inaudibly

If you want the quietest user experience possible, an ultrasonic design is generally going to be your best bet thanks to its novel usage of inaudible, high-frequency vibration.

This vibration is created by a piezoelectric transducer, and while this is technically a mechanical vibration, it occurs at frequencies of between one to two Megahertz, making it virtually inaudible despite being high energy.

Since then, Singh has mumbled along inaudibly, knowing power only as a gift bestowed, not as a responsibility earned.

Indoors, all things on which Gertrude laid her hand slid sweetly and inaudibly into their place.

The boy murmured so inaudibly that Margaret barely heard, and she rose and followed him as he led the way to the elevator.

He stopped and looked intently at his mother's picture, while his lips moved inaudibly.

The stranger approached and half knelt as the old man spread his hands over his head, muttering inaudibly.

Almost inaudibly Thea began to hum the air, though she was thinking of something serious, something that had touched her deeply.