Is classically found in bone conduction audiograms of patients with otosclerosis. This is actually a dip centered around 2000 Hz.
Some authors consider carhart's notch to be an artifact. This notch is closely related to the carhart's effect. Carhart's effect: was initially described following successful stapes surgery. There was an over closure of air bone gap following successful surgery. Classically this effect lead to an improvement in hearing levels particularly at 2 KHz frequency levels.
How carhart's effect is created?
When skull is vibrated by bone conduction, sound is transferred to cochlea via three routes. i.e.
1. By direct vibration of skull
2. By vibration of ossicular chain which is suspended within the skull
3. By transmission via external auditory canal (normal route)
In conductive hearing loss routes 2 and 3 are affected, but can be regained following successful stapes
surgery. Hence bone conduction thresholds improve around 2 KHz frequency range.