He felt a subtle vibration on his cheeks. Index appeared to be silent at first glance, but her mouth was opened ever so slightly and she was taking long breaths. That was the source of the vibration.

She was singing.

Just as ultrasonic waves produced ripples in a cup of water, her song was beyond the range of human hearing and yet she was providing information to Birdway in front of her. Given the situation, she was clearly providing the knowledge of her 103,000 grimoires.

Toaru Majutsu no Index New Testament vol.10 ch.17 pt.4