(1) Because the M88 hard helmet's earmuff part is oversized, when use like on gun, hard helmet's earmuff department will withstand gun's butt part.

(2) L. F. noise can be better attenuated so as to obtain good attenuation characteristics over the entire audible frequency range by using active noise reduction technique in the closed-air type earmuff.

(3) The optimum design of the controller of compound active noise cancellation earmuff is discussed.

(4) Objective To develop a new type of feedback active noise cancellation earmuff(ANCE) for aviator's helmet.

(5) Noise is another hazard. Working with noisy tools such as jackhammers requires proper, effective use of hearing protection. Ear plugs and earmuff type hearing protection is recommended.