Casework: bigger drivers are slower than smaller drivers

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

  1. Vaughan Odendaal

    Vaughan Odendaal Second Unit

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    Jon,

    Certainly the heavier moving assembly, like the heaver automobile, cannot accelerate as quickly.

    Yes. But as you said, unless I'm understanding you incorrectly, the deep bass does not require instanteous driver acceleration, therefore great motor strength isn't really required, right ? But then what happens to that same bass range if you use a heavy cone with an undersized magnet ? Perhaps I'm not understanding things here.

    At some point a host of other variables come into play, making this less than a linear relationship, but for the purposes of this discussion, for identical force:mass ratios, you'll get the same result.

    I think this is what I am not getting. If I take an 18" driver and a 12" driver, all things being equal, are you saying that both drivers will accelerate at the same force in their respective bandwidth's (again, from 80hz and down ) ?

    Thanks.

    --Sincerely,
     
  2. Jack Gilvey

    Jack Gilvey Producer

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  3. LanceJ

    LanceJ Producer

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    So other than for styling purposes, it looks like those old-skool speakers yours truly keeps yakking about are still viable in today's skinny/mini speaker world (though I'll admit many of the slim dual-woofer designs look quite elegant compared to some of the single 10" & 12" designs).

    Here's a beautiful speaker that combines both ideas:

    Vienna Acoustics Strauss
     
  4. Jon Lane

    Jon Lane Stunt Coordinator

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    Vaughn, I posed this as conventional wisdom, not as fact:

    I think we need to change our terms here; please correct me if I'm wrong: At the same level and frequency, the larger diaphragm experiences lower peak accelerations with proportionally lower forces applied.

    At some point an actual physicist is going to show up and refine (or refute!) some of my basic points, but for our purposes, the larger diaphragm is more than compensated by it's superior acoustic coupling and lower excursions. As I said earlier, moving something that weighs twice as much only a quarter the distance is a net benefit to the heavier system, theoretically making it "faster". It's "headroom" is superior, not inferior.
     
  5. Elliot_W

    Elliot_W Agent

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    Vaughan, completely unrelated to the topic:

    Are you of any relation to Robert Odendaal...an executive at Bell in Canada? Just curious, I work for the company...and know he lived in the RSA.
     
  6. Jon Lane

    Jon Lane Stunt Coordinator

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    Great essay. Note that the driver tracks the input. Mass affects output level and bandwidth, but within the driver's bandwidth, the coil tracks the signal just fine. There is no mass-related "speed" penalty there.

    (Which, by the way, also tells us that an ultra-light planar speaker diaphragm with a bandwidth limited at 10kHz is slower than a conventional tweeter with a limit of 20kHz. Poof; another myth out the window. Why do panels sound "faster"? More overshoot and self-noise; or more "detail". On the other hand, why do ribbons sound "faster"? With a upper limit of 40kHz, probably because they are...)
     
  7. Jack Gilvey

    Jack Gilvey Producer

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    A bit OT...I think that the dipole radiation of panels, and the way they interact with the room, has far more to do with their perceived "speed" (certainly in the bass) than any intrinsic "speed" of the planar drivers themselves. My dipole sub pretty much makes the other subs sound more like one another than they do it.
     
  8. Jack Gilvey

    Jack Gilvey Producer

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  9. Vaughan Odendaal

    Vaughan Odendaal Second Unit

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    Nope, no relation to Robert I'm afraid. Guys, thank you very much for your advise. I really appreciate it.

    Jon, thank you for answering my questions.

    Cheers.

    --Sincerely,
     
  10. Vaughan Odendaal

    Vaughan Odendaal Second Unit

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    Nope, no relation to Robert I'm afraid. Guys, thank you very much for your advise. I really appreciate it.

    Jon, thank you for answering my questions. Although to be honest with you, I am still having a bit of a tough time understanding why heavy mass cones with their associated motor assemblies (working in their range) won't, as you say, affect speed compared to a light 12" cone with associated motor.

    Why is it that when bandlimited, mass doesn't affect speed ? This is something that I need to understand. I'm sorry if I seem to be a nuisance asking these questions.

    Thank you.

    --Sincerely,
     
  11. Jack Gilvey

    Jack Gilvey Producer

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    Check out that Adire article.
     
  12. Vaughan Odendaal

    Vaughan Odendaal Second Unit

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    I did. Thank you for posting it

    It tells us that inductance is the key. But what is confusing me is that now mass isn't a problem at all because apparently massive drivers are just as quick as smaller drivers in their respective passband.

    I would like to understand why that is. If someone could please explain why mass isn't a concern at frequencies from 80hz to 20hz I would appreciate it.

    Some people tell me that heavy drivers can be compensated for using the appropriate BL motor force, and then other people tell me that it's not even required with the frequencies we are talking about.

