Product Manual

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.

Appendices Welch Allyn

OAE Hearing Screener