speaker magnet size - difference?

[moody]

Hi.

 

I'm just wondering in general how a different sized magnet on the same size speaker effects the sound. I presume higher power handling but other than that how does it effect bass, mids, etc?

 

Looking at say the difference between a 15" speaker with a 6" diameter magnet compared to a 9" diameter magnet.

 

This is based on the fact that I have a couple of Jbl k130's and a couple of big E-tone woofers that all need re-coning and I can't afford to do them all at the moment.

 

Probably most needed for bass cab duties in the near future.

[gadget69]

Hi

The motor structure as it is called provides the bassis for the speakers power handling capability. The size of the magnet is less important than its magnetic strength. Some of the best material for speaker magnets is too rare and expensive for economical use, cobalt is one of these superior magnets, and due to the supply of cobalt being in a third world country and controlled by a corrupt govt. the supply of this material is non existant. Also you will find that a number of speakers are going to neo dymium. A great, very light material that is however sussectible to total loss of magnet strength with heat as low as 100 degrees celcius! Thus the large heat sinks. That is the reason you see mostly large ceramic magnets on speakers.

 

One of the limiting factors in speakers however, is the voice coil. If you take the same basket structure the same magnet, and increase the voice coil size from 3 to 4 inches, you have increased not only the power handling, but the speakers ability to reproduce large fast transients.

 

Yes the magnet size matters as it increases the gauss strength the voice coil moves through, but the voice coil matters as it increases the speakers ability to reproduce what it is fed information wise. There are other factors here too we won't get into, spiders, surrounds, the Xmax of the driver... that all figure into a speakers ability to reproduce a portion of the audio spectrum.

 

I have found that the 4" voice coil cast frame woofers are much more stable bass reinforcement drivers in 15" and 18" drivers. BUT the smaller the driver the faster it moves the more realistic and even lower the freqs it can reproduce. I provide as an example the LABsub, EAW's supersubs and the like, they are horn loaded speakers that will go lower louder and with less distortion and power consumption than their bigger counterparts. Its like taking a 428 cobrajet engine and putting it in a 1 ton pickup. the other in a 1965 AC cobra body, which will start and stop faster?

 

The driver for the LABsubs is a 2.5" extended voice ciol 12" driver rated @ 400watts that has an Xmax of around 1.5", where most 15" and 18" drivers only travel 1/2" or less.

 

Thats my .02 worth

Be well

Gary

[agedhorse]

Magnet size is not important... the strength and how well focused the field's energy is within the voice coil gap. It's all about efficiency.

 

Also, a smaller voice coil will generally be more efficient than a larger V.C. with a given magnet size/strength. The flux density will be higher due to smaller gap area. The problem with smaller voice coils is the limited power handling... so you need to factor in the maximum desired SPL into the equation as a smaller VC may be the better performer for that application.

 

Regarding the comment about N-Dym magnet material being heat sensitive around 100 degrees C, that's pretty hot... boiling water hot... and the adhesives holding the speaker together would fail before you got to that point. Besides you wouldn't be able to tough the magnet at that temp.