Speaker magnets

Vintage Altec's and JBL's used large magnets for high density magnetic flux. Better control of voice coil movement! This is important for bass drivers which have large cones that need sufficient current from amplifiers to control movement!

@jasonbourne52

I have a pair of 8" Kevlar speakers with magnets that are 2/3 the diameter of the speaker, so presumably they will be better at low frequencies than my pair of 8" vintage Wharfdales that only have around a 2" diameter magnet, or is it more complicated than that? I can’t test as they are just the speakers, no cabinet. The reason for the questions is that I have access to a variety of speakers from 8" down, and thought I would do some reading (well a lot) and have a go at building some of them into a cabinet just for the fun of it, then experiment from there.

OP if you have any information on the drivers, that would help YOU build the correct size enclosure.

Second if you don’t have or can’t find the specs on your drivers, there are tools to help figure out certain thing about drivers. Parts Express has some great software and package deals to fix you right up..

Yes magnets wear out. In your lifetime probably not..
Some flake apart, they can loose their mojo, BUT there are some tricks..
There are ways to revitalize the magnets and even change them if you know what your doing.

Sound crazy, but I’m careful around very strong magnets or just a bunch warehoused in tight quarters. Weird vibes so to speak.. LOL

Regards

@oldhvymec3
I’ll have to go with your investigation suggestion as the speakers have nothing on them besides the Wattage and Ohms.

The 8" Kevlars are mental, if you get them stuck together they are one hell of a job to separate.

I believe the stronger the magnet the more efficient the speaker.
Also of interest is a technology called "Field Coil" speakers.
This technology uses the current from your wall outlet to create the magnetic field eliminating the magnet.
From dagogo.com: 
"A field coil speaker uses two coils; the voice coil and a field coil. Instead of a magnet, DC is applied to the field coil creating a magnetic field. This takes the place of the permanent magnet in creating the fixed magnetic field."

I have a pair of 1980s Gale GS302 speakers that have damaged woofers, the tweeters are okay. They are a two way system + a passive radiator. I have stripped them out and they are made of nice thick material with plenty of bracing and doubling up inside.  I may go with them for a first project, I have had thoughts about them before. I had a plan to fit a midrange where the passive radiator was, but inside its own isolated stuffed tube, somewhat like the B&O S45 II. Then make a cut out in the rear and fit a modern passive radiator at the back. Use the 8" Kevlars and a pair of 6.5" for the mid range, then connect them up with a decent quality crossover unit. They would look a little odd as the woofer is mounted in the centre. That means it would go tweeter, woofer, mid range from the top down. I have no idea if swapping the normal woofer mid range layout would have a detrimental effect.

@dweller

To my mind that would just introduce the possibility for another source of noise?

Driver motors, of which the magnet are the key part and the coil being the other, are essential for controlling a driver's motion.  The two are the "main dance" pair, with nearly infinite possibilities of what these two can do.  These motors can lower distortion or improve efficiency, give more output or reduce non linear behavior. Its the engine of the speaker, so it can be optimized in a million different ways to achieve what you want.  Just as there is no single or universal "best engine" for every vehicle or machine, some types of motors work out better for different uses.  A diesel with high torque at low RPM vs a gasoline motor with more horsepower at high RPM is a good parallel for the concept.  So driver motors vary a lot in their "goal".  It is not a simple device.

Field coils were the first drivers, I have a 1939 Stromberg Carlson Radio with a field coil true coax speaker,  mounted to a transmission line.  It was high end audio in 1937 to 1940 or so.   Field coils are not new or a step forward, but a throw back.  From an engineering perspective, Neodymium magnets and other super rare earth magnets are what's being used now to design the new generation of driver.  Smaller, lighter and more powerful.   Its what makes the speaker in your iphone or your computer work.

Brad

Check out the magnets on ATC speakers.  The you tube video of the factory is informative as well.

Magnet material plays a role as well.  

this may help

https://www.tonmeister.ca/wordpress/2014/01/31/bo-tech-how-to-make-a-loudspeaker-driver-a-primer/

Field coils have been around a long time.  They add a lot of weight to motors and generators/alternators.  A 150lb starter of old is 40 lbs at most now. Fixed neo magnet starter motor, the solenoid is almost as big around as the motor.

They say FC speakers are some of the best ever made.  They have to be expensive to make too. Interesting critters!!

Regards

Generally the bigger/higher gauss the magnet has, the better. Yes over time the permanent magnets do get weaker, but they can be reenergized. I still have some Altec and JBL from the 60-70's, great speakers, Alnico magnets. The newer Neodymium magnets are great and very hard to demagnetize, expensive, but a little goes a long way. Look at the gauss rating if specified, helps control the cone movement. 

