noun
hearing aid
A small electronic device worn in or behind the ear to amplify sound for people with hearing loss.
祖父は補聴器を使っている。
My grandfather uses a hearing aid.
補聴器の電池が切れた。
The hearing aid battery has run out.
集音器 is a sound collector or personal sound amplifier, often used by people with mild hearing difficulties or in specific situations, but it is not a medically prescribed device like 補聴器.
Compound of 補 (ho, 'supplement') + 聴 (chō, 'hearing') + 器 (ki, 'device'). The word is a modern coinage for the medical device.