Plantronics H31CD is a special modified version of the H31. Plantronics H31CD boost the incoming audio level to make listening easier in noisy environments. Plantronics H31CD (part# 43674-01) is recommended for dispatch operators, air traffic controllers, military, stock exchange, 911 dispatch operators. Features include: Quick Disconnect feature, Flexible Eartube style, the ultra-lightweight, StarSet headset rests comfortably behind the ear for all day comfort----easy on, easy off. Plantronics