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Casework: bigger drivers are slower than smaller drivers

Discussion in 'Speakers' started by Vaughan Odendaal, Mar 11, 2006.

  1. LanceJ

    LanceJ Well-Known Member

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    As I used to tell my students, when it comes to physical concepts, many times a certain effect just IS and trying to dissect it will only result in confusion (and this has nothing to do with the don't-question-authority concept - this is just the universe at work).

    Some of the simplest concepts are the basis of many of our most complicated technologies. Common example: the binary numbering system. Just two digits, 1 and 0, representing zero volts and five volts enable people from widely-separated locations around this planet to discuss this issue on this forum, or send a robotic probe to Jupiter that can make many decisions on its own. But still, no matter how complicated the system they are part of, that 1 and that 0 just are.
     
  2. gene c

    gene c Well-Known Member

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    I think I embraced this theory around 11:00 AM yesterday [​IMG] .
     
  3. LanceJ

    LanceJ Well-Known Member

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    Gene: the following (almost literally) happened a lot when I was taking my own physics classes:

    [​IMG]

    ......and I finally learned once that occurred, don't touch it! [​IMG]
     
  4. Mark Seaton

    Mark Seaton Well-Known Member

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    Back at it...



    I'm not sure I follow your question. Very high BL or not, the output is determined by the movement of the cone. The effect of the motor strength for either size driver will be entirely dependent on how it interacts with the box size and moving mass of the driver.
     
  5. Vaughan Odendaal

    Vaughan Odendaal Well-Known Member

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    Mark Seaton, thank you very much for your time. I will be asking you some more questions if you don't mind. . soon. [​IMG]

    And a bit off-topic, but a question about amplifier damping factor. Is it as important as we are led to believe ? Or is the mechanical damping of the driver more important overall ?

    That is another thing that my manager always harps on. That the amplifer completely controls the drivers ability to stop and start.

    Thanks again.

    --Sincerely,
     
  6. MikeNg

    MikeNg Well-Known Member

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    Great thread! Lots of good stuff here. First thing that came to mind was the illustration of little car/little engine vs. big car/big engine. It's simple but makes sense to me. The geek chatter here helps me to have an understanding of what's actually happening with the drivers - nice...

    I too would like to hear what the experts have to say on amp damping factors. I've read that ultimately any 'true' rating over 200 is more than enough in the real world, and specs that report '2000' is no more than marketing hype. I've also read that in order for the driver to take advantage of the amp's dampening factor, the appropriate (or adequate) speaker wire guage must be applied (too thin a wire reduces the amp's ability to stop/start). I think this came out of some docs issued by Crown. I'll post a link if I can find it...

    Have I been mislead by the internet? [​IMG]

    Mike
     
  7. Vaughan Odendaal

    Vaughan Odendaal Well-Known Member

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    Now we have to start considering the low frequency resonance and how the enclosure affects that. There is this pesky concept known as Hoffman's Iron Law.

    I understand Hoffman's Iron Law. But is it true that subwoofers don't need to move instanteously to reproduce deep bass transients ?

    I guess what I am having a difficult time grasping is the fact that motor BL doesn't affect acceleration in the drivers passband and yet SPL does.

    But then what about 18" drivers that actually are "slow" using inadequate magnets and voice-coils ?

    I would caution you when looking at BL. BL is not force (N). It is Newtons per Amp.

    I don't understand. If an 18" driver weighs more than a 12" driver, it will require more force to overcome that mass.

    Correct ?

    F = m x a. This tells us that so long as mass doesn't change, the acceleration will be proportional to the force applied. And the motor assembly is what gets the driver moving.

    This is why people keep on using car analogies to describe the whole bigger versus smaller driver debate. BL is a measure of motor strength. This is in the Vance Dickason Design Cookbook.

    Apparently you could move a 100" driver just as quickly, in pure physical terms, as an 8" driver even, by using a strong enough motor assembly.

    Correct me if I'm wrong here.

    --Sincerely,
     
  8. Mark Seaton

    Mark Seaton Well-Known Member

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    Read the rest of what I wrote below the comment you refer to. BL is only a direct indicator of motor strength when Re or Rdc of a woofer is constant. That's why I specifically use the term motor strength and not just BL. Again, we're interested in force per watt of power input.