    I am getting a little confused here.

    --Sincerely,
     
  13. Mark Seaton

    Mark Seaton Supporting Actor

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    Wow Jack, Good find. That goes waaaay back with some names I haven't chatted with in a long time, and I'm young! [​IMG]

    I'm going to copy a post I just made to Vaughan's thread at AVS. Hope you don't mind geek'n out a little on the tech stuff...
     
  14. Mark Seaton

    Mark Seaton Supporting Actor

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    Hi Vaughan,

    I did get your PM (via AVS), and frankly didn't know where to start. At least in a thread more people will get to read whatever is posted. Obviously at this point there's still a lot of confusion.

    The problems in explaining this lie in the question. You say "speed" but what do you REALLY mean by speed. Your boss is obviously referring to the mythical subjective bass quality and nothing tangible. Audiophile folklore is much more comfortable with these vague notions.

    That's really what this is, folklore. It has a valid basis in reality though. As with so many examples in audio, the confusion stems from an observed characteristic that is incorrectly attributed to a design method, rather than the poor design choices often made. Horn loudspeakers are probably the worst represented in this regard.

    So let's break down the concept of speed... The first, and probably most important issue to understand is how a common loudspeaker operates. Let's stick with sealed to reduce needless confusion in examples. For an acoustically small speaker (say
     
  15. Jack Gilvey

    Jack Gilvey Producer

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    You can be so geeky.
     
  16. Jon Lane

    Jon Lane Stunt Coordinator

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    Heh, it's always hard to follow Mark up. [​IMG]

    Here's one more mental experiment for the layman. (I thought this had been said here before but it must have been another thread[​IMG]

    If Driver A was slower than Driver B when both were subjected to the same input, wouldn't Driver A merely output a lower frequency? Speed = frequency, right?

    I.e., if the massier driver was slowing the input, wouldn't we observe a whack of harmonic distortion?

    On the other hand, if Driver A was mass-limited vs. Driver B, wouldn't we really see that the motor simply couldn't move it as far per unit of time? More mass = less efficiency but at the same frequency.
     
  17. Mark Seaton

    Mark Seaton Supporting Actor

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  18. Mark Seaton

    Mark Seaton Supporting Actor

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    In the referenced discussion, Vaughan asked:

    Sure, mass plays a factor. A needed factor. In some cases, you NEED more mass. In some cases you need less mass. One of the best sounding, really deep extending, box subwoofers I've heard was a prototype using a pair of 18" driven cones which each had more than 2kg of mass on them! Yes, they had a LOT of motor strength behind them.

    What is important is the relative balance of cone size, motor strength, moving mass, and box volume. Note I mean all of them, not just two or three, they are all interdependent, and the proper balance depends entirely on what the desired performance is.

    "Tracking the input signal" is another statement that really needs to be qualified. If we look at an impulse response, the stuff that "moves fast" is in fact high frequency. As outlined in the Adire article, in perspective to the driver, this is first limited by inductance. Of course in the case of a home theater subwoofer, the low pass of the subwoofer crossover in the bass management largely sets this limit of "how fast" things can change in the signal you are trying to "follow." To get any useful information, an impulse of the ideal signal you want to follow has to be compared, not to that of a full range impulse.

    There are of course damping issues that come into play with the low frequency corner of a subwoofer as well. This is again dependent on the relative factors I mentioned above. Don't blame the driver for the designer making design choices that compromised the result. Put that big driver in a suitable enclosure or change the driver characteristics and it can perform just fine.
     
  19. Vaughan Odendaal

    Vaughan Odendaal Second Unit

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    Mark, could you give me an example of why you would need (or want) more mass as opposed to less mass ? Just a couple of questions. Deep bass. Cones aren't required to move or accelerate quickly to reproduce deep bass frequencies.

    Correct ? Please correct me if I'm wrong.

    A high BL will increase efficiency, but to a point, as you say. But in order to compensate for cone mass, the cone would require a motor assembly with a high enough BL to counteract it.

    Correct ?

    But this extra horsepower will not really be used to accelerate the driver any quicker than if it didn't have the extra BL because acceleration is a function of SPL. Correct ?

    Now what happens if a 12 " driver with a very high BL, with very high excursion levels must compete against an 18" driver with lower excursion levels overall, so moves less air overall ?

    What would happen then ?

    Thank you for having the time to answer my questions. I really appreciate it. I'm about to get my "Master Handbook of Acoustics", in about a week's time, so my knowledge hopefully will increase exponentially. [​IMG]

    --Sincerely,
     
  20. Mark Seaton

    Mark Seaton Supporting Actor

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    Thanks Jack... I think. [​IMG] :wink:
     

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