There are mainly four types of earplugs for hearing protection: Foam earplugs: Mainly made from either polyvinyl chloride (PVC) or polyurethane (PU) (memory foam), which are compressed (rolled) and put into the ear canal, where they expand to plug it. Silicone earplugs: Which are rolled into a ball and carefully molded to fit over the external portion of the ear canal. Flanged earplugs: Including most types of musicians' or 'Hi-Fi' earplugs. Custom molded earplugs: Made from a mold of the wearer's ear and designed to precisely fit all ear canal shapes. Custom molded is further divided into laboratory-made and "formed in place". Ear Plugs may be either disposable or reusable, with foam and hand rolled silicone ones generally being disposable or for use a relatively limited number of times. Custom molded are non-disposable and made from either acrylic, vinyl, or silicone materials. Custom molded come as either vented (for communication) or non-vented (for high noise). A variation of the traditional foam earplug is the no-roll foam earplug that uses a built-in central stem to push the foam plugs into the ears. These earplugs achieve a seal due to their tapered shape, rather than expansion after being rolled.