Hearing loss can range from mild to profound. This graph shows which sounds can or cannot be heard depending on the degree of hearing loss. For example, someone with a severe hearing loss can hear an airplane or a motorbike close-by. (Very loud, hence lower end of the graph) However they will not be able to hear normal-level speech or birdsong.
Frequency and amplitude of a variety of common sounds.
A birdtweet is a high-pitched tone, therefore on the right of the graph; low-pitched tones are on the left.