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Microphone Magnets
Name: Tania G.
Status: Student
Age: 15
Location: N/A
Country: N/A
Date: June 2004
Question:
What kind of magnets are used in microphones?
Replies:
Many kinds of microphones do not use magnets: electret, electrostatic, carbon, piezo-electric, capacitive.
The kind that does use a wire-coil and magnet is called a "dynamic" microphone.
Different parts of the sound wave are properly funnelled onto the front and back of a diaphragm.
The air pressure of the sound wave pushes/pulls on the diaphragm, which is attached to a coil of wire,
which then moves tiny distances sideways in a magnetic field.
The average little loudspeaker is almost the same thing, but you can see all the parts on a speaker.
For these mikes and speakers, getting a strong magnetic field is fairly important for getting good sensitivity.
Permanent magnet materials by themselves have "built-in fields" of 2000-5000 Gauss (0.2-0.5 Tesla).
The best "Rare Earth" magnets have about 5000 G, and don't de-magnetize over the years, and are optimally light-weight for the strength.
But there is another gimmick which does 4 times more, giving up to 20,000 G.
Passively-magnetic Iron (the kind that is not a magnet, but magnets stick to it) can be polarized up to 20,000 G,
using electric current in a coil.
If there is a thick circular loop of iron magnetized to 20,000 G, and you saw a narrow gap through it for the wire-coil to slip into,
the field will still be almost as strong, say 15,000-19,000 G.
Passively-magnetic iron also "conducts" magnetic fields with fairly low loss,
so you can use it to make a "funnel"
to collect magnetic field lines from wide and thick chunks of magnet,
and concentrate it onto a small and thin air gap at almost 20,000 G.
If a magnet is 2x2inches and 1 inch thick and has 3000 Gausss built-in,
thick iron plates covering each pole-face can funnel its magnetic flux lines
into an air gap of 25% the area and 25% thickness, and the field there will be over 10,000G:
"M" is the permanent magnet; "N" and "S" are North and South pole-faces.
It needs to have a narrow air-gap for the moving coil of wire to slip into,
because having a wide gap (or just being alongside) will greatly weaken the field.
The advantage is, the quality of the magnet no longer matters so much.
Cheaper 3000G ceramic magnets are good enough, if you use large blocks of it.
It may be unappealing for a hand-microphone be large and heavy.
So manufacturers are likely to go to some trouble to include rare-earth magnets,
even with passive-iron concentrators.
My magnetic units (all Gauss) are slightly oversimplified, but they work pretty well.
Jim Swenson
The type of magnet that is used depends on the manufacturer. The
manufacturer will often tell you in the product description what kind of
magnet is used. Neodymium-iron-boron would be typical. The material
typically consists of 14 parts of iron, 2 parts of neodymium and 1 part of
boron. Other combinations are possible.
Bob Erck
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Update: June 2012
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