    The car analogy is a loose one that has conceptual problems and holes. The biggest confusion lies in envisioning the motion of a cone. Again, the position of a cone DOES NOT follow the pressure changes we observe, nor does it follow the electrical signal. There is effectively a squared transfer function inbetween.

    Try thinking about this extreme case as a conceptual example: Let's say we wanted to produce a positive DC signal in open space. What does the electrical signal look like? ...A constant voltage. If we look at this from what a microphone must observe, the pressure must stay constant, which means air must push continuously like from a fan to maintain pressure. In order for that to happen, the cone would have to start and then not stop moving forward so long as the signal was produced. This is why a woofer with any excursion limit cannot produce DC signals.

    While the example is not directly related to reproducing music, it should be clear that the cone does not follow the electrical signal, so we have to think of speed and movement in a bit different perspectives.
     
  9. LanceJ

    LanceJ Well-Known Member

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    (Mark or Jon: if I've screwed up something in the following explanation, please say something) As an obvious example of this technique, and because I always talk about these things [​IMG], check out the woofer of this Advent from the mid 70s: the Small Advent. FYI: all Advents during this time are sealed designs. That huge dust cap you see is not just used to keep dust out of the voice coil, it is also used to significantly increase the moving mass of the 9" cone (yes, nine inches - Henry Kloss hardly ever stuck to conventional thinking). The cap is made of a thick felt material & is very securely fastened to the cone.* He wanted a speaker that had about the same bass capability as his Large Advent, a design that used a more conventional 10" woofer in a much larger enclosure, but one that also took up less space.

    So Henry came up with this woofer that was quite heavy for its size and since it can resonate at a lower frequency than a "normal" woofer of the same size, it is also able to generate bass to much lower frequencies (kind of like how a Cadillac DeVille bounces slower after hitting a road bump than a BMW Mini does). Owners of both Advent models say Kloss easily achieved his design goal (some think the Smaller actually goes lower than the Large). But here's the tradeoff: this thing is a major power sponge. That heavy woofer + the sealed design can really tax a lightly-built amplifier. And to make things more stressful, the system has a 4 ohm impedance rating. For better-built amps that means they will nearly double their power output, helping to increase the owner's ability to crank up Sonic Youth to politically-incorrect levels. [​IMG] But for entry-level amps, that could mean protection relays will kick in much sooner.

    Other sealed speakers of that era like Acoustic research, Infinity, Genesis Physics, EPI and many others used a similar design philosophy - and since so many people were buying them, this was a major reason for the receiver "wattage wars" that were occurring back then. Mid-fi manufacturers like Pioneer and Technics were trying to outdo each other with these monster receivers, like with Technics' SA-1000 with 330 watts per channel. And back then those were usable wattage ratings, not the inflated crap you see on most receivers' spec pages nowadays.

    All this talk about heavy things also applies to most bass-reflex designs. That's why among subs that use "X" size of driver, the sub that also uses a larger enclosure with that same driver usually goes lower because of the increased mass of air inside the enclosure and/or porting system (this doesn't include subs that use electronic systems to extend their frequency response). Passive radiators also have their place in this discussion, but I don't want to derail this thread anymore than I already have. [​IMG]

    * I actually own a pair of these, bought from a resale shop for $10/pair. But the 32 year old woofers need to be totally refoamed & I haven't been able to get this done yet. Both tweeters work perfectly though. And yes, that is the way the factory constructed most of these funky cone-dome tweeters (i.e. the tape, the metal grill and the offset square gizmo).
     
  10. Scott Simonian

    Scott Simonian Well-Known Member

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    Woofer? Fast bass? Er, uh tweeter? No...fast bass!

    Hmmm.

    I love threads like these.

    [​IMG] [​IMG]


    *voice in the wind* "...Le......Inductance....rise time" *voice in the wind*
     
  11. Vaughan Odendaal

    Vaughan Odendaal Well-Known Member

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    I think I am even more confused now than when I started. [​IMG]

    Almost everyone on the DIYaudio site has used car analogies to demonstrate how flawed the whole myth is. After reading the Loudspeaker Cookbook, I understand BL as motor strength.

