From the Acoustic Society of Japan..
(sorry for my clumsy translation)
Q: It seems a balanced armature type has been more utilized on insert earphones lately- how is it different from a dynamic type?
A: A balanced armature transducer is a type of electro-magnetic transducers, and is fundamentally different from a dynamic transducer in principle. An electro-magnetic transducing mechanism is an adaptation of Flemming's left hand rule, which is a product of the force of magnetic field, current, and the length of a coil, caused by the perpendicular force generated when current is applied to the coil set in perpendicular to the direction of the magnetic field. The force, which is in proportion to the change in current, drives a diaphragm, and generates acoustic pressure.
Meanwhile, an electro-magnetic type utilizes any changes of magnetic pull, by alternating magnetic field generated at the magnetic gap. Consequently, since magnetic field strength B is proportional to its force B2, an electro-magnetic transducer must contain some second-harmonic components inevitably. In order to minimize this harmonic distortion, a balanced armature type has been developed. As Figure 1 shows, the design of a balanced armature consists of a magnetic armature in the center of the magnetic gap stabilizing the pulling force of direct current, while canceling the second harmonics that are proportional to square of the pulling force. Because the direct-current magnetic field and the signal magnetic field on the one side of the gap are in same direction, while it is in opposite direction on the other side of the gap with the force in opposite direction as well, the force square of driving pressure on the armature nullifies. Moreover, using magnetic ferrites, or other rare earth materials with less magnetic permeability, might slightly change the size of the magnetic gap, but the acoustic sensitivity should be enhanced around 6 dB with driving force now doubled, while the value of B remains pretty much constant. In practice, by comparing the sensitivity of a dynamic transducer and a balanced armature transducer (in reference to IEC 60711), the former type is 102 ~ 106 dB, while the latter type is 112 ~ 116 dB. Due to its high sensitivity, a balanced armature type has been used in applications in which more volume is required with less amplification, such as hearing aids, but recently, it has became the subject of interest to audiophiles. It seems there are only upsides, but of course, there are downsides as well.
1. Compared to a dynamic type, a balanced armature is high in cost due to more strict limitations in designing. Normally it is 3 times more expensive.
2. As the vibrating system is made out of an iron-related material, its mass is heavier; Thus, normally acoustic sensivity beyond 7 kHz range decreases.
3. Since the location where the direct-current magnetic pulling is stabilized is at the center of the magnetic field, even a slight displacement would cause the armature to stick to either side of the magnet. Increasing the stiffness of the vibrating system definitely helps resisting the magnetic circuit's pulling force. However, due to the presence of the magnetic gap between the ear, low frequency playback becomes harder when there's an acoustic leak.
Ōhira, Ikuo (Advisor at Ishda Sound Inc.)