Hearing levels of adults by education, income, and occupation, United States, 1960-1962 ; prevalence rates for hearing threshold in the better ear in excess of 15 decibels above and 5 decibels or more below audiometric zero, as determined by pure-tone audiometric tests at frequencies of 500, 1000, 2000, 3000, 4000, and 6000 cycles per second. (The Center,, 1968)