Magnet size and speaker sensitivity...

Docfoster

To what extent is the size (mass?) of a magnet the arbiter of a driver's sensitivity?

uglymusic

If you're nice to it, Ben, you'll not have any problems :-)

cj66

I have a feeling PAC is your man on this one.
It's my understanding that it is more to do with the force of flux generated by the magnet around the voice coil that has an effect on sensitivity, larger voice coils needing more flux. Therefore a large diameter bass cone requires a larger coil which requires a larger magnet or one made of a higher flux metal.....I think! (Although there is a lot more to it)

Q) Are high sensitivity speakers better at playing soul music?

Docfoster

cj66 said:

I have a feeling PAC is your man on this one.
It's my understanding that it is more to do with the force of flux generated by the magnet around the voice coil that has an effect on sensitivity, larger voice coils needing more flux. Therefore a large diameter bass cone requires a larger coil which requires a larger magnet or one made of a higher flux metal.....I think! (Although there is a lot more to it)

CJ,
Like you I have an expectation that PAC will know the answer to this.
But already, you yourself have advanced my understanding. Thanks for introducing voice-coil size into the thread and hence into my understanding!

PAC

Did someone call? ]

Sensitivity is sort of related to efficiency but the simplest way to think of sensitivity (or more accurately, relative sensitivity) is to consider the BL rating for the drive unit, the number and hence length of voice coil windings in the gap between the pole piece and the magnet and the strength of the magnet itself.

You can have a drive unit "motor" with a higher rated voicecoil made from thicker wire (hence able to handle more current) and an increased X-Max (length of voice coil minus gap height) in a magnet of a specific gauss rating, and it will have a lower sensitivity than a thinner but longer voicecoil with a reduced X-max in the same magnet/pole piece assembly.

Generally speaking, lower power rated voicecoils/speakers using larger drive units with shorter throws will be higher in sensitivity than longer throw units with higher power ratings.

The reason is simple.  More of the wire length for the thinner voicecoil is present within the pole piece and magnet gap, and remembering that the voicecoil is an electro-magnet, this means that for a similar input power it will produce more output due to having more windings in the motor gap.  That simple.

Docfoster

Thanks Paul. Exactly the sort of informed answer I was hoping for.
And CJ, you were right too.
So is the size of the magnet determined in sympathy with the length of the voicecoil wire...?

PAC

Hi Ben

the "motor" assembly is designed around the final "Q" desired for the speaker, the power handling, excursion, sensitivity desired and final use.  Horses for courses.  Magnet material and type varies and some are far more efficient than others yet some of the older weaker magnetic materials such as AlNiCO still sound the best to my ears.  There are modern very efficient magnets which allow greater BL rating thanks to high Gaussian strength, allied to sufficient windings and power rating. Think of BL rating (in Telsas) as the power of the motor to resist inertial forces of the cone mass to retain high control.

Big magnets don't automatically = high BL rated loudspeaker units.  That also relies on the magnet material (ie strength) and length of voicecoil wire, as the rating is simply the relative amount of  applied current to resist cone movement under an applied mass. 

Docfoster

"For every complex problem there is an answer that is clear, simple, and wrong."
(H. L. Mencken)

This quote springs to mind when I consider my previous and erronious "larger magnets = higher sensitivity" thinking.
I love it that in fact overall not only is magnet size not the arbiter, but that even with regard to magnets specifically the situation is further complicated by the variables of different "types of magnetic material".

Thanks Paul ("For many complex problems there is an anwer that is even more complex, but correct." ;-) ).