Probe Microphone Placement for Real Ear Measurement

Probe microphone measurement systems allow determination of sound pressure level (SPL) within the external auditory meatus (EAM). EAM geometry and middle ear impedance contribute significantly to the acoustic properties of the EAM, and consequently sound pressure distribution along the EAM is not uniform. Accurate measurement of SPL at the tympanic membrane (TM) requires probe microphone placement within 6 to 8 mm of the TM. A method of probe placement using the acoustical properties of the EAM (Sullivan, 1988) was investigated in 6 adults. Sullivan's acoustic probe placement method was found to overestimate probe distance from the TM, but with minor modification the acoustic method could be used to place the probe so that real ear measurements accurately predicted SPL at the TM. The difference between TM SPL and SPL at probe position was compared for the acoustic method and a constant insertion depth (25 mm) method. More accurate estimates of TM SPL were obtained with the acoustic method.