By Knowles from http://www.knowles.com/eng/premiumsound/What-is-balanced-armature
Knowles is the leading manufacture of balanced armature drivers. These drivers, originally developed for hearing aids and other critical listening applications, offer improved fidelity, realism and detail over traditional dynamic drivers. They are incredibly small and lightweight, and provide greater output with less power.
Balanced armature drivers use an electronic signal to vibrate a tiny reed that is balanced between two magnets inside a tiny enclosure. The motion of the reed is transferred to a very stiff aluminum diaphragm. This diaphragm is free of resonances in the audio band, allowing it to produce excellent clarity.
Balanced armature drivers are incredibly small. This leaves more room for other components inside the earphone, and can allow multiple drivers to be combined to get the best sound and performance from a particular device. And they’re also virtually weightless — an important consideration for fitness wearables or any other time comfort is a concern.
Balanced armature drivers are so small that multiple units can be combined in an earphone for true premium sound. A balanced armature driver can also be combined with an existing dynamic speaker in what is commonly called a hybrid earphone. In this configuration, the balanced armature drivers focus on the critical high-end portion on the audio spectrum, bringing a new level of clarity.