Sonic Warfare #2: Curdlers and Repellers

“In 1972, 13 choppers were equipped with Sound Curdler Systems (costing $50,000 USA each).  The Curdler is a small oscillator unit that is capable of hundreds of hours of uninterrupted operation from a simple 9v battery.  It emits a shrill pulsating shriek at 120 decibels at 10 metres.  (Standing on a runway during takeoff of a 747 measures 112db; in other words, potential aural harm.

The system is also called a ‘People Repeller’ when used with a hand portable US HPS1 Public Address System with a 350 watt amp and anti-feedback microphone.  This system delivered intelligible speech over 2.5 miles.  The system also came with an optional siren system with built-in wail/yelp audio frequency oscillator with adjustments to frequencies from 500-5000hz...

The 10,000-watt airborne systems were mounted on the choppers so as to best project sound downward in a pyramid apexing at 3,500 metres altitude.  This was the technological heart of psychological warfare.”

Plantenga, Radio and Aural Destabilization, 2001, pp 3-4