Welch Allyn OAE Hearing Screener - User Manual - Manual (Page 58)

Appendix C: Test sequence
A complete test sequence consists of an:
• AutoStart
• Calibration
• Test phase
AutoStart phase
The AutoStart phase determines when the calibration phase should proceed. Artifact
rejection is employed during the test phase to reduce the effect of transient noise
bursts.
Immediately after you press the test button, the AutoStart phase begins. AutoStart
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.
Calibration phase
The calibration phase calibrates the level of the tones that will be applied during the
actual test phase. The calibration phase automatically measures the response obtained
from a sequence of calibration tones and calculates the voltage needed to obtain the
desired intensity levels. If the desired peak pressure cannot be obtained, the unit will use
the maximum voltage. A successful calibration leads to the actual test phase.
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.
Variations in the SNR Estimate
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 you perform a test with
the instrument’s probe placed in a test cavity, theoretically 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.
54 Appendices Welch Allyn ® OAE Hearing Screener