The term ‘in-situ’ in the context of audiology is often used to refer to when a hearing instrument or other device is being worn. So, in-situ SPL would refer to the characteristics of a sound (sound pressure level at different frequencies), at or near the tympanic membrane when a hearing instrument is in place in (or behind) the ear.
An associated term is in-situ audiometry, which is determining the hearing thresholds of wearers while they are wearing their devices, using stimuli that are generated by the hearing instrument (rather than the audiometer via headphones or inserts). This technique can bypass the calibration errors that might arise when indirectly calculating the SPL at the tympanic membrane using someone’s thresholds as measured in dB HL (since an average value is often used in this calculation). Acoustic characteristics of the hearing aid and its coupling (vents, leakage and ear mould characterstics, for example) are also taken into account automatically. Set against these technical advantages, the technique has a potential time penalty in that thresholds might have to be measured twice (e.g. once for diagnosis of hearing loss, with threshold in dB HL via headphones or inserts and once again for verification of the hearing instrument with threshold in SPL via a hearing instrument.)