OtoRead Screener Appendix A: Test Sequence Page 50
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.
To Next Page
Loading...
