An audiometric finding characteristic of otosclerosis is an increase in bone conduction threshold with a peak at 2,000 Hz known as Carhart’s notch (Carhart, 1950). Although the notch occurs at 2,000 Hz, a reduction in bone conduction sensitivity is seen from 500 to 4,000 Hz which is, on average, 5 dB at 500 Hz, 10 dB at 1000 Hz, 15 dB at 2000 Hz, and 5 dB at 4,000 Hz (Carhart, 1971). Carhart attributed this phenomenon to “mechanical factors associated with stapedial fixation.” The Carhart notch is not a true indication of “cochlear reserve” and this apparent bone conduction loss may be corrected by surgical intervention (Tonndorf, 1971). Carhart (1971) and Tonndorf (1971) provided a review of several theories to explain this phenomenon. Further, Tonndorf (1971) provided a summary of the results of his studies of stapes fixation in various mammals (cats, dogs, guinea pigs, rats, and humans). Tonndorf (1971) explained that the middle ear contribution to the total bone conduction response consists of an ossicular inertial component and a middle ear cavity effect. He found the magnitude of the Carhart notch depended on the extent the middle ear contributed to the total bone conduction response in each of the species tested. Further, he explained the frequency of the notch varied depending on the resonant frequency of the ossicular chain for bone-conducted signals. The resonant frequency of the human ossicular chain was at a relatively high frequency compared with other species. Among the species he studied, this frequency was lowest in cats and highest in rats. Based on the work of Tonnndorf, it appears the Carhart notch peaks at 2,000 Hz due to the loss of the middle ear component close to the resonance point of the ossicular chain.
why does the carhart notch appear at 2000hz?