Headphones Transducer technology-Planar magnetic and Balanced armature

Many types of headphone transducer technology play an important role in the sound quality of headphones. In the previous article, Headphones Transducer Technology: Moving Coil, Electrostatic, and Electret, we learned about three of these headphone sensing technologies. This article focuses on two other types of headphone transducer technology: planar magnetic and balanced armature.

Planar magnetic

Planar magnetic (also known as morphodynamic) headphones use similar technology to electrostatic headphones, with some fundamental differences. They operate similarly to planar magnetic loudspeakers.

A planar magnetic driver consists of a relatively large membrane that contains an embedded wire pattern. This membrane is suspended between two sets of permanent, oppositely aligned, magnets. A current passed through the wires embedded in the membrane produces a magnetic field that reacts with the field of the permanent magnets to induce movement in the membrane, which produces sound.

Balanced armature

A balanced armature is a sound transducer design primarily intended to increase the electrical efficiency of the element by eliminating the stress on the diaphragm characteristic of many other magnetic transducer systems. As shown schematically in the left diagram, it consists of a moving magnetic armature that is pivoted so it can move in the field of the permanent magnet. When precisely centered in the magnetic field, there is no net force on the armature, hence the term 'balanced'. As illustrated in the right diagram, when there is electric current through the coil, it magnetizes the armature one way or the other, causing it to rotate slightly one way or the other about the pivot, thus moving the diaphragm to make a sound.

A custom in-ear monitor that uses 8 balanced armatures in a triple crossover configuration (4 low/2 mid/2 high). Headphone designs often use multiple balanced armatures to provide a higher degree of fidelity.

The design is not mechanically stable; a slight imbalance makes the armature stick to one pole of the magnet. A fairly stiff restoring force is required to hold the armature in the "balance" position. Although this reduces its efficiency, this design can still produce more sound with less power than any other. Popularized in the 1920s as Baldwin Mica Diaphragm Radio Headphones, balanced armature transducers were refined during World War II for use in military sound-powered telephones. Some of these achieved astonishing electro-acoustic conversion efficiencies, in the range of 20% to 40%, for narrow bandwidth voice signals.

Today, they are typically used only in in-ear headphones and hearing aids, where their high efficiency and diminutive size are major advantages. They generally are limited at the extremes of the hearing spectrum (e.g. below 20 Hz and above 16 kHz) and require a better seal than other types of drivers to deliver their full potential. Higher-end models may employ multiple armature drivers, dividing the frequency ranges between them using a passive crossover network. A few combine an armature driver with a small moving-coil driver for increased bass output.

The earliest loudspeakers for radio receivers used balanced armature drivers for their cones.