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2 different cab size & tone??

Discussion in 'Amps and Cabs [BG]' started by perutxo, Apr 2, 2009.


  1. Hi everyone,

    I have an ashdown mag c115-300 combo and the size of the cabinet, in the combo, is waaay smaller than the 115 ashdown cab on its own.
    So, would the 1x15, being in the combo's smaller enclosure, give me more midrange and less bottom?
    Would adding the same model 1x15 (as an extension cab to the combo) give me more bottom, getting in this way a more balanced tone?
    Any risk of getting some phasing, being exactly the same model of speaker but being in 2 different sized enclosures?

    Thanks
     
  2. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    If the 15" woofers are identical, and just the box size is different, then what you describe sounds practical to me. I can't say how much of a difference adding the extension cab will make.

    There will inevitably be areas of constructive and desctructive interference beween the drivers, which I'd call "comb filtering" instead of "phasing", but that happens anytime you have two wideband drivers relatively close to one another (and happens a lot in a 4x10). If you place the two 1x15 cabinets side-by-side, then the comb filtering will be in the horizontal plane and in general will result in radiation pattern narrowing and in particular reduction off off-axis highs in the horizontal plane. If you place the cabinets one on top of the other, the comb filtering will be in the vertical plane which will not introduce as much variation into what the audience hears. And, you'll hear the speakers better as well be cause the top one will be closer to ear level.

    If the two enclosures are tuned to different frequencies, there may be some cancellation in the bass region. Perhaps this is what you mean my "phasing". The port's output is out-of-phase with the woofer's output, so you'd want both ports going out-of-phase in synch. Check with the manufacturer about the tuning frequencies; if within 15% of one another then I don't think you'd have a problem (and I invite correction on this ballpark guestimate from anyone who knows better) . Or if one of the boxes is sealed and the other is ported, you won't have a problem.
     
  3. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Not to get off on a tangent here, Duke, but any particular reason why you call it "comb filtering" over "phasing?" You're one of the speaker geniuses on here whose info is always right on, so I'm very interested in your reply.

    BTW, anyone ever tell you that you look like Jon Bon Jovi? ;)
     
  4. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Comb filtering is in general a high-frequency concern, phase interactions occur across the entire passband. Combing occurs with cabs or drivers placed side by side, phase interactions are pretty much unaffected by cab/driver position.
     
  5. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    Good question JimmyM. I'm not entirely sure my usage of the terms is precise, but here goes:

    Comb-filtering refers to a path-length-difference-induced interaction pattern between two sources, or a source and its reflection. When you have two side-by-side woofers operating fullrange, and you go off to one side, the closer woofer's output will arrive first. Depending on the frequency and the geometry, the output from the farther woofer will sometimes reinforce (arrive in-phase-with) the closer woofer's output, and will sometimes cancel (arrive out-of-phase with) the closer woofer's output. So there is definitely something to do with "phase" going on, but it's path-length-induced and geometry-dependent rather than driver-phase-induced.

    When the effect of this type of interaction is plotted, it looks sort of like a comb whose teeth are getting narrower as we go up in frequency:

    [​IMG]

    At low frequencies, the outputs of the two woofers arrive essentially in-phase. The first notch is where the path length difference is 1/2 wavelength, or where the output of the far woofer arrives 180 degrees behind that of the near woofer. Successive notches occur at odd-number multiples of that frequenency; the peaks where the outputs are reinforcing one another are at even-number multiples of that initial notch frequency. You don't have to remember that. Just remember what the pattern generally looks like. The whole pattern will shift higher or lower in frequency as the geometry changes - that is, as you walk around the room for instance. In-room sound is a complex subject, beyond our scope here.

    Fortunately, the ear is pretty good at averaging out closely-spaced peaks and dips. But the first few notches are too far apart for the ear to average them, and that can be a source of audible coloration. Also, the average energy at high frequencies is clearly less than the average energy at low frequencies (3 dB less I believe), so as a result the tonal balance is modified by comb filtering.

    As for my not using the word "phasing", well in my opinion it's not precise enough. It could mean comb filtering, or it could mean the issue with ports and woofers that I mentioned previously, or it could mean a phase difference at the drivers themselves (not likely here if the drivers are connected with the same polarity, but common in a two-way or three-way system).

    As for the "genius" part, most of the time I'm either bluffing or plagarizing.... seriously thank you, but it's just that speakers have been my hobby, vice, passion, and/or job for a long time. I consider what musicians do to be the real genius.

    Jimmy, I appreciate your passion for the "why" behind the sound. I see that you've learned a lot from Bill, and so have I - he's among my main sources for plagarizing!
     
  6. Thanks a lot guys!!
     
  7. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Ya, I'm trying to make a go at learning acoustics and amp technology. I ignored it for so long and it really did me no favors. So guys like you and Bill who patiently answer our cab questions on the net are a godsend. Wasn't long ago that you'd have to spend huge time in a library or send away for books and you still wouldn't get access to guys like you and Bill unless you took a course. And Duke, your answer would have gone completely over my head a year ago, but today I understand it perfectly, so maybe I'm getting the hang of this stuff. Of course, I'm back in love with my 810, so maybe I haven't quite learned everything ;)
     
  8. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    Nope; we're the ones who haven't. What matters most to the ears is not yet fully understood.
     

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