inaudible
adj. B2 Upper Intermediate US //ˌɪˈnɔdəbəɫ// UK //ɪnˈɔːdəbəl// in·audi·ble
adj. impossible to hear. You use this to describe a sound that is too quiet or too high for your ears to catch.
adj. unable to be heard; falling below the threshold of human hearing or obscured by other noise. Often follows a linking verb such as 'become' or 'remain'.
His voice was almost inaudible over the loud music.
The speaker's microphone failed, making her closing remarks completely inaudible to the people at the back of the room.
While the high-frequency whistle was inaudible to the human ear, it immediately caught the attention of the guard dogs stationed at the perimeter.
Usage
Often used predicatively after linking verbs like 'be', 'become', or 'render'.