Fit - All HPDs must be properly fitted when they are initially dispensed. The correct size pre-molded insert will always be a compromise between a device that is too large and therefore uncomfortable, and a device that is too small and therefore provides poor protection. The appropriate compromise can often times be achieved, but only with care and skill. Not all HPDs are equally suited for all ear canal and head shapes. Certain head contours cannot be fitted by any available muffs and some ear canals have shapes that may only be fitable with certain inserts or canal caps or sometimes not at all.
Earmuffs can only work well when their cushions properly seal on the head.
Eyeglasses, sideburns, or long or bushy hair underneath cushions will prevent this and will reduce attenuation by varying amounts. Readjustment - HPDs can work loose or be jarred out of position during the day. It must be remembered that laboratory tests require the subject to carefully adjust a device prior to testing. Under typical use, wearers will eat, talk, move about and may be bumped or jostled, resulting in jaw motion and possible perspiration. These activities can cause muff cushions to break their seal with the head and cause certain inserts to work loose.