PEI (Polyetherimide) is the ideal material for tweeter domes due to its light weight, high tensile strength, high stability and tolerance for high temperatures. Inside the tweeter, the small diaphragm (the dome opening) produces high frequency tones. These PEI domes can capture even the highest musical notes to make your music textured with warm, full-range treble. The dome shape also spreads the sound around so that you can hear the high notes from every seat in the car.