What is the Carhart notch in Pure Tone Audiometry?

Stimulation of the ear via bone conducted vibrations is a well-established way to differentiate conductive from sensorineural hearing loss – but paradoxically, the accurate testing of bone conduction thresholds depends on normal middle ear function. Impairment of bone conducted vibrations, for example by stapes footplate fixation, results in raised thresholds by bone conduction. This effect arises across the frequency range and is related to the resonance properties of the ossicular chain. The result is a “notch” in the bone conduction audiogram which is more pronounced at 2 kHz as described by Carhart in 1950.

References 
Carhart, R. (1950) Clinical application of bone conduction audiometry. Arch Otolaryngol 51, pages 798-808