Best audio perfomance from 8Ω or 16Ω loaded compression drivers?

[JeffW]

Ah, found it, post #106

There is one thing which has always been apparent and is again shown by some studies with these and some other compression drivers- neodymium magnet structures always skimp on the magnet. There are several reasons for this but suffice to say neodymium is never worth the money...so far. These neo transducers seem to almost always lead to much less than ideal results than their ceramic magnet cousins and could lead to false conclusions about driver usefulness and behavior. Use the ceramic for more accurate understandings until the neo magnet type are of the same or better performance than there presently superior cousins.

Not sure about the guy's credentials, and all he ever linked to was Parts Express cut sheets. It'd be interesting to hear what JBL's transducer engineers have to say on his opinion.

FWIW, JBL lists the 2446 and 2450 as both being 19,000 gauss. I guess you'd need to measure it to see if they skimped on the 2450.

[Eaulive]

I won't read the whole thread again but the neodymium magnets have more power per pound than ceramic and even alinco, having them smaller is not to "skimp on the magnet" this notion is wrong.

When they switched from alnico to ceramic, this was a good opportunity to skimp, some did maybe, JBL didn't that's why we ended up with woofers twice the weight.

At the end the important thing is the flux inside the gap.

[1audiohack]

At the end the important thing is the flux inside the gap.

A possible consideration is flux stability or the magnetic hysteresis of the motor. One could argue it's field coil, AlNiCo, ferrite, then Neo, and sheer mass may be a part of that. Not my speciality, just curiosity.

[Eaulive]

One could argue it's field coil, AlNiCo, ferrite, then Neo,

You mean from best to worse?

[tomt]

Ragnar Lian, co-founder of Scan-Speak,
said that speakers with alnico magnets had 10x less distortion than ferrite,
and neodymium had about 20x less.
If a copper ring is added to a ferrite magnet,
it can approach neodymium specs but only above a certain frequency,
below which the ring becomes ineffective.

http://audioroundtable.com/Speaker/messages/223.html

Greg Timbers on magnets -

Ferrite magnets do not demagnitize with time or drive. They are affected by temperature but that is reversible. They will return to normal when they return to room temperature. Ferrite is basically a lousy magnet material for speakers but it is cheap and readily available. JBL has done a ton of things within the magnetic circuit to make the material behave in a more stable manner. At 100 degrees F, a Ferrite motor will be down about 1.5 dB in level which means the midband of the woofer will be lower by that much and there will be increased output around the system resonance. The TS parameters will be completely different - as though the BL was reduced by about 18%.

Alnico magnets, by their nature are easy to demagnitize with drive. They will not change with time and their dependence on temperature is really small - maybe 1% at 100 deg.F. Alnico stability and resistance to back EMF is really good. This is why they make the best sounding magnetic structures. Unfortunatelly, given a big enough pulse of magnetic energy, they will demagnitize by up to 3 dB. The sensitivity to demagging is dependent on the specifics of the magnetic circuit and the length of the coil providing the field. Underhung woofers (LE15 and such) midranges, tweeters and compression drivers do not have sufficient back EMF fields to push the operating point of the structure below the knee. They are essentially stable regardless of input signal. The short gap-long coil speakers are the ones that have a problem. A 2235 can take a hit of up to 3 dB if a big enough hit of current takes place. 1.5 dB to 2 dB is more common. The effect does not get better or worse with time, it solely depends on how much current is driven through the coil. The more current, the more field. Once the field is bigger than a certain number, some amount of demagnitizing occurs. It is perminent (until externally recharged) and will only increase if a larger sustained current hit occurs.

Therefore, if you have a qualifying alnico woofer and you have played it loudly you have some damagging. You can have the unit recharged and it will be fine until you play it again. Exceed the critical level and it will start happening. If you never do, it won't ever demag. Most of these designs trace back to the 50's and 60's where 15 - 30 watt tube amps were the rule. They didn't have the current capability to hurt anything. With the advent of big solid state amps, the current levels went up and the problems started to surface.

Most of the qualifying 4" motors will loose 1 - 1.5 dB unless they are pummeled. Some of the older 3" with really short magnets, like the 2213A and 123A will typically be around 3 dB down. They go really easily. The old decade woofers (116A and 127A) only had to see an amplifier in the room and they got really nervous. FYI, the new 1500Al used in the S9800 can take continued pulses of 5000 watts and loose no more than 1%. The test can only be done a few times before the coil is destroyed, but the magnetic assembly is totally stable.

http://audioheritage.csdco.com/vbull...522#post110522

[1audiohack]

Yes, from best to worst. It's not my held theory but I have read it. This could very easily go somewhere I didn't intend and I just don't want to go.

I am looking to quantify if possible, to show and compare with hard numbers and graphs, what it is that I discern by listening in hopes of getting the absolute most out of what I have and can afford.

