Over 400 people were involved in his research, and all apparently heard the voices. This culminated in the 1968 publication of Unhörbares wird hörbar (“What is inaudible becomes audible”). The book was published in English in 1971 as Breakthrough.
Over 400 individuals were involved in his research and all reportedly heard voices, resulting in the publication of "Unhörbares wird hörbar" ("What is inaudible becomes audible") in 1968, which was later translated into English as "Breakthrough" in 1971.