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displacement as a function of cabinet preferences

Discussion in 'Amps [BG]' started by makohund, Jan 13, 2013.

  1. makohund

    makohund

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    Given a choice between higher surface area but low excursion, or smaller surface area but higher excursion, which would you choose, and why?

    Or would one consider the question irrelevant, as there is not enough information? (Of course there isn't, so lets at least assume they both "sound great", while of course different.)

    Some scenarios:

    1. Resulting Vd of the smaller surface area cab is much larger.
    2. Resulting Vd of both cabs about the same.

    I think I know my choice, but am genuinely curious about others.
  2. craig.p

    craig.p

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    I'd choose higher surface area with low excursion, because then the air you're "connected to" is a higher impedance than it'd be with the alternative.
  3. Rockin Mike

    Rockin Mike Supporting Member

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    If they both sound great, my preference would be driven by things like size, weight, price.
  4. makohund

    makohund

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    The smaller surface area would likely take up less baffle space. Might by itself indicate a smaller, lighter cab? (Perhaps not always, but pretty likely?). Might mean fewer drivers, which impacts cost. Though that might be offset by the drivers being more costly per unit.

    Things to consider.
  5. makohund

    makohund

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    In both cases?

    Either way, interesting. Do you mean interfacing with a larger unit of air and moving that some, as opposed to a smaller unit and moving it more? The larger unit providing more resistance, and the smaller less? Something like that?

    Tell me more. :cool:

    Any idea how the two translate into sound pressure differently (or not), especially in a case of similar Vd?
  6. Downunderwonder

    Downunderwonder

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    Smaller surface area because it beams less, unless you then box them in a 4x or 8x, then I want my 15 back.
  7. craig.p

    craig.p

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    For maximum power transfer between a source and a sink, you want their impedances to be equal. This is the reason for an output transformer in a tube amp -- to match the tube output stage's high impedance to the relatively low impedance of the speaker cab.

    Close your fingers and push on the air in front of you. It takes almost no effort. This is because the air in front of you has a very low impedance.

    Now push on the cone of any bass driver. It takes far more effort. This is because the bass driver's suspension has a very high impedance -- again, in a relative sense.

    How do we minimize that mismatch between the high impedance of the bass driver and the low impedance of the surrounding air?

    Well, we can not change (decrease) the impedance of the bass driver to accomplish that. But we CAN change (increase) the impedance of the surrounding air.

    How? By cheating! By increasing the total area of the radiating surface, which will automatically increase the amount of air that radiating surface interfaces with. Which in turn increases that air's total impedance -- by that I mean the impedance "seen" by the driver/drivers.

    Here's a real-world example. Say a truck goes by your house and its exhaust note happens to match the resonant frequency of the house's exterior+framing+wallboard. Inside the house, you can hear a low, loud "wooooom" as the truck passes. What you're hearing is the wallboard vibrating over its entire surface. Its actual displacement, though? As close to zero as you can imagine. High surface area, low excursion. A wall would be a great bass radiator -- up to a certain frequency past which you couldn't cycle it fast enough. In which case you could break the wall up into discrete radiators, each separately-driven, so each radiator's decreased mass would allow it to get up into the mids. You'd still be using the entire wall, but you'd be driving it as an array of adjacent panels.

    Any SVT cab owner should be nodding his head at this point and saying, "Told ya so."
  8. KJung

    KJung Supporting Member

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    Interesting thread. I am, in general, a fan of more drivers, with each driver voiced a bit more toward the middle of the midrange, with less deep bass extension. Yes, there are trade-offs in dispersion once you get up to the upper midrange (given that many drivers with superior low end specs sound like dog up top and require a mid driver), but for my personal tone goal of a rig slotting itself into a mix similar to a mastered, high quality recording of a pop band (i.e., with the bass slotting above the kick drum in most cases, with more midrange response in that key 100hz to 600hz region), I'm willing to deal with a touch of beaming nearfield.

    That being said, there are some interesting designs coming out (the best example being Roger Baer's cabs IMO, and also some of Duke's Audiokinesis models with his OEM drivers and amazing crossover) that are kind of a 'best of both worlds', which I appreciate. In other words, a high quality driver that isn't designed to be more of a sub-woofer type PA driver but still with very good mechanical specs, and a mid driver that is chosen and voiced to sound more like the top of a good midrange driver loaded bass cab, and crossed over a bit higher than the pure technical specs would recommend but still MUCH lower than the typical two way cab with tweeter.

    Trade-off's like any design, but kind of a 'best of all worlds' to me.... i.e., a warm, punchy midrange complex tone with lots of wump from a relatively small cab with a single woofer, but not the 'playing through a PA cab' type tone that, at least for some, is a bit too clean, wide, and a bit sterile sounding.

    Lots of ways to skin this cat IMO. That all being said, I've rarely heard any cabs that sound or perform as well as various Bergantino 212's:D It is all about balancing those trade-offs to get the performance that speaks to you as an artist, and realizing what those trade-offs are, so you can make a reasonably good purchase decision:)
  9. Foz

    Foz

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    I like turtles...
  10. Arjank

    Arjank

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    Ok, lets consider it's about pure bassreproduction here and the best possible sound quality.
    My answer would be larger cone area with lower excursion.
    Why? Because overall distortion is a lot lower.
    Ok, modern xmax monsters have been improved a lot lately but there's nothing better then lots cone area then even more cone area to get solid bassreproduction.
    Same as with engines, in the ol' days you had the Chevy V8 bigblock, nowadays they try to get such power out of smaller engines but most will agree that it's not the same.
  11. 1958Bassman

    1958Bassman

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    FWIW, the distance a cone moves due to a specific amount of force is called the 'compliance'.

