Passive headsets cancel sound by blocking sound waves. Sound can be blocked by 3 methods: Barriers, Absorption Material, and Damping Methods.
In our aviation headsets, we use the ear cup as walls to enclose the ear and act as an acoustical reflector. The ear cups act as a barrier against the path of noise and eliminates energy of the noise wave. The hard barriers target high frequency noise. We also use different types of acoustical foams to absorb the sound waves and eliminate more energy from the sound waves.
Vibration isolators which include spring steel equipment mounts, plastic and rubber based bushings and grommets all contribute to noise canceling by damping the noise. These vibration isolators absorb noise coming in through the headband and ear cup stirrups and also noise waves that are bombarding the ear cups.