Neodymium and Ferrite motor structures on tweeter drivers

[felipe]

Hi everyone. I'm starting to learn a little bit on the motor structures of drivers..tweeters specifically. When I had my Pioneers I knew that those used ferrite magnets for better thermal capabilities, better efficiency,etc. With the Polks that I now have, I noticed they use Neodymium magnets. From the research I've done so far, Neodymium is more costly, and it's thermal capabilities aren't as good. Why would certain companies use neodymium and others use ferrite? What are the advantages/disadvantges of the two? Forgive my lack of knowledge as I'm still learning..

 

[TLS Guy]

Neodymium is an expensive rare Earth metal. It has a much greater flux density per unit if weight, and is less prone to demagnetization over time. This means that you get get much greater flux density in the gap, and therefore greater efficiency. It is lighter which reduces size and shipping weight.

These advantages give much greater flexibility is V/C gap design. Over all it is worth the extra expense, as the end result is likely to be a better tweeter in the higher end units.

 

[felipe]

Thanks for the info TLS Guy . With the greater flux density in the gap, what does that do for heat managment? I learned that some attach a heatsink for "better" thermal capability. Does that mean that neodmyium generates more heat? Does the xover frequency of the tweeter determine whether to use ferrite or Neo (or attach a heatsink to a Neo)??

 

[BoredSysAdmin]

I am curious, is this is an honest attempt at education or a fishing expedition which OP hopes to rate speakers based on type of tweeter motor?

Please notice the word "likely" TLS Guy Used.
He's coming from scientific background and he's correct that speaker using neodymium tweeter does not necessary equate to better speaker, but it could ....
Case in point are your Polk speakers, despite using better tweeter motor, it does not make them better speakers. If you search this site, you'll notice that in exception of a occasional troll, no one recommends to buy Polk.

If you love them - good - enjoy them, but my advice to you: don't try to judge speakers by their specs or even worse components - both can be useful to a degree, but also in same time very inaccurate and irrelevant.

 

[felipe]

This is not a fishing expedition BoredSysAdmin. I'm learning about the different designs and why some use one over the other for educational reasons and curiousity ONLY. I'm not trying to judge the Polks or any other speakers..only learn about them. Thanks for the advice anyways.

 

[DukeL]

With the greater flux density in the gap, what does that do for heat managment?

The greater flux density tends to translate to higher efficiency, which means less power is needed to reach a given SPL, which in turn helps to offset the reduced heat sink capacity of the neo motor.

The kind of thermal compression most people are familiar with is the kind that results from the motor heating up and losing strength. Less well known and less well documented is what I'd call "thermal modulation", a very rapid-onset (or "short time constant") type of power compression that is primarily a function of the voice coil heating up virtually instantaneously when hit by a burst of power, which causes its resistance to rise.

Floyd Toole had this to say on the topic, in a post in another thread on this forum:

"The audibility of power compression in its many variations probably could use some more research to define what is audible and what is tolerable. The magnet heating that you describe is important in pro audio sound reinforcement systems where the loudspeakers are required to work at or close to their design limits for long periods. Such heating and cooling has a very long time constant. This is not the case in most home systems. Although the modification of motor strength through magnet heating is a factor, most of the audible effects are from voice coil heating, which has a much shorter time constant. I just saw a test of a high-end audiophile speaker that in going from an average level of 70 dB (loud conversation, background music) to 90 dB (a moderate crescendo, or foreground rock listening) lost about 4 dB in output over about 3 octaves in the mid-high-frequency range. It became a different loudspeaker at different listening levels."

If I understand correctly what Toole is saying here, rapid-onset voice coil heating is probably more of an issue in home audio than is magnet heating. So assuming the same thickness of voice coil wire, the more efficient neodymium-motor driver should exhibit less thermal modulation due to voice coil heating for a given sound pressure level.

That's probably a much longer answer than you were looking for.

 

[felipe]

Thanks for the descriptive explanation DukeL . You know your stuff .

 

[TLS Guy]

Thanks for the info TLS Guy . With the greater flux density in the gap, what does that do for heat managment? I learned that some attach a heatsink for "better" thermal capability. Does that mean that neodmyium generates more heat? Does the xover frequency of the tweeter determine whether to use ferrite or Neo (or attach a heatsink to a Neo)??

Well if you have a lighter smaller magnet it will adsorb less heat unless the material has a much higher specific heat. Heat generated depends in the power consumed. This depends on efficiency and crossover frequency. The lower the crossover frequency the higher the power consumed by the tweeter and the larger the quantity of heat generated. Rare Earth magnets tend to higher efficiency and therefore less heat generated for a given spl.

Tweeters these days have ferrofluid in the VC gap. This is very effective at transmitting heat from the VC to the magnet. Obviously if the magnet is small and light, then a heat sink can help.

The lowest crossover point of a tweeter is largely determined by free air resonance and total Q (Qts) and also the mechanical limits of the driver which includes VC heating and therefore thermal compression issues.

