It plays pure tones at 1 kHz (check) and then 8 to 20 kHz in one ear at a time. Each trial has two intervals of about one second, with three short tone pulses in only one of them, chosen at random. You answer in which one you heard the tone. Two correct answers unlock the next frequency; a wrong answer or an "I heard nothing" ends that ear, and the last frequency you got right becomes the result.