hearing aid
The National Institute on Deafness and Other Communicative Disorders describes a hearing aid as; "a small electronic device that you wear in or behind your ear. It makes some sounds louder so that a person with hearing loss can listen, communicate, and participate more fully in daily activities. A hearing aid can help people hear more in both quiet and noisy situations." Source
In anime and manga characters with deafness are quite rare, but ones that do have the disability may be presented with hearing aids in their ears and/or using sign language.
