Interacoustics OtoRead Instructions For Use Manual page 61

OtoRead™ Instructions for Use - US Page 55
7 Appendices
Appendix A: test sequence
A complete test sequence consists of a probe check, calibration, and test phase. The probe check phase
determines when the calibration phase should proceed, while the calibration phase calibrates the level of
the tones that will be applied during the actual test phase. Artifact rejection is employed during the test
phase to reduce the effect of transient noise bursts.
Immediately after the test button is pressed, the probe check phase of the test begins. The probe check
phase checks both the quality and stability of the seal by measuring the response obtained from a
sequence of test tones. The stability of the seal is determined by comparing the responses obtained over
time. When the level of the response is within an acceptable range and is stable over time, the unit
proceeds to the calibration phase.
FOR DPOAE
The calibration phase automatically measures the response obtained from a sequence of calibration
tones and calculates the voltage needed to obtain the desired pressures. If the desired peak pressure
cannot be obtained, the unit will use the maximum voltage. A successful calibration then leads on to the
actual test phase.
The test phase consists of measuring the response obtained from the pairs of test frequencies (f1, f2)
applied to the receivers. Two receivers are used, with each receiver generating one frequency in order to
reduce intermodulation distortion. Frequency domain estimates of the actual L1, L2, distortion (DP) and
noise floor (NF) are obtained via the discrete Fourier Transform, with a bin resolution of approximately 31
Hz. The NF estimate is obtained by averaging the power in the 4 closest (+/-2) bins to the DP bin.
FOR TEOAE
The calibration phase automatically measures the peak pressure obtained from a sequence of clicks and
calculates the voltage required to obtain the target peak pressure. If the desired peak pressure cannot be
obtained, the unit will use the maximum voltage.
The test phase consists of measuring the response obtained from repeated sequences of clicks applied
to the receivers. The click sequence is 3-1-1-1 repeated twice. Signal and noise floor estimates are
obtained by adding/subtracting the two response sequences, respectively. The energy of the signal and
noise floor estimates in various frequency bands is obtained in real time and displayed once per second.
The average peak pressure of the stimulus is calculated after completion of the test.
Artifact rejection is employed during the test phase to reduce the effect of transient noise bursts by the
use of an adaptive rejection threshold. The unit attempts to accept the quieter sections of the test, while
rejecting the noisier portions of the test. When the noise level is approximately constant during the test,
the instrument will tend to accept most of the data in the test. However, as the level of the noise becomes
more variable over time, the instrument will attempt to accept the quieter portions of the recording. Noise
estimates are obtained approximately 32 times per second and a suitable threshold is estimated from the
data. Data segments with a noise floor above this threshold are rejected, which tends to lower the noise
floor of the test. In order to reduce the possibility of obtaining an artificially low noise floor, the minimum
threshold level is limited.
Comment about variations in the SNR estimate
The user needs to be aware that the SNR estimate has an inherent statistical variation due to the effects
of random noise, especially when no emission is actually present. If a test is performed with the
instrument's probe placed in a test cavity, it can be shown theoretically that the SNR will be greater than 6
dB approximately 7 times out of 100. This is not a limitation of the instrument, but a fundamental property
of the method used to estimate the SNR in all emission testing. In order to reduce the occurrence of this
"false" emission, the instrument limits the minimum value of NF, which has the effect of reducing the SNR
for tests that have a low noise floor. As the noise level of the test increases, the user will notice that more
"false" emissions will appear, which is to be expected.