# Casework: bigger drivers are slower than smaller drivers

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

1. ### Vaughan Odendaal Second Unit

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I've heard this many times. At my work, my manager tells me that an 18" driver can never move as quickly as a 12" driver. And that for simulated effects, like explosions, very big drivers can't move quick enough.

I hate it that I couldn't explain this misconception, this myth. I just disagreed but I know that it's wrong. I recently got my "Louderspeaker design cookbook", and I am learning a lot (only on page 22, ). But I would like to learn how and why bigger drivers are not slower than smaller drivers.

His response to me was that the 18" driver simply moves too much air. He also said to me that it's a physical impossibility that an 18" can move as quickly as a 12" driver.

I would like to clear this issue up once and for all. All those who are technically inclined, please offer your advise and explain away this misconception. Please.

Thank you.

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Anyone ?

--Sincerely,

3. ### Jon Lane Stunt Coordinator

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He's right. But only technically and only within a context not applicable to the actual use of either the 12" or the 18" driver. Short version: Within the operating bandwidth both drivers track the input signal in real time. There is no speed penalty for the bigger driver at all.

Where a driver's mass limits it's "speed", that driver exhibits band-limiting as a lowpass function. IOW, when the driver cannot track the input due to mass, it attenuates its output, rolling off the higher frequency bandwidth.

Driver mass and size affecting "speed" (as in, the smaller, lighter driver is "faster") is a myth when you consider the operating range of the driver. If the driver is at reference efficiency at some point in its bandwidth, it's tracking the input at that same frequency virtually perfectly.

Where your friend is likely getting hung up is in the assumption that woofers must accelerate nearly instantaneously. They do not, or they'd not be woofers, they'd be flashlights. Instead, they track large sine waveforms at a low enough frequency to make "speed" irrelevant.

4. ### gene c Producer

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I'm no techie but I have to agree with your boss on this one. You can't expect a larger, heavier object to move as fast as a smaller, lighter one given the same amount of force applied to them (power). Whether the slower speed results in a noticable difference in sound is another thing. But isn't this why tweeters are the smallest, lightest drivers in a speaker? Because they have to move so much faster than bass drivers. Might be a reason for that . If I, and your boss, are proven wrong, a whimpering apology will be issued. Eventually. But don't hold your breath waiting.

5. ### Jon Lane Stunt Coordinator

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Let me take another shot at this:

The idea that a larger or heavier diaphragm is slower is a myth. The comparison the OP posed involves two drivers used over the same or similar freqencies, say, below 80Hz. The 12" driver has no advantage related to speed whatsoever.

Lowpass frequency for lowpass frequency, large woofers are simply not slower than small woofers.

6. ### gene c Producer

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This is a good point I didn't consider in my first post. This is why some people use two subs. A larger one for the lowest frequencies and a smaller one for the upper bass range. I don't think I, or your boss, fully understood the point you were trying to make. I obviously didn't consider their operating range but I didn't realize that Vaughan was. But Joe did a great job of clearing it up. At least for me.

7. ### gene c Producer

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I understand what you are saying Jon, (our posts are over-lapping) But am I right in thinking as the frequency goes higher (instead of lower) that the driver must move faster and therefore a smaller driver has the advantage? Help me out here. I'm trying to weasel out of an apology!

8. ### Jack Gilvey Producer

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Nice posts, Jon. This comes up every so often and it's always fun to watch engineering and intuition parry.

9. ### Jon Lane Stunt Coordinator

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We're all on the same page, provided we use the same context. The benefit to faster diaphragm motion is either higher frequency output or higher amplitude output or some combination of the two.

For all intents and purposes, however, the larger/heavier diaphragm is not slower, it's just more bandwidth limited on the top end. Apology averted, although the false intuition that small=fast is still prevalent in listeners.

10. ### Jon Lane Stunt Coordinator

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Incidently, there is a rough correlation between diaphragm area, frequency, amplitude, and velocity that raises yet another counter-intuitive point.

