at one time or another. Can't be having any muddy bass. I have two RMX1850HD's available for the job of powering them, so juice is definitely not an issue. Would you consider the kilomax as maybe more of a dance hall sub?

    Also, do you consider the results you get from pbox(the one you have downloadable on your site) to be generally accurrate enough to get started tuning a box, or should I use my cheesy program averaging method? Thanks for the help:)
     
  4. I can't help you with A/B comparisons. There are very few places where you can have access to a correctly enclosed and tuned Kilomax Pro 18 or a Magnum.

    For me, the Magnum is a newer design. More important, the Kilomox Pro 18 is a high Qts (0.560) which reduces the transient response, according to Dickinson's book. The high Qts goes hand in hand with the +3dB hump at the rolloff point. The Magnum 18LF has a much lower Qts of 0.320, making it cleaner for transients and not as boomy in the bottom end.

    The EBP of the Kilomax is only 57, suggesting it is better suited in a sealed box. Running the numbers shows an immense box is required. The 18LF has an EBP of 97, indicating very suitable for a vented box.

    The magnetic motor strength (Bl) of the Magnum is much higher than the Kilomax, and the Bl:Mms ratio is about 33% higher also. This means the Magnum exerts tighter control over the cone because it has more magnetic strength in ratio to cone mass.

    I like the 18LF because it works in 4.0 cubic feet tuned at 32 Hz, and it produces higher SPL for less power.
     
  5. Ian Hall

    Ian Hall

    May 31, 2002
    Rialto,CA
    Are there any other subs in the same price range as the magnum and kilomax that you would consider better suited for a subwoofer in a small pa system reproducing multiple instrument signals? Manufacturer is of no importance to me, only quality.
     
  6. hi,
    i was jsut wondering all this talk about cubic feet, what is that in cubic meters/litres???? because it is a lot more complicated than just multiplying it by 3 like you would if it was just lineal.

    i have a cab of 130 liters, and i am wanting to put a sub in it. i was thinking about car audio subs as they don't need a huge enclosure. i emailed eminence and they couldn't recomend a sub with port design for my 130 ltr cab to get down near 30 Hz
     
  7. Ian Hall

    Ian Hall

    May 31, 2002
    Rialto,CA
    bassman, do an internet search for "Metric Conversion" and you should come up with some sites that have converters(javascript or program) that will convert ft3 to liters3 and inch2 to cm2 and that sort of thing. Car audio subs are an idea but they tend to be more ineffecient(car audio is definitely more of my forte than pro audio). That doesn't mean they won't work, you'll just need more power. They also tend not to sound so great in the higher registers.
     
  8. i really only want it for a sub woofer, as i have an 800 watt 410 + horn. also i was thinking because car audio subs don't need a big enclosure i could divide my 13 liter cab up so that the 15 inch car audio sub is in its optimum size enclosure and then still have enough room for say another 10 or 8 + 5 inch for the hi-mids and highs, and then put in a passive 3 way crossover. as i said the cab is 130 liters, it is the same size as my 410 so they stack exactly, and if i can't get a pro audio sub that can perform in 130 liters then i might just use a car audio sub and make a full range cab.
     
  9. geshel

    geshel

    Oct 2, 2001
    Seattle
    I think there are 28.3 liters per cubic foot?

    The Magnums do look like pretty nice drivers. Fiddling with WinISD, it looks like if you put the 18LF in a bandpass box, 4 ft3 rear chamber and 2 ft3 front chamber tuned to 65 Hz, you basically get the same response as a simple vented box of 4 ft3 tuned to 32 Hz, except the response rolls off above 100 Hz, and it (the bandpass, suprisingly) has better transient response. I don't know about the power handling though, my copy won't do that.

    Put the 18HO in a similar BP box, but 5 ft3 and 3 ft3 respectively, and you get the same exact output but with about a 3dB increase in efficiency. The ports for this one would be 4 x 4" dia x 7.65" long.

    8 ft3 is "pretty big" in my book, but it certainly beats 12 or 17. . . :)

    All of the above are down about 9dB at 30Hz though - I have a pretty hard time getting a design with much higher output than that at 30Hz. They'll all do OK at that freque(again, not sure of the power handling, Bruce can maybe help out there).
     
  10. My spreadsheet has all the metric to imperial conversion factors. No math required, plug one in, get the other out. I use cubic feet because it presents me with a visual image that I do not get with metric measurements.

