Can the BL of a driver be increased?

GestaltH

Member
2008-01-21 6:44 pm
Hi,

I was looking around for good and cheap horn drivers. I've found some that are pretty good such as the Eminence Alpha-6A:

http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=290-400

I has high BL for its size, low mms, and low Le, but I was hoping to get the BL even higher. I have heard that adding a bucking magnet will increase the sensitivity a little bit, but it seems like attaching a magnet in opposite polarity would actually decrease the magnetic field. Am I thinking about that right, or is there some effect where it actually increases it? Even if it increases it by, say, bending the magnetic fields to where they are more useful(which seems unlikely to me because of superposition), wouldn't it be even better to put one on with the same polarity ? Is there some reason that wouldn't work?

I'm aware that adding more magnets will change some of the other parameters, but which ones? Also, is there a cheap way to test a driver for its parameters?

How about just swapping out the magnet for a more powerful one? Would that be reasonably possible?

Thanks for any info.
 

ricobasso

Member
2007-07-12 9:24 am
A bucking magnet on external ring designs (rather than internal pot) would work by reducing the external leakage of the main magnet. Whether any of this reduced leakage ends up usefully in the voice coil gap is another matter. That would depend on how close to saturation the steel pole pieces are in the first place. Fitting a magnetised magnet to another is not easy. Beware of pinched fingers. :bawling:

Short answer; try it and see.
 

GestaltH

Member
2008-01-21 6:44 pm
critofur said:
If you "compress" a magnetic field it will yield higher strength:area ratio.

A bucking magnet could have some effect (in doing that, giving you a higher BL), but, I'm guessing not significant in this case?

The thing that wasn't making sense to me about this was that magnetic fields obey superposition, so you can just add the vectors at any point to get the resultant vector. If you add the vectors in the gap from the original magnet and a bucking magnet, don't they tend to cancel and not add? The whole idea of a bucking magnet is to cancel fields.

ricobasso said:

Short answer; try it and see.

I think that I will have to try it. Empirical data would probably be better than theory in this case.

sreten said:
Hi,

The short answer is an external magnet will gain you less than a dB.
Changing the magnet to a bigger one ? how would that fit ?
The easiest route is buying the correct driver in the first place.

http://eminence.com/proaudio_speaker_detail.asp?web_detail_link=LA6-CBMR&speaker_size=6&SUB_CAT_ID=2

[IMGDEAD]http://eminence.com/images/la6-cbmr.jpg[/IMGDEAD]

This goes up to 11 (said nigel .....again)

:)/sreten.

I'm not sure how I would get another magnet to fit. I don't have any drivers nearby at the moment to figure out if a swap would be possible. There are two reasons that I decided not go with the driver that you suggested. The first is that the Fs is well into the region where I was planning to use it. The second is that the xmax is only 0.2mm and I wanted to use it to 100 hz or lower.


wg_ski said:
The Alpha 6 does a respectable job to 3KHz as-is if you're not looking for the ultimate 110dB/W efficiency. On an exponential flare you should get to 3k if you're ok with 100dB/W.

Thanks for the advice. I was planning to have it be closer to 105db/W. I agree that the drive should do a good job by itself. I was just interested in seeing if I could improve it at all. I only need it to go to about 1500-2000Hz, so I should be able to pick up some efficiency from a smaller bandwidth. I might end up just using them as is, but it would be interesting to see what is possible.
 
It is easy to fit another magnet if you want to; just put a little epoxy one side of the piggy back magnet and plonk it on, the difficuly is trying to make sure that the added magnet is simmetrical.
Bucking magnets feilds opposed, piggy pack magnets feilds aligned.
I tried it a while ago with salvaged magnets, you can hear a very small difference; as Sreten said only +1dB, but sometimes plus 1dB may be worth it. Do a cost benefit analysis, maybe you have some old buggered drivers you can get the magnets off to try
Regards Ted
 

GestaltH

Member
2008-01-21 6:44 pm
Moondog55 said:
It is easy to fit another magnet if you want to; just put a little epoxy one side of the piggy back magnet and plonk it on, the difficuly is trying to make sure that the added magnet is simmetrical.
Bucking magnets feilds opposed, piggy pack magnets feilds aligned.
I tried it a while ago with salvaged magnets, you can hear a very small difference; as Sreten said only +1dB, but sometimes plus 1dB may be worth it. Do a cost benefit analysis, maybe you have some old buggered drivers you can get the magnets off to try
Regards Ted

Ok cool. Its nice to hear from someone that has tried it. The piggy back method does work then (if only a little). I will definitely try this. Was the setup that you used this augmented driver in horn loaded? The reason I ask is that I wonder if it would have more of an effect when in a horn.

Another question, how would I go about measuring the BL? For that matter, how do I measure the rest of the parameters too?
 
Those questions I can't help you with, I'm not an experimenter, I'm a fiddler and tinkerer, when I tried it I was trying to get a little more performance from some cheap woofers, sealed boxes, and while it did work to some extent i ran out od X-max very quickly, perhaps better suited to boosting a Mid where it will be used below the resonant frequency.
To date i haven't tried it in a mid, some-one more skilled than I am may be able to answer that part of the query.
Regards Ted
 

ricobasso

Member
2007-07-12 9:24 am
Originally posted by GestaltH

The thing that wasn't making sense to me about this was that magnetic fields obey superposition, so you can just add the vectors at any point to get the resultant vector. If you add the vectors in the gap from the original magnet and a bucking magnet, don't they tend to cancel and not add? The whole idea of a bucking magnet is to cancel fields.

It's not a simple as that because the magnetic properties of steel are non-linear. There is often a bottleneck in the magnetic path which includes the voice coil gap. The steel will be close to saturation in this bottleneck so any attempt to apply more magnetomotive force will result in saturation. At this point the extra flux will ignore the steel and will complete its loop in the air. An air path of course is not restrained and no flux concentration can take place. This means that you will get a minimal effect in the vc gap. It may be that the only effect sought was to reduce the external field to make the speakers more home theatre crt friendly. In that case the extra magnet will have done its job and all is well.

Without a magnetic simulation software package you can't predict anything. I've spent many hours designing loudspeaker magnets with such a package and never failed to be surprised by the results with bucking magnets and screening cans. Without a software simulator, you will only be able to "suck it and see".