A laboratory was established at the Johns Hopkins University in 1924 for study of the causes and prevention of deafness. The plan of investigation was patterned after that used so successfully in general pathology, i. e. the correlation of impaired function as determined by clinical tests with the location and nature of the causal lesion. Systematic adherence to this plan for fourteen years has resulted in the accumulation of approximately 15,000 records of hearing tests, illustrating every type and degree of deafness, and has led us to conclusions that could not have been forecast. This communication deals with one phase of our investigation, which we believe is of the first importance in the recognition and prevention of the commonest type of deafness. A preliminary report on this subject was published in 1937.1 It was essential that the hearing test should be as accurate as possible and that each patient