I also have a bent for absolutely having to know how things really work. Sometimes it's a curse, I am not an engineer or scientist and as such have the expected limitations. A problem with our hobby is anecdotal information is prevelent, but real knowledge is often much more rare than many are willing to admit. Some answers are not easily found in the general literature so I search still. I had hoped that somehow there was a solid answer to the original question and that I had just missed it.

Hopefully I will have something usefull to add in the future, for now I have far more questions than answers so I'll pipe down.

Thanks all,
Barry.

[JeffW]

Don't stop on my account, you never know what'll pop out of a discussion. The way I look at it, JBL wouldn't waste a chance to make a 476Be with something other than neo if something else would have been better. When you make a $$$$ compression driver with a pretty small production number, cutting corners on magnets doesn't really make any sense. The D66000 weighs over 300lbs each as it is, surely a few more pounds of magnet wouldn't have made them scrap the design.

It doesn't really matter to me in a practical sense, I'm stuck with neo-based drivers now anyway. They'll just have to be good enough

[1audiohack]

Oh I'm not going to stop. I'm just getting started.

[Mr. Widget]

I think the design integrity and quality of manufacture are more important than the material the magnet is made of... as Ragnar Lian and GT suggest in the post above some of the short comings of ferrite can be over come by clever engineering as can the short comings of alnico... even the inherent design of the driver affects this, (GT's comparison between the alnico mags used in the underhung vs. overhung designs)... another data point, TAD uses the same diaphragm in the 4001 and 4002 compression drivers... the 4001 is alnico and significantly costlier than the 4002 which uses neodymium. So, is neodymium only meant for a lighter and cheaper design in TAD's belief? No, their TOTL 4003 is also a neodymium driver... I think it is safe to say that simple ratings of different drivers based on their magnetic materials is a pointless exercise.


Widget

[pos]

The 2452 would also be an interesting case to study: it has the same flux density and specifications than the 2451 (also neo) but half the weight!

[Robh3606]

There is one thing which has always been apparent and is again shown by some studies with these and some other compression drivers- neodymium magnet structures always skimp on the magnet. There are several reasons for this but suffice to say neodymium is never worth the money...so far. These neo transducers seem to almost always lead to much less than ideal results than their ceramic magnet cousins and could lead to false conclusions about driver usefulness and behavior. Use the ceramic for more accurate understandings until the neo magnet type are of the same or better performance than there presently superior cousins.

Don't put too much weight in some guys statement on the Internet. There is nothing wrong with Neo, Alnico or Ferrite that a good transducer design cannot work around. You need hard data and it would be very foolish to think JBL or Tad don't already have it. I have read many times that Neo is Superior to alnico and ferrite. From my point of view I don't care as I have drivers using all three materials that all "sound" quite good in actual use.

Rob

[4313B]

Don't stop on my account, you never know what'll pop out of a discussion. The way I look at it, JBL wouldn't waste a chance to make a 476Be with something other than neo if something else would have been better. When you make a $$$$ compression driver with a pretty small production number, cutting corners on magnets doesn't really make any sense. The D66000 weighs over 300lbs each as it is, surely a few more pounds of magnet wouldn't have made them scrap the design.

It doesn't really matter to me in a practical sense, I'm stuck with neo-based drivers now anyway. They'll just have to be good enough

Yep.

While there is an obvious advantage to "AlNiCo done right" in the case of the 1500AL, 1500AL-1, and 1501AL, JBL must not have thought that the same advantage existed for the 476BE. The only cause for pause seemed to be a four slot phase plug versus a five slot phase plug. We all know how that worked out - we have a four slot phase plug. While the 476BE doesn't have the mass that the 2441 sports, it is definitely not merely a scaled up 435BE.

The 2452 would also be an interesting case to study: it has the same flux density and specifications than the 2451 (also neo) but half the weight!

If I remember correctly JBL stated that the 2452 was simply a 2451 manufactured with lighter, less expensive materials. Same MSRP though.

[1audiohack]

I think it is safe to say that simple ratings of different drivers based on their magnetic materials is a pointless exercise.
Widget

I absolutely agree.

[1audiohack]

You need hard data and it would be very foolish to think JBL or Tad don't already have it.Rob

I agree completely again.

So, riddle me this gentlemen, I have measured the D16R2441's in all four pairs of the test drivers and the SPL variations and the impedance peak magnitude at and below 700 Hz is best behaved in the 2441, a little worse but line on line in the 2445 and 2446, and markedly worse in the 2450. Is it irrational to wonder why?

[yggdrasil]

I agree completely again.

So, riddle me this gentlemen, I have measured the D16R2441's in all four pairs of the test drivers and the SPL variations and the impedance peak magnitude at and below 700 Hz is best behaved in the 2441, a little worse but line on line in the 2445 and 2446, and markedly worse in the 2450. Is it irrational to wonder why?

What about the size of the rear chamber?