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Sixth Order Alignment?

Discussion in 'Amps and Cabs [BG]' started by alexclaber, Apr 16, 2005.


  1. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    According to the manual, Acme cabs use a 6th order alignment. The -6dB point is 31Hz and -3dB is 41Hz. They're rated at 350W RMS and 300W sine-wave @ 31Hz. Apparently cone excursion increases rapidly below 30.87Hz - would this be the tuning frequency?

    What is sixth order alignment and what bearing does it have on the LF roll-off rate and group delay?

    Alex
     
  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    If what you' re refering to is an acoustic 6th order alignment (there is also an electrical 6th order) it's simply a dual chamber box with both chambers vented, as opposed to only one in a 4th order alignment. 6th order alignments have two tuning frequencies, one for each chamber, and the box rolls off at 24dB/octave below the one and approximately 12/dB above the other. LF group delay isn't significantly different from other vented alignments. The advantage is the ability to achieve higher SPL, at the cost of a narrower passband. Hoffman will not be denied.
     
  3. That's what I thought but how does this apply to Acme cabs?
     
  4. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Hmmm, well it's definitely not that kind of enclosure, it's a 'simple' 2x10" ported cab, with low Fs woofers plus a cone midrange and dome tweeter (with a nice 3-way passive crossover dividing the frequencies). I was assuming that this was an alignment like SBB4 or QB3...

    Any other ideas?

    Alex
     
  5. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The manual isn't on line that I can find, so I couldn't venture exactly what they are talking about. But a typical single chamber vented enclosure is a 4th order device irrespective of the alignment. Not that it would matter, as a 6th order BP is also 4th order in the HP function.
     
  6. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    I wonder what he meant by that? My definition of 6th order alignment is a bandpass cab as described by Bill. Does the website have any more information on this?
     
  7. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    I think this is what he's talking about. To summarise what the pdf says, this uses a simple EQ circuit to provide a 6dB boost restoring flat response. Apparently the box alignment is critically damped (down 6dB @ Fb) and produces extremely tight sounding bass. The 6th order alignment gives a lower cutoff point than a max flat alignment and does this in a smaller box. The tradeoff is the 6dB boost at Faux (Fb x 1.07), which requires 4 times the amplifier power at Fb compared to a max flat alignment.

    This all rings true with my Acme experiences - extremely fast tight and deep sound but real power sucking ability. And the cabinets are remarkably small. The pdf mentions that the EQ circuit includes a high pass filter - I suspect Andy left this out because he warns of introducing sub 30Hz frequencies into the system because of the lack of control at such frequencies. I always run high pass filters on the amplification which solves that problem. Also Andy mentions that the passive EQ required in the Low-B1 was somewhat challenging because of getting the speaker to work in such a small enclosure without the impedance dropping too low for most amplifiers to handle.

    Enlightened! :)

    Alex
     
  8. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Right then, passive EQ. Would I be right in guessing that this involves changing the impedance that the amplifier is seeing at a given frequency? My understanding of impedance ratings is that the nominal impedance quoted on a cabinet is the lowest impedance that an amplifier will see, with the impedance rising to a high peak at the tuning frequency. So this passive EQ which is at around the tuning frequency ensures that the impedance only rises to a 1/4 of what it would in a conventionally tuned cab, thus demanding 4 times the power from the amp at these low frequencies. (Is this not unlike the effect of a transmission line in also getting more power from the amp by a smaller rise in imepdance?) Hence the infamous power sucking ability of the Acme cabs (but incredibly deep response).

    So, as I've discussed in another thread, if you put a high pass filter set above these frequencies in line with the amp, the Acme's apparent efficiency and maximum output will substantially increase. (This begs the question, why? Because they're not just great at the ultra lows, they do a very nice job of smooth clear response right up to 22kHz).

    Alex
     
  9. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Still impossible to say without something from the manufacturer what it is, 6th order acoustic (I suspect not) or 6th order electronic ( I suspect so), which obediantly obeys Hoffman's by trading extension/smaller cabinet size for efficiency. The EQ is most likely a part of the amp, and not a passive component. Although it can technically be done that way the cost of passive components that would be required to work at these frequencies would be prohibitive, literally a hundred dollars or more, while active components in the amp would cost a buck or two.
     
  10. If this is a burning question, call Andy Lewis and ask him.

    :D
     
  11. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    It's not. And you know what they say about curiosity...

    Alex
     
  12. Andy Lewis is a stand up guy. I had a long tech conversation with him, and he is very sharp.

    As for old quotes, my favorite is: "No matter how thin you slice the baloney, you can still throw a brick through a plate glass window."

    :D
     
  13. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Confirmed. It is a 6th order alignment as described in that pdf. To get a true flat response you have to provide the 6dB boost @ 30Hz before the cab, there is no passive EQ on the woofer.

    Alex
     
  14. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Thus the box is a 4th order acoustic alignment with a bass boost in the amp to get the extended low end. You can do the same with any 4th order box simply by pushing up an EQ slider, the difference being that the EQ circuit in this amp is precisely tuned to the particular requirement of the speaker to derive the desired response. It's a good method of getting low bass from a small box, the trade-offs being the size of the amp required to drive it and the excursion required of the woofer to handle the power.
     
  15. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    What's interesting, Bill, is that the cabs have far more extended and full lows than pretty much every other cab out there, without any need for the bass boost to flatten out the response.

    I presume most other 4th order bass cabs use a woofer with less low bass oriented characteristics and then go for a non-flat alignment with a hump in the mid-bass/low-mids, and a much higher LF roll-off.

    Alex
     
  16. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    It does have bass boost, built into the amp, you're just not aware of it because there's no dial to adjust it. Use the speaker with a different amp and it won't sound the same.
     
  17. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    It doesn't, it's just a 3-way passive cab, which I drive with an SWR GP into QSC PLX 3002. All controls set flat.

    Alex
     
  18. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    If it's a cab only and 6th order electrical, not acoustical, then it's using passive components to provide the bass boost, but it wouldn't be a bass boost, it would be a cut of everything else. The net effect is still increased power draw. If there are no passive components in the cab altering response then it is a 4th order box.