Wiktionary
subsonically
adv. In a subsonic manner.
Usage examples of "subsonically".
She reached for the MAC-11, rolled onto her stomach again, aimed for the second-from-bottom step and squeezed off a withering blast of rounds that turned her immediate target into an explosive spray of blown-out wood chips before powering subsonically through the hollow interior and ripping out the other side, only slowing marginally as they zip-drilled the flesh, sinew and bones of the man crouched there.
Meanwhile his real voice, which the device rendered into frequencies a human could hear, vibrated subsonically out of his feet and up into the man's bones.