    BL according to Vance is a measure of motor strength. And what moves the cone is the strength of the static magetic field combined with the alternating electromagnet field. Correct ?

    The stronger the magnetic field, the more the cone will move. The motor will determine how much it can move the diaphram in a similar way to "horsepower" or "torque". Now this is all wrong ?

    What is the function of a bigger motor ?

    I need things to be put into perspective, if at all possible. I apologize if you are getting a bit frustrated trying to explain this to me but please understand that I am getting conflicting opinions from pretty much everywhere I go.

    Thanks.

    Is it possible to move a 60" driver as quickly as an 8" driver given a big enough motor assembly ? Still, I don't understand some things.

    So deep bass doesn't require fast accelerations and big drivers or small drivers aren't required to have fast acceleration times in order to reproduce them ?

    Correct ?

    If so, why ? Perhaps if you can explain this a little bit easier I might understand it better. Again, thank you very much for putting all the effort into replying.

    I appreciate it alot.

    --Sincerely,
     
  12. LanceJ

    LanceJ Well-Known Member

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    I think when some people say "fast bass", they are referring to non-boomy/non-bloated bass, and not literally its speed.

    Correct me if I'm wrong, but the lower the frequency, the more chance for room-related problems such as boominess/bloat. So subs (or conventional speakers) that don't go as low as another sub have less chance of boom/bloat so these are thought of as sounding faster. I'm also basing (haha) this on the fact that many audiophiles are fans of small standmounted monitors (something with 6" or smaller woofers) because they say these types of speakers have "very tight & accurate" bass compared to large floorstanders.

    Personally I think what is actually happening is a lot of people just aren't spending enough time positioning those larger speakers and/or their subwoofers to find their optimum locations, giving their owners the mistaken impression these cannot provide accurate bass.
     
  13. Scott Simonian

    Scott Simonian Well-Known Member

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    BINGO!!!
     
  14. Mark Seaton

    Mark Seaton Well-Known Member

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    I'll echo Lance and Scott's comments. The problem goes back to what I noted in my first post. The mythic, subjective descriptor of "speed" or "fast bass" has ZERO basis in physics or the physical operation of a woofer.

    I guess we should have just stated this up front. The problem is that many jump to all sorts of creative conclusions about what causes the subjective sound they hear based on the examples they are exposed to. It is two very different things to say "I've never heard an 18" produce clean, accurate, or articulate bass," vs. saying "An 18" cannot provide clean, accurate, or articulate bass."

    To use the car analogy, if your only exposure to V-8 engines was big trucks and SUV's, you might conclude that V-8 engines can only push lots of weight but can't really go fast. Owners of exotic sports cars may give you a fight on such a statement.
     
  15. Jon Lane

    Jon Lane Well-Known Member

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    Vaughan, there are two ways to present views of this phenomenon. I've limited mine to a subjective, layman's view in order to argue from an easily grasped perspective. At times this can include turning questions related to perception (large = heavy = slow) into other questions: Why is speed not actually just function of frequency?

    Mark has weighed in with the actual physics of the thing; thus giving us a solid backstop if we choose to use it. Either way you have insurmountable evidence that simply says that quickness is nothing much more than the frequency being reproduced. Raise the "quickness" in this context -- your boss's perspective -- and you raise the frequency. Drop the "quickness" and you naturally drop the frequency.

    This is obvious and it's completely intuitive. It turns the question around to, why/how would a heavy moving mass be "slower" instead of constantly assuming that it merely would only as a function of it's perceived or actual mass.

    Here's a good example of a tributary of this assumption, one that keeps popping up and one that shows us the powerful impact of preconceptions:



    Let's turn this around once more as well:

    1. Why would you need to move a 60" diaphragm as quickly as an 8" diaphragm?

    2. Why would a larger motor be necessary to move it at all?

    The first answer is that level-for-level and frequency-for-frequency, you wouldn't have to! Take 50Hz and 80dB outputs from each and the 60" driver moves a miniscule fraction of the 8" driver's excursion. I.e., it simply doesn't have to accelerate to the same peak velocity.

