The apex-m20 module houses the apex™ technology. Apex features a pneumatically interactive vent that minimizes ear fatigue which results in a safer listening experience and a more rich and spacious sound. This module is designed to work with the A & U-Series earphones (also backwards compatible with all previously sold A & U Series earphones) and EP-Custom Earplugs. The m20 module provides -20dB of noise isolation. Modules are sold in pairs only.