Fiio F9 SE Dynamic Hybrid Earphone - Black has two balanced armatures and a separate dynamic driver. The 9.2mm dynamic driver is specifically tuned to produce bottomless lows while the two BA drivers precisely portray extended upper-mids and highs. Intelligent Control - Hi-Resolution Audio Certified for frequency response up to 40kHzwhich can fully reproduce the audio and give you a natural and life-like listening experience. PEK polymer nanocomposite driver for heightened detail. - The F9 SE utilizes a dynamic driver made of PEK (polyether ketone) polymer nanocomposite known for not only being tough but also being light.