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cabinet design question

Discussion in 'Amps and Cabs [BG]' started by basss, Sep 2, 2005.


  1. basss

    basss

    Aug 27, 2001
    NYC
    I'm having a cabinet built for me for a driver that I already have (Beyma 12LW30/N). The main goal for this project is portablility and I'm trying to fit the speaker into a small box while still retaining some performance.

    The internal dimensions I have worked out are 14"H x 14"W x 9"D giving me 1.02 cu. ft. internal volume. I've subtracted .14 cu ft. to account for driver, port, and cup handle displacement giving me .88 cu ft.

    With a circular port of 2" in diameter and 5" depth I get a tuning frequency of 38 Hz for the box giving me SBB4 alignment.

    I wanted to check in with the speaker design guys here to see what they thought of this and make sure there was nothing I'm overlooking. Also if someone could give a general discription of how box volume and tuning effect efficiency that would be great. Thanks
     
  2. basss

    basss

    Aug 27, 2001
    NYC
    T/S parameters for this driver

    Resonant frequency, fs 38 Hz
    D.C. Voice coil resistance, Re 6.1 ohms.
    Mechanical Quality Factor, Qms 6.6
    Electrical Quality Factor, Qes 0.3
    Total Quality Factor, Qts 0.29
    Equivalent Air Volume to Cms, Vas 109 l
    Mechanical Compliance, Cms 267 ?????m / N
    Mechanical Resistance, 2.4 kg / s
    Rms Efficiency, ?????o (%) 2
    Effective Surface Area, Sd (m2) 0.0540 m2
    Maximum Displacement, Xmax 5 mm.
    Displacement Volume, Vd 268 cm3
    Voice Coil Inductance, Le @ 1 kHz 1.4 mH
     
  3. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Don't tune the box to 38 Hz, there's no point because you're not going to get anywhere near that low anyway with this driver in that size a box. You're attempting a hi-fi tuning and it won't work very well. 55-60 Hz will give you much better response above 60 Hz, which is pretty much as low as the box is going to work effectively anyway. You also need to use at least a 4" d. duct or you'll have all sorts of port noise. If you want to see how box size and tuning affect things just do a WinISD simulation, you can see instantly that you should consider doubling the box volume you propose.
     
  4. basss

    basss

    Aug 27, 2001
    NYC
    I play a 5 string - won't tuning the cab to 60 hz risk damaging the driver when I play low notes? I thought that I needed to tune it that low in order to protect the driver. Would tuning the cabinet to 60 hz necessitate the use of a high pass filter?
     
  5. First, the application calls for a 5-string. Your bass may, or may not, generate more 2nd harmonic, but it will still generate the 31 Hz fundamental to some degree.

    Operating a vented box below its tuning frequency is not recommended by anybody, including JBL, et al. The reason is the 24dB/octave rolloff and acoustical unloading that occurs below the tuning frequency. Cone movement increases 4x for every octave down, at the same loudness level. Cone movement is almost nil at the 38 Hz tuning frequency, and rises quickly on above and below this point. Under power, a strong 31 Hz fundamental combined with a 60 Hz tuning frequency is a recipe for a blown driver, IMO. Your mileage may vary, because the Avatar cabs are tuned in this range and they seem to survive a 5-string.

    The SBB4 tuning of 38 Hz and 1.41 cubic feet (net) volume will protect the driver from exceeding Xmax in the 30 to 50 Hz range. Excursion peaks at 60 Hz in this tuning. Changing the tuning to 60 Hz shifts the over-Xmax point to 50 Hz and below, at full rated power.

    If your fundamental is 10dB less than the 2nd harmonic, this means low B is mostly 62 Hz and higher. But if your fundamental is nearly the same as the 2nd harmonic, that is a whole lot of signal applied at 31 Hz to a 60 Hz tuned system. It all boils down to how much fundamental vs 2nd harmonic is present in your bass. I can't answer that question.

    The QB3 alignment (1.23 cubic feet at 51 Hz) is somewhat of a compromise. Xmax is exceeded below 45 Hz at full power. If the fundamental is weaker than the 2nd harmonic, this tuning will work for you. The F3 is around 62 Hz for both QB3 and Optimal/Flat tunings.