    The answer to the second question is that given that we've already determined that speed itself is irrelevant for these real-world examples, motor strength must affect some other output characteristic, in this case efficiency and ultimately, bandwidth.

    My point with these two limited examples is that when you turn the most basic questions around and pose them back at the presumption that heavy = slow, while the underlying physics won't leap out at you, at least you're starting to question the intuitive but incorrect assumption that so many have about bass speed.

    In other words, try looking at why the premise is physically impossible in order to understand why the explanation being offered here makes sense.

    Frame the questions differently and place them in the light of some basic, realistic obversations about bass system behavior and you'll probably come to a different conclusion before you find those pesky intuitions leading you off down the same paths.
     
  16. Greg_R

    Greg_R Well-Known Member

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    Vaughan, I think this is your big misconception. "Instantly" = approached the speed of light = go get your Nobel prize. All audio drivers start and stop "quickly" and hopefully "quickly enough" (i.e. have good transient response in their system).
     
  17. Jon Lane

    Jon Lane Well-Known Member

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    I'd add that there's another enormous issue with bass systems in the real world: An almost universal lack of proper setup options that make subwoofers integrate correctly with their fullrange loudspeaker partners.

    Put another way, would we haphazardly wire a tweeter onto our $3000 main speakers with no knowledge of speaker design? Well, I propose that that's what hundreds of thousands of well-intentioned folks have done by splicing a ~80Hz active subwoofer onto their systems.

    I hesitate to make this claim because doing so isn't going to make me any friends ... or sell any more of my products. But unless the summation between main speaker and sub reflects a bona fide crossover theory and design, predictable and sensible response characteristics, and reasonably flat acoustical power, it's going to sound like, well, a subwoofer and a main speaker instead of like music.

    And the effect is therefore going to be subjectively described as "slow", which really means pronounced, non-flat, and ill-damped, all of which are precisely what you get when that sum isn't perfect.

    (If you need me, I'll be running for the door... [​IMG] )
     
  18. Vaughan Odendaal

    Vaughan Odendaal Well-Known Member

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    I'll echo Lance and Scott's comments. The problem goes back to what I noted in my first post. The mythic, subjective descriptor of "speed" or "fast bass" has ZERO basis in physics or the physical operation of a woofer.

    Okay. Could you explain to me why that is ? I just need to understand the reason why the physics in fast or slow bass is not a physical operation.

    If I used the heaviest magnet and slapped it onto an 18" driver which, likewise, was very heavy, are you trying to tell me that it's ability to track the input signal would not be hindered ?

    About the whole "instantaneous" thing about woofer acceleration. I just wanted to know that with low bass, and with simulated effects, like explosions in films, woofers aren't required to move or accerelate extremely fast ? Not a rhetorical. [​IMG]

    Thanks.

    --Sincerely,
     
  19. Vaughan Odendaal

    Vaughan Odendaal Well-Known Member

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    So Jon, if I am getting this correct, the mass of the driver will affect efficiency, but not acceleration. SPL is a function of how quick the driver will react to the input signal.

    And a heavier driver won't hinder that operation. Just lower it's Fs ? And any perspection on the slowness of the driver is just the room acoustics, alignment. . .etc ?

    On the SPL affecting acceleration, is there an article on that that I could perhaps read ?

    Thank you.

    --Sincerely,
     
  20. Mark Seaton

    Mark Seaton Well-Known Member

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    The only final addtion might be how "fast" a driver has to change/switch force back and forth is a matter of frequency. Remember that the force is in the opposite direction for each 1/2 cycle. BL is N/A, so to change force, current has to change directions. Inductance restricts changes in current. Greater inductance has more resistance to change in current, so inductance of a voice coil limits how well it responds to high frequency signals. Fortunately we don't put very high frequency signals into our subwoofers. If we low pass them at 80Hz, they only have to "change direction" so fast. If we crossed them at 200Hz, they would have to change directions more often or more quickly. As such, a driver only has to change directions fast enough to respond to the signal being fed to it, which is defined by the crossover frequency and slope.
     

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