[Scotty]

Something to think about. A driver only has to be fast enough to accurately
reproduce the bandwidth of the signal that is assigned to it. Woofers don't have to be "fast" compared to a tweeter. What they should not do is store and re-release energy later in time. You can see the problems this criteria poses for a full range speaker. It's next to impossible for a full range driver  8 or 10in. in diameter to be a woofer and a tweeter at the same time. The laws of physics dictate that it's performance will be compromised at the frequency extremes.
If you look hard enough you can find speakers that sound fast at all frequencies
and by extension  more closely approach the sound of reality. I think planers,ribbons and electrostatic loudspeakers may sound faster or more life-like
because there is very little mass to accelerate and decelerate and there is also very little stored energy in these drivers.
Scotty

[macrojack]

Wouldn't greater surface area allow for lower excursion while still moving as much air? That would explain the fact that you really don't see movement in planars like you do in most cones. My knowledge of physics is less than rudimentary so forgive me if this is a childish assumption. I have speakers with full range 10 inch drivers which don't seem to suffer at all from compromise at the frequency extremes. They have phase plugs and whizzer cones and are rated from 40 hz to 12 khz. They have to be played VERY loud before the cone movement is visible. I would characterize them as fast drivers but the manufacturer said that fast is not a music term when I asked him about that aspect of their performance.

[MaxCast]

What if your driver goes too fast? 

[PaulHilgeman]

Be sure to get All the moving weight when making calculations, there's more moving mass there than just the cone.

The 8g-30g numbers above were entire moving masses, the spider and surround partially contribute to mass as well.

More later...

[zybar]

What if your driver goes too fast? 

George

[JoshK]

When I think of this problem from a physics 101 perspective here is how my thoughts go....

As a driver has to move in and out very quickly it has to acheive a lot of acceleration.  Remember velocity is speed with direction, so a change in direction (in vs out) even at constant speed equals acceleration (think of a car going around a turn at a constant speed, the driver still feels a pull to the outside (the body can 'feel' acceleration)).

F = MA  Force equals mass * acceleration.   We have already been discussing moving mass, that is your M.  A is the acceleration needed for the transient response.  So the greater the mass, the greater the force needed to maintain the acceleration.  So this doesn't explain the difference by itself, if one could presumeably use a stronger and stronger magnet with greater moving mass.

I still believe it is primarily how the driver parameters interact with other affects in the system.  A bass driver with larger group delay with sound slower than one with less group delay.  I think this is explained better on SL's site.  In my simplistic thinking, this is why ported systems sometimes sound slower than sealed systems.

As macrojack pointed out, a greater surface area (Sd) can move as much air with using less excursion.  However, greater Sd usually comes with other tradeoffs, like typically higher moving mass, lower Fs (good thing typically), and lower frequency at which the driver starts beaming.  It is all a juggling act.

[Dan Banquer]

A fast driver is typically a driver with a wide bandwidth.
                      d.b.

[PaulHilgeman]

You often hear of planars, ribbons and electrostatics being labeled as fast. The transient response is superior to a cones due to little moving mass. It's a tiny fraction of what a cone driver uses. The mass of a cone can be ten or more times higher than a ribbon or planar.  The planars on my speakers weigh 1.5 grams. Compared to a cone plus of course it's voice coil it's a small fraction of the equivalent cone.

True that they are often 'labeled' as fast.

However Moving mass has little to do with it.  It is a combination of things, and I have my opinion of what 'sounds' fast, but it has little to do with the physical properties of the speaker, mms, compliance etc.

Ribbons and planars do take advantage of the fact that the diaphragms are light weight, however they use very small magnets.  Most of what you see in a ribbon back ends are the impedance transformer.  Most planars typically use bar type magnets across the back of the diaphragm. 1-2mm in diameter and the width of the diaphragm.  Likewise, heavier dome type tweeters use larger magnet systems.  However, it is certainly possible for a dome tweeter to have the high frequency extension of a planar or ribbon.  Of course HF extension, rise time, impulse/step response etc. are all one in the same.

I would make a good guess that the total moving mass of one of the big Magnepans is not that much different than a dynamic driver speaker that has similar bass performance.

The bottom line that I am not so eloquently getting at is that 'speed' is merely a fair term to describe the sound of a speaker by, but really has little to do with the drivers, and especially their moving mass.

To widen the discussion a little bit, I will give some of my opinion of what makes people thing something sounds 'fast'.  First lets think about 'slow' and one thing that I have heard, and I think some will agree is that ported systems can sound slow, where sealed can sound quicker.  While this is partially true, this comes from what the High-Pass function does to the sound.  The steeper the High-pass is, the more delayed the frequencies at and just above the roll-off are delayed and the more that they 'ring'.  A sealed system has a 2nd order (12dB/octave) roll off and a ported system tends towards a 4th order system  (24 db/octave) roll off.  Some ported systems can be tuned to have a hump, and will in some cases, initially roll-off at something greater than 24dB/octave, giving them more boom, poorer transient response and more group delay (1/f * phase shift(f)).  So, among other things, speed can be had by properly designed ported systems as well as sealed systems.

Also, LF drivers that have too much output in the 60-100Hz region can sound fast and punchy, but this is just a tonal issue, while this can sound exciting and fast on rock, jazz and pop music, things like chamber reverb and deep orchestral notes will be washed out and out of balance on a system like this.  Single instruments that play through this region will also sound incorrect, and can lead to say a thin sounding stand-up bass.

-Paul Hilgeman