Gould was widely known for his unusual habits. He often hummed or sang while he played, and his audio engineers were not always successful in excluding his voice from recordings. Gould claimed that his singing was unconscious and increased in proportion to his inability to produce his intended interpretation from a given piano. It is likely that this habit originated in his having been taught by his mother to "sing everything that he played", as his biographer Kevin Bazzana puts it. This became "an unbreakable (and notorious) habit". Some of Gould's recordings were severely criticised because of this background "vocalising". For example, a reviewer of his 1981 re-recording of the "Goldberg Variations" opined that many listeners would "find the groans and croons intolerable". Gould was renowned for his peculiar body movements while playing and for his insistence on absolute control over every aspect of his environment. The temperature of the recording studio had to be precisely regulated. He invariably insisted that it had to be extremely warm. According to another of Gould's biographers, Otto Friedrich, the air-conditioning engineer had to work just as hard as the recording engineers.