    Also, when a truck makes a wall vibrate, it's not just the wallboard, it's the framing and insulation across any span that's not rigidly fixed in place, like door and window frames & where it's nailed to the floor and ceiling. Windows are another great radiator.

    As far as high surface area vs low excursion, I wish I could have kept my 30" EV woofers. Styrofoam cones, light weight, low excursion and in the large box (76ft³), the F3 is 17.5Hz. They were originally used for the bass pedals of a church organ.
  12. 1958Bassman

    1958Bassman

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    That's the reason for the saying "There's no replacement for displacement".
  13. Bmorefoozler

    Bmorefoozler

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    The bigger cone would almost certainly be more sensitive, and then given the same displacement, louder as well. Furthermore, it is far more likely to have a better midrange response.

    Consider this like a BP102 vs a EVM 15L... Which one sounds better ina single driver cab.
  14. barryaudio

    barryaudio

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    Authorized Builder: fEARful bass, greenboy designs, Bill Fitzmaurice
    ^ this.

    I can appreciate what craig.p said about surface area vs. displacement, but practicality always wins. Yes, walls can push a decent amount of air with small excursion, but so can an 8" sub. I'll take the size, weight, and cost advantages of less drivers and bring more power to the table.
  15. CL400Peavey

    CL400Peavey Supporting Member

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    +1

    And if I am debating between a big cab, or cab with a lot of excursion, then the only choice is a big cab that has a lot of excursion.
  16. fdeck

    fdeck Supporting Member

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    HPF Technology: Protecting the Pocket since 2007
    From a technical perspective, sound pressure at a given frequency is determined by the air volume displaced by the cone. Since volume is a product of area and linear displacement, these two things trade off exactly.

    Nevertheless, there are many practical considerations as others have pointed out. Historically, drivers had limited displacement due to materials available at the time, and also voice coils with limited thermal power handling capability. These factors would both have weighed in favor of larger cone area due to the use of multi-driver systems.

    Today, my design approach would be to figure out (if possible) how much sound I really need, and try to get that amount with a single driver before resorting to multiple drivers. That approach is based on expectations for things like cost and size.

    Another factor is that area and width are related, so an increase in area lowers the "beaming" frequency. This directly affects tone quality and potentially system complexity if a design necessitates a midrange driver and crossover.
  17. KJung

    KJung Supporting Member

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    This was kind of the point of my post. They might both sound 'great', but they will both sound quite different. Any time you change a parameter of a speaker system, you will get a significant change in tonality in performance.

    So, the OP's assumption that they 'both sound great' makes the discussion kind of impossible. Most cabs actually do sound 'great', but they sound 'great' to different people depending on the tonal profile, distortion levels, range and a zillion other non-linear interactions between cab components, amplifier power, voicing, distortion levels, EQ design, instrument, player and tone goals. And, often, the best spec 'on paper' is not close to the targeted tonal performance, unfortunately.

    +1 If they both sound similarly good to you, no brainer... go for the cheaper, lighter, louder design:p
  18. Bassmec

    Bassmec

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    When technology invented a way of getting more power per forty footer,
    Who are we mere motals to our Gods to translate that into automatic downsizing, sod the economy, lets just call it added headroom and keep the same scale as the good old days.:D:bassist:
  19. makohund

    makohund

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    Not impossible, as evidenced by the existence of replies. ;)

    Yes, they will sound different. Yes, every little variable will change sound, and we're not looking at enough for a whole picture. I'm talking purely theoretical here, out of intellectual curiosity... not actually cab shopping or anything.

    But I'm curious about the impact of just this one aspect, and the tradeoffs between the approaches. Yes, no replacement for displacement. But displacement is three dimensions, Assuming we're dealing with speaker cones, the variables for displacement (and displacement alone) are:

    1 - Surface area of the cones
    2 - number of cones
    3 - xmax

    And perhaps to some degree "arrangement" if multiples are in play.

    Basically... Bigger bore, or bigger stroke? Or "negligible difference, if equal other variables will take precedence".

    Craig.P's thing on air impedence matching is very interesting. Fdeck's comment on on greater surface area likely resulting in lowered beaming frequency (and inherent tone & performance differences that brings) is pretty insightful.

    I agree with most that practicality wins, and likely points to higher xmax (and the smaller/lighter package it usually brings). And also subscribe to cl400peavey's "gimme lots of both" philosophy.

    Which makes Craig.p's take on it all the more interesting to me... Not something I've thought of or considered before, thanks. Anyone else have any more insight into how much weight that difference carries in the land of tradeoffs that is cab design?
  20. CL400Peavey

    CL400Peavey Supporting Member

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    My take on it is is this, there are lots of guys hauling around an 810 and it works great for them. I used to be one of those guys, but I couldn't get what I wanted totally out of it. So now I have cab that is smaller and significantly lighter than my 810. It's actually lighter than my old 410. In shedding the weight I did not loose capability, I in fact gained more than my 810 could achieve. I am fortunate that this design also meets my tonal needs.

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