If your very large area diaphragm weighs 100 grams and moves 1/4" to get the job done (pick a reference output of 90dB) and your smaller diaphragm weighing 50 grams has to move 1" to produce the same output, then the smaller diaphragm is obviously experiencing substantially higher peak velocity, ie, speed of four times that of the larger diaphragm, and it's therefore accelerating twice the mass per unit of time.

All other things being equal, in this scenario the smaller driver actually runs out of pistonic bandwidth sooner than the larger driver, and by our speed/frequency construct, would technically be slower.

Do I have that right?

Dontcha love hifi?

11. ### gene c Producer

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Thanks Jon, for allowing me to escape from the dreaded apology (sorry Vaughan ) but this gives me yet another chance. If a driver has to move faster in order to produce a higher frequency, then doesn't smaller (lighter) = faster, at least eventually?

12. ### Jon Lane Stunt Coordinator

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Smaller=lighter=faster in the context of band-limited woofers (I assume the OP meant woofers and not midrange drivers) is a myth.

It's all about the context of intended use. Below a frequency commonly used in hifi for 12" and 18" woofers, smaller=lighter=faster is a myth.

Band restrictions aside, does small=light=fast? Absolutely, or we'd not have 25mm silk dome tweeters.

13. ### gene c Producer

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Thanks Jon. I appreciate the effort. Jack, I find intuition mostly harmless. It's ignorance that usually gets me into trouble :b .

14. ### Vaughan Odendaal Second Unit

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Wow. So many responses already. Jon, excellent points. My manager is an engineer who has been in the field for 30 years.

So it disturbs me when I hear how wrong he is. Because he told me that it is physically impossible for an 18" driver to move or accelerate as quickly as a 12" driver.

He tells me that for simulated movie effects, like explosions, you need a bass driver with quick transients with quick rise times. He tells me that an 18" driver simply will move too much air which will cause it to move too slow.

So he recommends a 12" driver. I also would like to know, if the magnet assembly was undersized for a given driver, how would that effect deep bass frequencies (80 hz and below) ?

I mean, there is a reason why some 12" -18" drivers use huge magnet's and voice-coils. I understand that BL is a far more important factor in determining acceleration. Mass is not the most important aspect to consider.

I just would like to know what kind of effect a heavy 18 " cone would be like (with a corresponding magnet assembly) when reproducing movie effects, or music in it's operating bandwith (80hz as an example, down to wherever).

Lastly, when it comes down to explosions, what kind of frequencies are required (and what type of driver) to accelerate quickly to give you the "kick" ? Is it the midrange, or the lower mid driver ?

Thanks a lot. I appreciate everyone's responses. I need to learn as much as I can

PS I see on DIYaudio.com, there is a debate going on concerning this very topic in the speaker section.

--Sincerely,

15. ### Lee-c Second Unit

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It depends how much force you apply to a given weight, if you apply enough force you can move
a heavier weight just as fast as a lighter one.

16. ### Jon Lane Stunt Coordinator

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Vaughan, it's counterintuitive, isn't it? Certainly the heavier moving assembly, like the heaver automobile, cannot accelerate as quickly.

Except, as Lee-c says, when we simply apply enough motive force, at which point even the mass of the heavier car is proportional to the force v. mass of the lighter one. The diaphragm can weigh a hundred pounds and as long as you apply as much force proportional to the transducer's output as you did to the 100 gram cone, they deliver basically the same result.

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.

But you've introduced another variable:

As motor strength (Bl) increases, so too does magnetic damping, limiting bass but enhancing efficiency. No free lunch here; draw a woofer's response curve in your mind and "slide" the output up in level but do so more or less on the slope dictated by the original system's parameters. For example, roughly double the output, but halve the F3. As I said, there are many other variables, such as alignment type, but for a simple closed box, this is approximately true.

How to get a lot of whallop from a woofer design? Again, counterintuitive: Enhance coupling and lower distortion by going to the diaphragm with greater area. For an extreme example of this, consider a bass horn. Frequency for frequency, the extreme acoustic coupling of the horn to the air in the room will flap your pants legs, and will do so at ultra-low distortion.

