Why Not Ferrofluid In PA Woofers?

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
With the emphasis on SPL per box size that PA speakers have, it seems a natural. When amplified equipment first appeared many years ago, watts were expensive and voice coils couldn't handle that many anyway. So the PA boxes had to be large and comparatively sensitive.

Now, with greater emphasis on SPL and bass, and watts comparatively cheaper to buy, why don't PA 12s and 15s have ferrofluid to cool them? You get more SPL or more bass extension without having to enlarge the boxes you're lugging around if you have ferrofluid cooling the voice coil.

The major objection to ferrofluid is that some think it doesn't sound right on expensive home equipment, but in PA you're not that worried about subtle difference.

It would make sense for all or most PA speakers to have ferrofluid, but they don't. Anyone know why?
 
Kelticwizard,
There are PA speakers that use Ferro Fluids, but you have to remember that in-order to use it correctly the drivers need to be designed with the fluid in mind. Proper gap design and venting is important to keeping the fluid where you want it to be. Just putting fluid into a driver that was not optimized for ferro fluid can be a waste of time. It is very easy to hydraulically push the fluid out of the gap, without venting below the fluid this is what will happen, the fluid will be pumped out and end up above or below the gap and then it loses all effect. There are many different viscosity's available and this must be balanced with the design and intended frequency response required. This is the simple answer to your question.
 
Changes

When selecting a ferro-fluid, the optimum values of viscosity and saturation magnetization for a particular transducer design must be determined. However, the quantity of fluid to be dispensed into the magnetic air gap in some ways may be the most critical value; since variations in ferro-fluid quantity influence thermal power handling, the impedance curve, and frequency response characteristics of a loudspeaker.

Over time, all these values will certainly change, due the effects of heat and the tearing action of large and quick voice-coil excursions occurring in the confined space of a magnetic gap. (The Silly Putty phenomenon is at work here). Given the high duty-cycles imposed on drivers used in P.A. venues, the use of ferro-fluids would seem to be contraindicated due to the accelerated fluid deterioration that may be expected.

Regards,

WHG
 

Brett

Member
2002-01-07 6:02 pm
You could get even better performance and longer life from drivers by designing or modifying them to have a heatsinking system that passively or actively removes waste driver heat to the outside of the box.

A passive approach was detailed by Wayne Parnham years ago, here and with more links here.

Bossobass also did active (fan driven) cooling of HT subs in his Raven.
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Well, I haven't had time to thoroughly read and digest all of the links and information here yet, (I'm slow that way), but I would just like to say how delighted I am with the wealth of info the posters have given here.

For instance, I now know that one reason few woofers have ferrofluid is that high excursion tends to throw the fluid out of the gap unless parts are machined to prevent this. I would love see the machining changes that prevent this, if anyone knows.

I did not know that viscosity and other parameters of the fluid would change more with high excursion drivers, therefore changing the parameters of the drivers themselves.

Finally, the idea of a heat sink for the pole piece is one that I thought of before, even looked up the thermal conductivity of permalloy, which is excellent, beforehand. But it is good to know that someone has already done it and to see how.

If anyone knows where I can find out about the machining changes to keep the fluid in, I would be grateful. But the heat sink idea looks like something a DIYer can do if he plans it out. Unless I am mistaken, I think there are some piezo driven fans that used to be available in the aftermarket for the old MacIntoshes way back when-as I recall the hotter they got the more power the fan put out, not that it was much.

I'll see if I can across something similar to those old piezo fans.

Much thanks for the links and info.
 
Hi keltic ..
Maybe instead of a liquid an inert gas could be used to more efficiently get the heat from the voice coil to the magnet structure.. Another thought , Insted of a heatsink attached to the magnet structure how about some form of liquid cooling with an external radiator and fan. Many years ago at a proffesional audio show there was a bass horn using a large servo motor to drive the cone this they claimed virtually eliminated power / thermal compresion . The speaker was built by a company called Comunity loudspeakers..

Best Regards Mark

Ps the said speaker was good to around 200 Hz... And finally aluminium alloy coned drivers maybe would help getting rid of some heat directly from the voice coil. as long as the coil former was constructed from a good heat conducting material that is ..
 
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A couple of points here. One is that it is next to impossible to modify a speaker after the fact to work well with ferrofluids. The problem becomes that you need to drill holes at the least to vent the magnetic assembly below the voicecoil and the magnet being charged would hold any metal chips in place and have fun getting them out of a charged magnetic circuit. The changes are really simple in that the basic structure can stay the same but you want a vent below the gap so that there is not a closed chamber below the voicecoil, much like a vented pole piece vents the air behind the dust cap. Another way to cool a magnetic circuit that is often over looked would be to add a heat pipe to the assembly, again hard to do with a magnetized speaker but it could be added to a vented pole piece. Heat pipes transfer heat from one place to another like air through a closed fluid in a sealed tube. Look up heat pipe or heat transfer pipe and you will see what I am talking about.
 
