Sound Powered Phones for Crystal Sets
Introduction Part II


What makes a "Sound Powered" element more sensitive than
a old style "Balanced Armature" element or a set of "Magnetic Headphones"?

Good question. There are several things that make it more efficient. Lets start with some basics.

Magnetic Headphones Basics

See figure #1
Magnetic headphones, new or vintage, work the same. Wire (a coil) wrapped around a pole (iron) which attracts a disk of metal when a current is applied to the coil (see "A" in figure #1).

Magnetic forces travel through Iron easier than through air. The "Reluctance" of the magnetic field to travel through air makes a headset less efficient than if it travels through iron. Therefore "A" is less efficient than "B" in figure #1.

"B" is called "bi polar"

Figure #1

A single pole ("A" in figure #1) design is less efficient than  a double pole design ("B" in figure #1) because it has less "reluctance".

The most "sensitive" spot on the disk being pulled by the magnetic poles is in the center. This is without a dough the most sensitive spot. Sensitivity drops off rapidly as you go to the sides of the disk.

"A" pulls from the very center and "B" pulls from the sides just a little, but the bi-polar design is still much more efficient than the gains you get from pulling from the exact center.
 
 

What we learn here is that the we need a design that pulls from the center  to be more efficient and more "sensitive".

Enter Balanced Armature headphones such as Nathaniel Baldwin's vintage headphones.

See Figure #2

Notice the reluctance for the lines of force for the magnetic field is very low. The magnet field has an iron path most of the way.

Also the "lever action" works on the center of the disk or diaphragm which also increases its efficiency.
 
 

Figure #2

What more could you want???

OK

 
 

Sound Powered Elements

Enter sound powered designs. See figure #3

This is basically a improved "Balance Armature" element with design improvements such as:

Duraluminum diaphragms (a more flexible disk, mechanically attached so it did not have to be magnetic).

The mass of the moving parts was reduced (less mass, more efficiency).

Coil windings were improved.

On some elements (RCA's) the magnetic fields are brought into play using laminated bars instead of solid iron.
 
 

These and other improvements were possible partly because of better machining of parts at the time.
 
 

Figure #3

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