However, not all vocal teachers agree with this view. Thomas Appell's 1993 book "Can You Sing a HIGH C Without Straining?" aimed to refute the theory that all singers switch registers at the same absolute pitch. Appell defined chest voice as resonance below the vocal folds and head voice as resonance above the vocal folds. He recorded examples of male and female singers changing from chest voice to head voice at different pitches in an attempt to prove that the transition pitch is a function of the intensity of the vocal tone and is not absolute. At higher vocal cord tension (intensity of singing), Appell shows that the pitch at which a singer transitions from chest to head voice will be higher. At lower vocal cord tension (intensity of singing), Appell shows that the pitch at which a singer transitions from chest to head voice will be lower.