Operating Instructions
68
Instructions for Averaging Results
For monitoring purposes, it is the DP level itself that should be averaged. The DP level indicates the
health of the outer hair cells and the middle ear, when testing in relatively quiet conditions. The SNR is
the measurement of choice in screening, where varying noise levels are present.
Average the DP level at each frequency of interest for the total number of tests performed during one
session. Repeat the same procedure for each subsequent session. Compare the frequency-specific
average from the prior session to the average from the current session using the critical differences
provided in Table 1.
Producing the Desired Eardrum SPL
Siegel (1994) reported that large differences between eardrum SPL and the SPL measured by the
microphone in an OAE probe could occur at high frequencies. In our own measurements, we have seen
occasional differences as large as 15 dB from the combination of wavelength effects and improper
eartip seating.
Following Harris et al (1989), Siegel recommended that at high frequencies the receiver drive required to
produce the desired eardrum SPL be predicted from a low-frequency probe measurement of ear canal
SPL (to adjust for individual ear canal volume differences) and a previously determined receiver-to-
average-eardrum calibration.
The ERO•SCAN follows Siegel’s recommendations, additionally imposing an upper
limit of 70 dB SPL
measured at the microphone of the probe in order to minimize spurious distortion products. The P1 and
P2 SPL values printed by the ERO•SCAN unit are those measured by the microphone in the probe in
order to minimize spurious distortion products. These provide a check that neither probe is blocked, but
as described above may differ from actual eardrum SPLs.
Summary of Contents for Ero Scan Pro
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