<prompt> Frequency of Carhas notch is ?1000 Hz2000 Hz3000 Hz4000 Hz  <response>: Ans. is b i.e., 2000 Hz Dip in bone conduction in otosclerosis Carhas notch - 2000 Hz Dip in noise induced hearing loss Acoustic dip - 4000 Hz In otosclerosis bone conduction is usually normal as would be in cases of conductive hearing loss. However, some cases show a dip in bone conduction on audiogram which is maximum at 2000 Hz Cahas notch. In NIHL both bone conduction and air conduction are defective as would be in SNHL and there is a typical notch acoustic dip at 4000 Hz, both for air and bone conduction. So, Cahas notch of otosclerosis is a dip for bone conduction, while acoustic dip of NIHL is for both air as well as bone conduction. 