Normal audiogram of the right ear. The vertical lines represent the frequencies that are tested from 125 to 8000 Hz. The horizontal lines record the threshold at which the patient states that the sound is heard. Normal thresholds are 0 dB + / − 10 dB. Patients with a hearing threshold below 20 dB are considered to have average or better-than-average hearing. The greater the dB, the louder is the sound and the worse the hearing. "O" is the standard symbol for air conduction of the right ear; "X" is the standard symbol for air conduction for the left ear. The " is the standard symbol for unmasked bone conduction for the right ear; " > " is the standard symbol for unmasked bone conduction of the left ear. The reason why both masked and unmasked measures are needed is to make sure one ear is not hearing the sound presented to the other ear (one ear is 'masked' so it does not hear the sound presented to the other ear, giving a false value).