You won't get nearly as much physical bass from the smaller, direct-radiating speaker as you will when you couple a larger effective area to more air. What's counterintuitive is that air has such low impedance that it takes quite a lot to "grab" and drive it. Wave your hand around and nothing much happens, but wave a sheet of plywood around and things happen ... along with your needing to put far more energy into moving the sheet of plywood against its air load than into your arm to move your hand.

More area, = better. Horns = best. It's about acoustic coupling.

17. ### Vaughan Odendaal Second Unit

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Alright, I understand. . sort of. But this begs the question, what would a heavy cone with a undersized magnet assembly (compared to a 12" driver with adequate motor structure) affect in the bass range ?

I hear that low bass does not need woofers to respond quickly, or instanteously, if I understand Jon correctly, because woofers are not quick devises.

So then my question is, why use bigger magnet assemblies for a given driver if it's intended application is to voice frequencies from 80hz and below ?

I just would like to understand this. Driver mass is not as important as motor force. Alright. Motor force is a function of magnet size, voil coil size (length of wire, amount of turns in the gap. . .or is that to do with linearity ?), the spider and surround play a significant part in this too (in terms of the restoring force).

I hope I have things correct here. Please correct me if I'm wrong. But if as Jon says that in the drivers bandwidth, speed shouldn't be a concern between a 12" and an 18" driver, then what benefit would I get if I use an 18" driver with an appropriate motor with the force equal to that of a 12" driver ?

Your response would be much appreciated.

--Sincerely,

18. ### Jon Lane Stunt Coordinator

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This question gives yet another way to sum all this up: A broad-spectrum, complex-signal input may contain enough energy at high enough frequencies to energize more than one driver -- or all drivers -- in a multi-way speaker system.

Let's say the input signal has fundamentals in the very lowest frequencies, and other fundamentals or overtones ranging up to 4kHz. If our system has a tweeter crossover at 2.5kHz, then indeed, all the drivers in the speaker will be used to recreate this complex signal's complete range of energy, including the tweeter.

The shock of the leading edge of this signal may require a sharp, high-frequency leading edge, and the sheer energy may simultaneously exercise a 18" woofer to maximum output, but at no time did the woofer cone exceed the acceleration dictated by it's lowpass frequency and output level (cone velocity). Therefore it's "speed" is completely irrelevant.

You thought you heard -- and maybe even saw -- the woofer attacking the signal in such a way that it appeared to move instantaneously, but it did not. Any accelerations beyond it's ability were displaced to higher range drivers until all of them were output elsewhere.

19. ### Jack Gilvey Producer

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Something to consider when looking at woofers vs. tweeters is that it's not all about small = light = fast, a typical tweeter is small largely for dispersion purposes. There are full-range 6" drivers (Diatone) that do quite well as far as "speed" up to 20k, but they beam (become as directional as lasers) severely up that high.

20. ### Jon Lane Stunt Coordinator

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By 'undersized' and 'adequate' I'll assume that the outputs from both systems are identical in level; they have the same efficiency because the motors are proportional to that output.

What happens is that the 18" goes lower. It's greater area gives it better coupling. In fact, you may have noticed that magnets may be visibly similar in size between say, 12", 15", and 18" drivers. This is because the larger driver needs approximately the same motive force to output the same level. Similarly, large speakers with a lot of piston area are more, not less efficient than smaller ones.

Conversely, if the motors are sized to deliver the same f3, the larger driver will be more efficient.

I don't mean to give the impression that light cones aren't attractive. In fact if you model simple bass systems, you'll eventually find that the ligher cone/smaller motor system (when output level and f3 are proportional to the heaver cone/larger motor) will be slightly more advantageous in terms of the three parameters we juggle when designing bass systems: net enclosure volume, f3, and efficiency. The problem is that light moving masses are at some point challenged by practical concerns like physical strength, especially when that strength must keep a very large cone from bending. A sheet of cardboard weighing as much as hockey puck isn't nearly as resistant to bending forces even though it does couple to air far, far better.

On the other hand, if I want a lot of bass out of a small enclosure, and if I have a lot of available power, the design that invariably wins is the small, relatively heavy cone driven by a very large motor and hundreds or even thousands of watts and a ton of signal EQ. But remember that we'll have a very inefficient system and unless we have a means to servo-control it, a lot of distortion.