WHY A NOTCH AT 4, 000 HZ? In humans, the frequency of maximum cochlear damage is one-half to one octave above the frequency of maximum stimulation. This phenomenon has to do with the angle of curvature of the human cochlea as well as less blood perfusion in the basal end of the cochlea compared to the apex. The human external ear (pinna and ear canal) influences the physical properties of sound outside the head (i.e., in the diffuse field) by resonating at frequencies between 2, 000 and 4, 000 Hz, depending on the volume and the length of the ear canal; for larger adult ears the maximum ear canal resonance, as measured with a probe microphone, is 2, 600 to 3, 000 Hz. In children, with shorter ear canals with a smaller diameter, this ear canal resonance is higher in frequency. This resonance serves to amplify sound by 15 to 25 dB relative to the diffuse field (for instance, as measured at the shoulder) at the resonant frequency. Acousticians and engineers have referred to this resonance as the transfer function of the open ear (TFOE) or the external ear transfer function and is known to audiologists as the real ear–unaided response . When fitting hearing aids, placement of an ear mold results in disruption of this normal ear canal resonance, resulting in insertion loss. The real ear–aided response (REAR) must provide amplification to compensate for the insertion loss, just to get back to the sound level that would arrive at the eardrum without the earmold or hearing aid in place. For broadband sound, the result of the TFOE is an overall level measured at the eardrum roughly 7 dB higher than measured at the shoulder. Given that most environmental sound is relatively broadband, the frequency range of maximum stimulation is roughly one-half to one octave below 4, 000 Hz. This is another reason why the 4, 000-Hz frequency region is the most susceptible to damage.