An FC mag puts heat around the VC something you don't want. It adds needless complexity and for some strange reasons sounds the best powered by a Tungar DC power supply. The older FC designs were overbuilt but I wonder if the heating-cooling cycles an FC produces would affect the longevity of the FC in modern FC transducers. That being said FC, Alnico, rare earth, Iron ferrite mags all can sound fine depends on the overall design of the transducer and imprementations when used in a finished loudspeaker.

There are magnets and then there are magnets.  The older speakers either used Ferrite or Alnico magnets.  The alnico magnets were used in some of the better speakers at the time, like JBL.

From what i understand the size is not necessarily the thing that make it better. For example High Q driver used in Open baffle designs can have small magnets but smaller gaps in the voice coil (closer tolerances from what i understand are part of what makes a higher efficiency driver hi efficiency).
 Magnets got really big in the desire for big bass from small drivers to get the control of the cone and maximize excursion (why small speakers usually need lots of power and damping). Large drivers may not need the excursion so the magnets may be smaller. so magnet size is not a factor in sound quality from my experience, its all in the overall design and intended use. 

Maybe a speaker designer will chime in and clear some of this up. 

Thanks guys,  there is plenty of info here to get me started in the right direction.  Looks like I have a bit of reading, accompanied by a fair bit of trial and error, good job I have plenty of free time :^)

There is always a trade off.  The magnet needs to match the designers goals.  Large magnets do increase efficiency.  They also roll off and limit low frequency extension (all other variables being equal).

Most modern speakers do not have anywhere near the size magnet or magnet strength of a good vintage speaker at all, nor anywhere near the voice coil size and type and amount of wire around the coil, which gives the vintage type driver a tremendous advantage in efficiency and control over cone motion. They use a type of magnet that does not deteriorate over time as well as a fiber surround that does not deteriorate either. I have 70+ year old vintage models that work and sound like they were made yesterday because of these reasons.

Plus 1 on weirdness and magnets. Can be very creep and/or surreal. Normal stuff like a fire truck using its flashing lights (no sound) and coming up the mountain road right by you, seems like a religious experience. But at least you realize why it’s happening (well placed warning signs that recommend limited exposure times). 

Bigger is not always better in woofer magnets. You need the right amount of gauss(related to magnet size) to give the right driver specs for the bass enclosure design to produce the bass control and roll off desired. Too small a magnet and you get boomy bass. But too big a magnet and you get over damped bass which starts to roll off too high and you end up with less bass.

The OP will need the Thiel-Small parameters for his drivers to design cabinets for them. These equations supersede the old cut-and-try method.

The Thiele-Small parameters are equations that will calculate the optimum box size for a particular woofer. Look up "How to measure Thiele-Small parameters". No need to futz around with old trial-and-error methods!

Tannoy still uses Alnico magnets in their top tier speakers....

I am led to believe that Tekton utilise this type of magnet in their TOTL Moab speakers....

https://happysouvenirs.com/en/products/souvenirs-fridge-magnet/

Hello,
This is very interesting. I like to learn new things. When I was sixteen. I designed my own speaker and enclosure from Radio Shack and the lumber store. Stupid me I forgot to create a crossover. Also, it was not cheap to do at the time. I should have bought the book they sold. No internet at the time since this was 35 years ago. Live and learn. 

JBL still uses Alnico 5DG magnets in their top of the line speakers. See the link below.

http://www.audioheritage.org/vbulletin/showthread.php?33557-1501al-2

I am led to believe that Tekton utilise this type of magnet in their TOTL Moab speakers....

I wonder what led you to believe that?
Tarot cards, ouija board, industrial strength recreational pharmaceuticals, recent alien abduction?

Tekton does not use Alnico magnets in any of their speakers.

While most big woofers have large motor structures, it’s not the sole determination on quality and output. The 8” woofers used in the KEF 104/2 look like anything but impressive, yet their low end is output and power handling is remarkable. It’s the same with Hafler 300 speakers with two 6.5” woofers. Out of the cabinet, their not impressive yet they manage to pump out a tremendous low end quality and volume. 

Did you folks know that a big sub magnet, juduciously placed,  will trigger a hotel safe to unlock it. 

I’ve been quite busy over the weekend :^)
https://cursorium.co.uk/speaker-project/

More work to be done.

Passive radiator rings CNC cut, drilled, painted and passive radiators fitted. URL as above.

Anisotropic Alnico magnets are theoretically the best magnets for subwoofers but they are more expensive so Ferrite magnets are much more common. Ferrite magnets work well and are harder to demagnetize they just have to be larger. 

’’Alnico magnets are theoretically the best magnets for subwoofers’’.

That Is why JBL chose Alnico 5dg magnet for the JBL 1501al-2 woofer and not Ferrite. Alnico Is the best for making permanent magnet speaker drivers. JBL engineers discovered that more then 75 years ago. Probably the only better driver Is a field coil. The JBL 1501al-2 cost is $1500.00 each.



http://www.audioheritage.org/vbulletin/showthread.php?33557-1501al-2