"Now stay inside, you fool," she muttered, her voice barely a whisper in his communication amplifier.
"Master of the Moondog"
Stanley Mullen
He tested the Marspeaker-an amplifier and speaker in another pouch, designed to raise the volume of his voice to a level where it would carry through even the air of Mars.
"Police Your Planet"
Lester del Rey
"You've got a pad on your tail," the kid said, again as low as his amplifier would permit.
"Police Your Planet"
Lester del Rey