A couple of points here. One is that it is next to impossible to modify a speaker after the fact to work well with ferrofluids. The problem becomes that you need to drill holes at the least to vent the magnetic assembly below the voicecoil and the magnet being charged would hold any metal chips in place and have fun getting them out of a charged magnetic circuit. The changes are really simple in that the basic structure can stay the same but you want a vent below the gap so that there is not a closed chamber below the voicecoil, much like a vented pole piece vents the air behind the dust cap. Another way to cool a magnetic circuit that is often over looked would be to add a heat pipe to the assembly, again hard to do with a magnetized speaker but it could be added to a vented pole piece. Heat pipes transfer heat from one place to another like air through a closed fluid in a sealed tube. Look up heat pipe or heat transfer pipe and you will see what I am talking about.

Liquid cooling.. just what i said
 
You could get even better performance and longer life from drivers by designing or modifying them to have a heatsinking system that passively or actively removes waste driver heat to the outside of the box.

A passive approach was detailed by Wayne Parnham years ago, here and with more links here.

A bit like Volt does with their radial drivers?
 
Been Done for Years....

Many of the models up and down Community's line are Ferrofluid Cooled. The Solutions Series in particular comes to mind. The sound very good and get unbelievably loud. I think the SLS960 can hit over 140 dB SPL. Even their old CSX series cheapo boxes had FF cooled woofers if memory serves.

B
 
davidsrsb,
Where do you come up with the solidification information for a ferro fluid? I find that hard to understand from a basic chemical process. The fluid is an oil and unless there was a loss of a component due to evaporation or some secondary source of contamination how do you turn an ester based oil into a solid? Do you have actual experience of this or is this anecdotal information you are reading somewhere? I am very curious where this information is coming from, there are way to many devices with ferro fluids in commercial use to believe this is a common occurrence.
 
I'm unaware of hardening of the synthetic "oil" base in ferrofluid.

Someone could inquire of Ferrofluidics or whatever the name of the company that bought them is...

As far as cooling using pole pieces, this is common practice now for most Neodymium drivers since the neo magnets do not do well with heat.

There is probably something online about the application of ferrofluid in woofers. No doubt underhung VC designs would work better than over hung high xmax designs. :D

_-_-bear
 
Bear,
He is the link to the only company that I know of doing it in the USA. I assume that it is a rumor myself that the fluids dry out. I think it is more likely that a poor designed motor assembly allows the fluid to be pushed out of the gap and then the voicecoil burns and sticks to the pole piece. I would think you would have to overheat the fluid by a large factor to break it down to the point of failure. The new Corvette and Cadillac and Ferrari uses ferro fluids in the shock absorber, would they do that if it was going to harden like that, and I can assure you those shocks get damned hot in use.

www.ferrotec.com/products/ferrofluid/audio
 
PeteMck,
One thing we learned in my college classes is that Wikipedia is an unreliable source of information and none of that information is vetted or confirmed. Anyone can add to Wiki, I could write just the opposite, that is just an opinion. I am not going to say that there may not be some bad ferro fluids out there from who knows where, but that is not American made ferro fluid doing that.
 
My TDLs had one tweeter with a completely locked dome and one locked at one point on its coil. Frequency response was vary odd with no lower treble and a strong peak in high treble. The harmonic distortion measured very high. Careful dismantling and wiping out the very sticky ferrofluid got the tweeters going again with reasonable measurements.
My Mission 752 tweeters have similar high harmonic distortion and loss of low treble, but I have not managed to dismantle the tweeters yet to inspect the coils. These speakers have NEVER been played very loud.
 
Davidsrsb,
The real question becomes where is the fluid that is used in these devices made? That is the real question. In the USA you can only use the fluid from Ferrotec, it is protected by patent. So my first response is that the fluid is a knockoff and though it may be a ferro fluid it has a problem with the chemistry. There are too many companies in the US that would be screaming if this was the case here as the warranty problems would be in the millions of dollars. There are to many speakers here that are decades old without the problems you speak of. I see the kits to inject your own fluid into a speaker and the price gives them away, they are priced below what you can purchase the fluid wholesale from the manufacturer here. There is a reason for that. This is counterfeit materials. I would bet money on that.