Behind-the-ear (BTE) hearing aids 

BTE hearing aids are worn behind the ear. This type of hearing aid is in a case. It connects to a plastic ear mold in the outer ear. These hearing aids are generally used for mild to severe hearing loss. They are typically used for young babies and children. Poorly fitted BTE hearing aids can cause an annoying whistling sound (feedback) in the ear. But all types of hearing aids may cause feedback if not fitted well.