Soon after he began his practice, his father began referring him opera colleagues with vocal problems. Tomatis soon discovered traditional treatments inadequate but also that there was very little research on the voice itself. He formulated the theory that many vocal problems were really hearing problems. His theory that "the voice does not produce what the ear does not hear", is the hallmark of his research and his method. He discovered that the voices of opera singers had damaged their own muscles of the middle ears. With damaged hearing, they were forcing their voices to produce sounds in registers they could no longer hear.