With a Penta Balanced Armature system including magnesium super tweeter, integrated magnesium inner housing and an optimized sound path, these stage monitor headphones faithfully reproduce even the most subtle sounds and emotions. For clean high notes, natural tones, and the pure emotion behind every performance, the IER-M9 has a Penta BA system including a magnesium super tweeter. Sony's original T-shaped Balanced Armature directly drives the diaphragm for more linear motion, and clean, faithful high notes. The IER-M9 super tweeter diaphragm is made of lightweight and rigid magnesium alloy to deliver the fast dynamic transient of performance. The unique BA unit design also benefits from extremely low resonance thanks to high internal loss. Internal wiring is upgraded with a silver-coated copper voice coil and gold-plated terminals, faithfully reproducing sound without sacrificing even micro signals so pitch, harmonics, and emotion are portrayed. The custom audio grade film capacitor in the cross-over circuit delivers much lower distortion. The audio grade capacitor of the IER-M9 delivers fine detail sound. Plus, audio grade solder also helps minimize any loss in signal path. The IER-M9 are designed to stay firmly in place with a snug fit for all ear sizes. With a firm fit and optimal housing shape, each earbud stays in the right position, even while moving on stage. Sony developed these in-ear monitor headphones cooperating with professional musicians and live-sound engineers. Their opinions are reflected in this product.