    Yet another solution is use a 31-band EQ, and just cut the nuts off everything below 62 Hz. A 60 Hz tuning would be perfect for this... but I dunno how the bass would sound without the fundamental.
     
  6. ESP-LTD

    ESP-LTD

    Sep 9, 2001
    Idaho
    I don't believe either factor effects efficiency. Efficiency is a property of the driver itself and does not consider cabinet design.
     
  7. ESP-LTD

    ESP-LTD

    Sep 9, 2001
    Idaho
    Then you most certainly have achieved your goal.

    As for tuning, you need to decide which evil you hate less- distortion/damage or weak low end, and tune for that.

    A popular rule of thumb is that port diamater should be at least 1/3 of speaker diameter, to keep the speed of the air flowing thru the port slow enough that it doesn't 'chuff'. 3-4" is a better idea than 2".
     
  8. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    True, but this is because sensitivity is typically defined at a frequency (1000 Hz) where box design doesn't matter. If sensitivity is plotted as a function of frequency in the range where basses make most of their noise, then box design matters a lot.
     
  9. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Not so. Below 200 Hz or so a speaker's performance is at least 75% cabinet derived and at at best 25% driver derived, and that's in a direct radiator. In a horn cabinet driver broadband efficiency versus a direct radiator alignment can be increased by a factor of at least 5x.
    I can, and a 32 Hz fundamental it will have at least 12dB less SPL requirement than the second harmonic at 64Hz, so 55-60 Hz tuning isn't really problematic in and of itself so long as you have plenty of headroom to begin with. However, if you're pushing the cab to the max at 64 Hz and you're down by 20dB at 32 Hz that isn't going to make it. 5 string really needs tuning at 45-50 Hz and least 2 cu ft of box.
    Fair enough, so long as you know going in that you can have it small and you can have it go low but you can't also have it loud. Two out of three is all you get.
     
  10. ESP-LTD

    ESP-LTD

    Sep 9, 2001
    Idaho

    I stand corrrected. I was assuming that efficiency and sensitivity were different aspects.
     
  11. basss

    basss

    Aug 27, 2001
    NYC
    Can you explain why this is? It seems like you are saying the percieved loudness of a 32 Hz fundamental is higher than the second harmonic of the same SPL. I thought lower freq's were of the same SPL were percieved as quieter (the Fletcher Munson thing).
    I guess I want small first, then low, then loud. I will be using this cab for small acoustic gigs and some light pop stuff and portability and sound quality are priorities before loudness.
     
  12. ESP-LTD

    ESP-LTD

    Sep 9, 2001
    Idaho
    I think I can get this one right :)

    If you were amplifying a trumpet, you could determine what the fundamental was and all 'important' harmonics, and create a system to make that range louder and be 'flat' in response; faithfully duplicating the original harmonic content.

    But an electric bass guitar doesn't have an analog in nature; it's timbre only exists as electric impulses from pickups. We can faithfully reproduce that with a system that is flat from 30hz to 10k or so, and in a perfect world that would be superior. Indeed, my last cabs were essentially flat from 30hz to 10k or so.

    What I found in a live situation, was that the audience couldn't here my 'flat' bass- they could FEEL the rumble, but it wasn't clear, and the room was booming with low frequencies. I discovered that I actually had to cut low end to control the boominess in rooms.

    What billfitzmaurice is suggesting (IMO), is what we have come to think of as 'good sounding bass guitar' does not have a flat response down to the fundamental; the fundamental is actually 12db down from the second harmonic. When reproduced in this way, there are less problems with 'boom' and more clarity for the audience.

    It isn't a perfect reproduction of the signal from the pickups, but it is a more useful one from a live performance perspective. I had a hard time with this one, but empirically, my bandmates and audiences are much happier with a more traditional and less flat system.
     
