Contemporary use of sound warfare dates back to World War II, when Albert Speer, Hitler’s Chief Architect, invented an acoustic canon that would create deafening noise by deploying 1,000 explosions per second. (It was never used). In 2003, the BBC reported that U.S. interrogators were subjecting Iraqi prisoners of war to long sessions of loud heavy metal music as a torture tactic.

Domestically, the U.S. police have used sound canons, or long range acoustic devices (LRADs), to break up protestors at the 2009 G20 in Pittsburgh, at Occupy Wall Street in Zuccotti Park, and, most recently, against protestors of the Dakota Access Pipeline. The devices can be square or round, propped up on a stand or held by hand. Before the police and military started using LRADs to dispel protestors, the Pentagon supplied them to U.S. cruise ships to use as a defense against pirates. Though range and decibel vary by device, LRADs can produce a sound beam that reaches 162dB of pure noise (any sound above 140 dB causes pain). When the Israeli Defense Force used LRADs to displace settlers from the West Bank in 2005, the Totonto Star’s Middle East bureau reported: “The knees buckle, the brain aches, the stomach turns, and suddenly nobody feels like protesting anymore.”

At this year’s Documenta 14 art exhibition, held in Kassel, Germany, and Athens, Greece, the artist collective Postcommodity co-opted these sonic weapons to create an artwork both inherently peaceful and strikingly beautiful. Titled The Ears Between Worlds Are Always Speaking, the piece uses two LRADs set up at the Lyceum in Athens to heal, not hurt—the devices broadcast an enveloping opera composed of immigrant stories, testimonies, and songs.

Postcommodity—made up of artists Raven Chacon, Cristobal Martinez, and Kade L Twist, all based in the southwestern United States—started working on the piece in 2016, after being invited to participate in Documenta, a 100-day exhibition held every five years. According to Twist, while LRAD dealers are reluctant to sell to individuals—they prefer to buy from military gear auctions and sell to police forces or municipalities—it only took a little persistence. The artists bought the military-grade sound cannons used in the piece off of eBay.

Postcommodity’s idea for the piece was to use the LRADs at low volume, positioned in different directions along the border of the Lyceum, to create what Martinez calls “stereo headphones” for the site. Though they broadcast only slightly above the volume of ambient noise of the site (around 75dB), their beams of sound can cut through it as long as there is a direct line of sight. They chose the Lyceum because of its history: it’s where Aristotle founded his Peripatetic school of philosophy, named for the philosopher’s tendency to walk the grounds while he taught. Postcommodity wanted to highlight a parallel between the history of walking the Lyceum during Aristotle’s lectures and the long walks and journeys that displaced people are forced to take in order to find refuge.

“We wanted to draw attention to people doing long walks as a part of neoliberal displacement, caused by warfare, economic collapse, globalization,” says Martinez. The collective had long been thinking about how LRADs have been used to facilitate war and silence dissenters. Their idea was to position them on the site so that they encouraged the visitors to walk around to get the full effect of the broadcasted score.