In various embodiments, a system is used to provide an apparatus configured to measure sound in an ear canal of a wearer's ear at a distance from a tympanic membrane of the ear. The sound is measured and received by the apparatus to produce a signal. A frequency analysis is performed on the signal to determine output as a function of the frequency and to determine the frequency of the minima (null). Further, a distance equal to a quarter wavelength of the null frequency is calculated. A correction factor associated with the quarter wavelength is retrieved and applied to the output to generate a corrected output. An estimated sound pressure level at the tympanic membrane from the corrected output is produced.