North Korea has cranked up the volume of its loudspeakers along the border with South Korea. But this time, instead of the usual decipherable propaganda, it’s blaring weird and disturbing noises that are upsetting South Korean locals within earshot. Using noise and loudspeakers as methods of coercion goes way back. Host Carolyn Beeler speaks with Toby Heys, the author of “Sound Pressure: How Speaker Systems Influence, Manipulate and Torture.” He’s also professor of digital arts at Manchester Metropolitan University in the UK.