Huh? What is an ear defneder?
we hear sound because of the physical movement of the molecules of air. loud sounds have a larger physical movement (to prove this to yourself , take the cloth front off a loud speaker - watch the movement of the speaker as you turn up the volume. it moves more at higher volume - you may also feel pulses of air coming from the base reflex port of the speaker at high vol.)
so to make an effective ear defender, you need to attenuate (reduce) the movement of the air. Thus the type of material should be designed to reduce the flow of air. one material that you see is expanded polyurethane foam - which consists of an air filled matrix of plastic. the small pockets of air do not allow for such large movement of the air - thus making the sound quieter. double glazed windows work on the same principle - by sandwiching a thin layer of air between 2 layers of glass the deviation is reduced. Another technique is to use a solid material - where the atoms are held and thus less able to transmit the vibration - this is of course heavier. Usually a mix of techniques are used. watch out for resonance in any design. you could experiment with materials as part of the project
BTW: the electronic sound reducers that you see cause the net wound to reduce by creating a copy, 180 degrees out of phase - thus canceling the motion of the air form the original sound. but these are typically not considered ear defenders.You're welcome