Middle-ear function is commonly assessed experimentally by measuring displacement amplitudes of its structures and associated phases as a function of frequency for constant SLPs. For cats, transfer functions so derived are independent of frequency within the low-frequency range, having a slope of −12 dB/octave at higher frequencies. This is in sharp contrast to the auditory threshold curve with its characteristic low-frequency attenuation. However, such considerations neglect the fact that the middle ear transmits mechano-acoustic power and not merely displacements. Power transfer curves, calculated for the ears of cats, are shown to increase with frequency at about 12 dB/octave at low frequencies, being essentially flat at higher ones. Their shape is very similar to that of auditory threshold curves. Considering the transfer of power, the low-frequency attenuation is simply due to the small size of the tympanic membrane. Further considerations indicate that the size of the tympanic membrane (and also that of the cochlea) appears to represent a compromise between at least five different factors to assure allaround optimal performance. This notion is supported by the findings that the tympanic membranes and the basilar membranes of different species grow relatively slowly with body size, specifically that their areas grow only as a single dimension of length.