Unless a tweeter has very low Qts, it can not be driven through Fs.

 

[felipe]

Very good info ...Thanks again TLS Guy . I guess that would(to some extent) explain the high efficiency of the Polks I have.
Further research into the Polks reveal that the crossover for the tweeters are at 2.5khz (12db/octave slope). Is this considered a high crossover point or average??

 

[TLS Guy]

Very good info ...Thanks again TLS Guy . I guess that would(to some extent) explain the high efficiency of the Polks I have.
Further research into the Polks reveal that the crossover for the tweeters are at 2.5khz (12db/octave slope). Is this considered a high crossover point or average??

It is a very average crossover point.

 

[felipe]

If you search this site, you'll notice that in exception of a occasional troll, no one recommends to buy Polk.

What's wrong with Polks? I find them to be open, balanced and natural sounding. Granted, I wouldn't pay full price for them as there are better options at those price points, but if you find them either used on CL or at a good discounted price on online retailers IMO they are a great bang for the buck . I guess you can consider me the "occasional troll" .

 

[BoredSysAdmin]

What's wrong with Polks?

Several years ago I went to audio store and auditioned every single model Polk bookshelf they had in stock - about 6-8 and I honestly went from hate (lower models) to indifference, then to "very nice" at the top end of range, which was LSi/9. Now you should know how expensive LSi/9 is and here's the kicker.
I know, not guess, than one could get as good or better sound than lsi series at significantly lower price point, Ascend CMT340SE for example - half the price and much easier to drive. or even new Dennis Murphys http://philharmonicaudio.com/aa.html monitor - only a bit under $200/pr

 

[felipe]

Several years ago I went to audio store and auditioned every single model Polk bookshelf they had in stock - about 6-8 and I honestly went from hate (lower models) to indifference, then to "very nice" at the top end of range, which was LSi/9. Now you should know how expensive LSi/9 is and here's the kicker.
I know, not guess, than one could get as good or better sound than lsi series at significantly lower price point, Ascend CMT340SE for example - half the price and much easier to drive. or even new Dennis Murphys http://philharmonicaudio.com/aa.html monitor - only a bit under $200/pr

Like I said perviously, I wouldn't pay full price for Polks. I have not heard any of the other Polk models, but I do agree that the LSis are overpriced for what they are.

 

[highfigh]

Like I said perviously, I wouldn't pay full price for Polks. I have not heard any of the other Polk models, but I do agree that the LSis are overpriced for what they are.

If you go to a site like Parts Express to look at tweeters, you may notice that several of the Neo models have a heat sink on the magnet. I can only assume that this sheds more heat than a plain magnet but in a small cabinet, the total heat lost is limited unless the air can be exchanged or caused to move enough to increase the loss.

 

[highfigh]

If you go to a site like Parts Express to look at tweeters, you may notice that several of the Neo models have a heat sink on the magnet. I can only assume that this sheds more heat than a plain magnet but in a small cabinet, the total heat lost is limited unless the air can be exchanged or caused to move enough to increase the loss and at some point, the rate of loss will begin to slow. By that time, the problem should be audible.

 

[DukeL]

If you go to a site like Parts Express to look at tweeters, you may notice that several of the Neo models have a heat sink on the magnet. I can only assume that this sheds more heat than a plain magnet but in a small cabinet, the total heat lost is limited unless the air can be exchanged or caused to move enough to increase the loss.

The heat originates in the voice coil, so assuming equivalent voice coils and equivalent efficiency, the amount of heat that's generated would be the same, but since the neo magnet is so much smaller, its temperature rises faster. And yes the fins are to help it dissipate the heat faster into the surrounding air. I would guess that a finned neo motor heats up the air inside the cabinet a bit faster than a ceramic motor (which would have more surface area and more thermal mass than the small finned neo motor).

Small sealed boxes are the worst from a heat management standpoint, as there is no air exchanged with the outside world and the air is not being circulated well within the box. The best air circulation would be from a port whose airflow is turbulent (chuffing), and the best exchange of air with the outside world probably comes from having two chuffing ports, one below the woofer's magnet and one above, to facilitate the rising hot air exiting through the upper port, thereby drawing cooler air into the lower port. Chuffing is of course undesirable from a sound quality standpoint, so the next best thing would probably be non-chuffing ports positioned high and low. I like to take advantage of this "chimney effect cooling" where possible in high-power applications, like prosound.

An even more effective heat management solution is the Heat Pipe invented by Wayne Parham, which he uses in his prosound horn subwoofers. Basically it's a large thermally conductive (copper or aluminum) pipe that conducts heat away from the voice coil area to a large aluminum heat sink attached to the outside of the cabinet.

 

[felipe]

Another informative post DukeL . I don't believe the neo magnets have heatsinks on the Polks. All that said, is it possible to attach a heatsink to the neo magnet?? If desired??