Dr. Gavreau designed and tested special horns and "whistles" of various volumes. These were each remarkably simple flat circular resonant cavities, having a side output duct. They were simply the large analogues of foghorns and police whistles. These flat forms were volumetrically reduced in successive design stages because it was found that their output was far too great. The infrasonic foghorns could produce a frightening two kilowatts of infrasonic energy, at a pitch of one hundred fifty cycles per second.