  13. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    No, what I'm saying is that the actual content of a low B note has the harmonics at a far higher level than the fundamental. It would only be otherwise if your low B string was at least a quarter-wavelength long. At 32 Hz that would be 8.8 feet, and you'd have to locate the pickup at the 12th fret. The only bass instruments capable of producing fundamentals at the same levels as the harmonics are large pipe organs. Even Tubas and Sousaphones with half-wavelength pathways lack sufficient mouth area to support linear response to the lower fundamentals.
    At least, depending on the frequency, at 32Hz it's likely considerably more, it shifts though as you go higher and the string length gets closer to a wavelength. Speaker response also enters the equation, as with electric instruments the speaker is part of the instrument. Today's subwoofers make possible frequency response impossible to achieve at significant power levels only twenty years ago.
    They are related. Efficiency is the percentage of acoustic watts output relative to electric watts input. The average direct radiator speaker runs 3-5% efficient, the average horn 25-40%. Sensitivity is the measured output in decibels relative to a given wattage or voltage input. The average pro sound direct radiator has about 95dB/1m/2.83v broadband sensitivity, the average horn about 106dB.
     
  14. basss

    basss

    Aug 27, 2001
    NYC
    Interesting. I always thought that the fundamental had the highest level and the harmonics were present to at varying but lesser levels. That clears up that.

    The reason I've chosen the dimensions that I have is that I want to be able to carry this cabinet in a shoulder bag and this is the biggest cab that I would be comfortable doing that with. The speaker should work well as it has a Neodymium magnet and is designed to work in a small enclosure.

    With the information I've given, what would you recommend for a port size? I realize I'm going at this backwards but I'm trying to make the best of the situation.
     
  15. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    If I had to go with a cab that small I wouldn't do it with a twelve. A ten could have a far flatter frequency response from the same size box. The ten could also get away with a three inch port diameter. I'd start playing around with specs of the various neo magnet tens out there to find the one that works best to a 50 Hz f3 in that box size.
     
  16. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Basss, I'm curious to see how that driver performs for bass, so please let us know. I've got it earmarked as one I'd lke to try out some time, so real world opinions will be greatly appreciated.
     
  17. basss

    basss

    Aug 27, 2001
    NYC
    I actually have the driver in another cabinet right now. It's a very nice driver for bass. It's not the most efficient speaker but it handles low end well has nice mids for a single 12.

    I will be sticking with this driver and cabinet. What do you think of a 2.5 x 5" port which would tune it to approx 45 Hz? I know I may get some port noise, which I do in the cabinet that I have now, but I've never noticied it in a live situation.
     
  18. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    That's nice to hear, especialy from someone who's profile indicates you've used some highly regarded 1x12's in the past. Do you think 2 of them would compete with a Top 40 type rock covers band?

    I ran the speaker through WinISD Pro. Ideal cab size should be about 40 litres (1.413cu.ft) so I wonder if your .88cu.ft might be a bit small. I see nothing wrong with the 45Hz tuning, although I too like to tune lower for 5 string. I guess it depends how much time you spend camped on the low B.

    A cab of 1.413cu.ft at 45Hz gives you a -3db point of 63Hz, which IMO isn't too bad for a speaker that light. At .88 cu.ft, the -3dB point is 77Hz. Is it too late to consider making the cab slightly larger?

    I also see no problem with a shelf port of 2.5" by 5". In the 088cu.ft cab, make it 15.92 inches long to get to 45Hz.

    From an excursion point of view, this speaker really shines compared other neo 12's. By my calcs you're able to give this speaker the full 450W all the way down to 38Hz, even though the speaker is -10dB at that frequency. So it's not a bottom end monster on paper which s why I'm leaning towards the slightly larger cabinet if it's at all possible. But you say there's enough bottom end for you with the speaker in it's existing cab, so this may not even be an issue?
     
  19. basss

    basss

    Aug 27, 2001
    NYC
    I only use mine for small acoustic/low volume gigs. The EA wizzy I have blows it away volume wise. But the Beyma cab does get a nice sound that works well for low volume gigs in a cab that I can lift with one finger. It doesn't have massive low end or big volume.
     
  20. An example for all us visual learners :D :

    [​IMG]

    This is the frequency response of the open B string on my old Ibanez EDA905. You can clearly see the fundamental at 31Hz is not as high as the first harmonic.