Estimation of the sound pressure at the eardrum

In hearing experiments or in audiological investigations, the results usually have to be referred to an appropriate input signal. Typically, the input voltage of a headphone or the sound pressure generated in a free sound field or under a different reference condition are used. In many cases, however, it would be preferable to know the sound pressure at the eardrum, as it is more closely related to the auditory sensation. If headphones are used, the reference to the pressure at drum can be approximately achieved by means of an ear simulator, such as the well-known ear simulator, according to DIN IEC 711. But the ear simulator can only reproduce the acoustical properties of an average ear. The resulting errors depend on various parameters, including those of the headphone used. A considerable portion of the error is caused by the individual ear canals. These errors alone can exceed 10 dB in the audiologically used frequency range up to 8 kHz. Some methods of estimating the pressure at the drum are investigated. Optimum results are obtained if both the sound pressure and the volume velocity at the ear canal entrance are measured. But fairly good results can also be achieved without measuring the volume velocity.