    A 6th order bandpass is a seriously difficult box to design. I fully believe the design programs will only get you within gross error. You will build, refine, build, refine multiple times. A complete waste of time for a one-time project. Worse, I cannot believe the transient response of a bandpass box is any good at all. Especially a 6th order which is a double-vented arrangement. Group delay gets worse as the acoustic load on the cone increases. A 6th order BP puts enormous load on both sides of the cone, to the point it can tear itself apart under power. YMMV, but they are not for me.

    I suggest not using the word "car" when used with subwoofers. A sub is a sub, no matter where it is installed. There are large cabinet, vented subs that are more efficient than small cabinet, sealed subs. Also, there is a negative image associated with "car" subs, that is based in ignorance. I have four of them (Rockford) because they have the electrical parameters I wanted for my bi-amp rig. The singular issue with subs is they are enormously power hungry and grossly inefficient. You need a lot of them to make the same loudness as only a few high frequency drivers.

    A vented box has no place in an automotive application because it generates too much bass. The vehicle has "cabin gain" that extends the response of a sealed box subwoofer substantially lower. A small, sealed box sub that sounds anemic in the open air has a massively solid bottom when installed in the vehicle.

    IMO, a portable PA system is no place for subs. A small PA should be for voice, horns or harp. When you want to DI the bass, mic the drums, etc, you are entering the world of the large PA. Multiple subs, midranges, horns, tweeters, EQ, etc etc. This defeats the purpose of "portable."
     
  11. geshel

    geshel

    Oct 2, 2001
    Seattle
    The boxes I was describing are 4th-order BP - sealed rear, vented front.

    The transient response is "better" imo because it is more even than the vented box's. In the passband the group delay ranges from 5 to 11 ms, whereas for a 5 ft3 box tuned to 32 Hz, ranges from 2 to 10 ms. (This is for the 18HO). The tuned box also peaks at 18ms at 25Hz - that's below the cutoff, but it also guarantees the sound would be mush if the system was EQ'd to compensate (given proper power handling). The bandpass box peaks at 11 ms at 40Hz.

    Also, the bandpass box provides loading below system resonance so is less susceptible to overpowering at low frequencies.
     
  12. Ian Hall

    Ian Hall

    May 31, 2002
    Rialto,CA
    Portability is directly proportional to how many ironworkers you know, and how much beer you can afford to buy them for moving your stuff:D There are two Main reasons for the existence of the subs in this PA-

    1. To generate bass as a backup to my drummers trigger setup.
    2. To generate bass for some of the intro samples that we play at shows(eerie sounds, hip-hop beats,etc.)

    This PA is meant only for very small shows and rehearsals. It isn't big enough for a large club, but isn't neccessarily meant to be. The bass or guitar would only be DI'ed in emergency situations, such as an amp failure.

    Bgavin, you are very right about the ported box having no home in a car. I have never installed a ported box in any of the installations I've done, and there have been quite a few.

    I have been drifting now towards the Sigma 18- It seems to like a 5.5ft3 box, and If I make two seperate boxes it would be much more portable than a Kilomax box. They are also less than half the price of the magnum. I dunno, guess I still have more thinking to do.
     
  13. geshel

    geshel

    Oct 2, 2001
    Seattle
    I'd heard this but didn't have a good idea how true it was until I saw this.
     
  14. inwbcstm do you work in car audio????
    i have been thinking lately about having say 2 12 inch subs in a sealed box in the boot, then have a 10 inch free air sub in the parcel shelf..but because the back of the free air speaker is open to the front of the 12's wouldn't they be out of phase and there fore have some cancelation??? or would you jsut reverse the polarity of the free air sub??
     
  15. geshel

    geshel

    Oct 2, 2001
    Seattle
    Neither one would be recommended. If the 12s and the 10 are in phase, then the 12s would be "assisting" the 10 - not that that's good, since two 12s would probably push the 10 past it's limits pretty easily. Out of phase, they'd work against one another, and it would have an effect like making the 10 feel like it was in a much smaller box. But both these explanations are surface guesses - in reality it would be much more complex given that the trunk isn't really sealed or completely rigid. But if you think of it like putting one speaker box inside another, you'll realize it's something strange and give up the idea. :D

    If you're going to put two 12s in the trunk, that should be sufficient. A 10 in the shelf wouldn't add much even if it weren't for all the weird interactions you'd get. If you want some midbass, you could hang an enclosure for the 10 from the shelf, but you'd probably be better off just sticking with what's in the doors for that.