31 January 2014
Vol 343, Issue 6170

About The Cover

Cover image expansion

COVER A circulator for sound. The arrows represent acoustic signals flowing through the device (diameter: ~20 centimeters), which circulates sound in a nonreciprocal fashion: Signals can flow from left to top, from top to right, and from right to left, but not in the opposite directions. This creates one-way communication channels for sound, allowing listeners to hear but not be heard in return. See pages 495 and 516. Image: Erik Zumalt, Cockrell School of Engineering, The University of Texas at Austin