Leakage in the chamber facing the eardrum. Free airflow out of the chamber at lower frequencies limits the bass response unless the driver is compliant enough that its excursion would increase in a leaky chamber to sustain low frequency pressure. If the stiffness of the driver is far greater than the cavity, the excursion does not significantly change and a large loss of bass can be observed. Opening the back alone does not. The ear cavity leak is the very reason for difficulties in pressurizing below the fundamental. This is deliberate in typical open backed headphone design. This may illustrate the point more clearly. The ear imposes its directional transfer function on the input sound. This is generally preserved in headphones, save some loading of the ear by closing it off with a headphone. If the canals are shorter than the average, the ear resonance is shifted upward when tested by a far off loudspeaker and when subjected to a headphone. The IEM must reconstruct the subject's open ear resonances that are not present. Time and frequency compose a conjugate variable pair, and their response functions are related one-to-one to the other by Fourier and inverse Fourier transform. While we tend to ignore the phase component in practice and only consider the magnitude, headphones operate in mostly minimum phase. A key property of conjugate variables is described by Heisenberg's uncertainty principle. A resonance that is sharply focused around around a particular frequency cannot be localized sharply in time, and vice versa.