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Old 15th July 2011, 12:50 PM   #1
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Default MMs / BL = dynamics?

I remember Mr. Brines talking about the relationship between moving mass, BL, and transient response. But is the same factor a way to predict the dynamic potential of a driver? For some now I have been operating on the assumption that it is, but I would like to know if there is any grounds for that or not. For example, looking at the Audio Nirvana 10" and 12" ferrite models, both units have approximately equivalent motors of 1400 and 1409g. But the 10" has 17g mms and a BL of 9 while the 12" has a moving mass of 28g and a BL of 8.86. This would lead me to assume that the 10" has greater dynamic potential as it is able to accelerate its 17 grams of weight more efficiently, producing more "slam." But, then again, maybe the 12" has greater slam potential by virtue of being so much larger.

Any value to making predictions such as these, or do Mms, BL, and other indicators fall short of hinting at how a driver will perform in real life?
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Old 15th July 2011, 01:57 PM   #2
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Depends. You're refering to the nominal Newtonian acceleration factor which is taken (using SI units) as BL / Mms, giving an acceleration factor in meters per second per second per ampere. It's vaguely interesting from an academic POV, but in reality, it's all but meaningless. BL strictly should be written as B*L*i where i is the current in the voice coil. Thus, from that you can see that acceleration proportional to the amount of current you feed the drive unit. Simple as that. The ratio of raw motor power to mass is just another way of expressing efficiency.

'Dynamics' is an often misused term; technically it simply means the dynamic bandwidth of the drive unit, i.e. how much headroom it has available to handle large dynamic peaks in the material. FWIW, if you're concerned about this, you may find the Volume Displacement (Vd) figure of a drive unit, which is Sd*Xmax to be useful. That tells you how much air the raw driver can potentially shift in a linear fashion.

If you're concerned about transient response, this is pretty much a non-issue for wide-band / full range drive units. Since transient response is linked to the bandwidth the drive unit is able to cover (the transient part of the information occuring at higher frequencies) by definition, they all possess good transient response. A wide-band drive unit with poor transient response is essentially a contradiction in terms, unless it has an apalling frequency response, with large holes all over the place.

Last edited by Scottmoose; 15th July 2011 at 02:11 PM.
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Old 15th July 2011, 02:43 PM   #3
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Really interesting Scott; thanks for the reply.
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Originally Posted by Scottmoose View Post
'Dynamics' is an often misused term; technically it simply means the dynamic bandwidth of the drive unit, i.e. how much headroom it has available to handle large dynamic peaks in the material. FWIW, if you're concerned about this, you may find the Volume Displacement (Vd) figure of a drive unit, which is Sd*Xmax to be useful. That tells you how much air the raw driver can potentially shift in a linear fashion.
So then a Mark audio driver will have better dynamics than a similar-sized Fostex driver by virtue of having greater xmax, and thus more ability to handle dynamic peaks? (Setting aside the variable of cabinet design.)
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Old 15th July 2011, 03:36 PM   #4
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Basically correct, but you need to account for the respective efficiencies of the different drive units as well.
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Old 15th July 2011, 04:10 PM   #5
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I don't quite agree with that. MMS and BL should indicate how fast the cone can move. A light cone and strong motor means the cone can be easily accelerated, all else being equal. This means it can retrieve little details that may be otherwise lost on more heavier sluggish cone/motor combinations.

With regard to dynamic capability, IMO efficiency is a more indicative parameter. And finally, Xmax indicates how loud the driver can play after that first watt.

How does all this play out in the drivers being talked about? The ANs are extremely good at detail retrieval. In fact, I'm willing to be they are in Lowther class. They also do dynamics pretty well, but that is only down to their high efficiency. If you don't listen very loud, or have a small room, or listen to only simple music, the ANs have plenty of dynamics and plenty of detail retrieval. But as you turn up the volume, the limited Xmax presents itself. I'm crossing them over at 300 Hz, but on Led Zepp, at loud levels, they simply cannot handle it. Distortion is too high.

If you instead took a B&C 8", ala Geddes, that marries high efficiency with decent Xmax, it would do far better than the AN when it comes to absolute loudness. Of course, it doesn't have the extension of an AN.

Fostex and other high-eff full range drivers could be classified in the same category. They will have some small differences because of whizzers, dust caps, surrounds, etc. But their sonic signature will be the same.

The other class of drivers are the MAs and Jordans, low efficiency but huge Xmax. The Alpairs are very linear over their total Xmax and low distortion too. This is immediately obvious. But they certainly don't have the liveliness and naturalness of an AN/fostex/Lowther. To me, they sound less dynamic and less musical compared to the high-eff drivers. Of course, if simplicity of build is taken into account, the MAs are much easier to get good sound out of than the ANs/Lowthers that basically cry out for horns.
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Old 15th July 2011, 06:07 PM   #6
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MMS and BL should indicate how fast the cone can move. A light cone and strong motor means the cone can be easily accelerated, all else being equal.

Bl/Mms is sometimes called the acceleration factor. It's correlated to efficieny but it's a fallacy to believe it's correlated in any way to the quality of the reproduction of transients by the cone. With IBs, in the piston range, cones having the same emitting area and delivering the same output SPL at the same frequency are submitted to exactly the same acceleration. If the mass of the cones is the same, the force on the cone is the same and if the BLs are different, it is the current in the coils which is different. The cone "sees" the same force, it does not "know" that the BLs are different. Great drivers can have low or high ratios of Bl/Mms, it is not a quality criterion.

Last edited by forr; 15th July 2011 at 06:22 PM.
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Old 15th July 2011, 06:56 PM   #7
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Efficiency is directly proportional to the square of the Bl product and inversely proportional to the cone mass (if I remember the formula correctly). Certainly, a lighter cone and stronger motor means greater efficiency, other things being equal. And what does more efficiency mean? Ability to play louder with less power? Yes. More headroom? Yes. Less power compression? Yes. And that to me generally means better sound. There are exceptions to the rule because there are a million different ways to make good sounding drivers.

If you look at nature, an increase in efficiency is generally accompanied by an increase in performance. That is not a hard rule, but it applies to drivers IMO. I've found that less sensitive speakers are the ones that sound dull, lifeless and are most certainly difficult to drive.
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Old 15th July 2011, 07:30 PM   #8
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It only means more headroom if you've got sufficient Xmax for a given application. You can have a reasonably high efficiency drive unit with almost no linear travel that will have less dynamic headroom than a slightly lower efficiency unit with far more linear travel.

Quote:
I don't quite agree with that. MMS and BL should indicate how fast the cone can move.
Logical fallacy I'm afraid. Acceleration is proportional to the current in the voice coil. No current, no acceleration, however light the powertrain or powerful the magnet.
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Old 15th July 2011, 07:53 PM   #9
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Hi Ra7,

Less power compression?

In domestic conditions, no study shows power compression

If you look at nature, an increase in efficiency is generally accompanied by an increase in performance.

Do you have an example ? I can't see how concepts of efficiency and performance exist in nature.

Last edited by forr; 15th July 2011 at 07:59 PM.
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Old 15th July 2011, 08:01 PM   #10
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[I]Do you have an example ? I can't see how concepts of efficiency and performance exist in nature.
How about that cat in your profile pic?
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