When Coherence Blanking is enabled, only portions of the measurement that are above the selected signal to noise ratio will be displayed. This is a helpful indicator of measurement quality.
For example, the measurement below was taken in the presence of external noise (someone talking). This significantly contaminated the measurement and unusable sections have been blanked. This measurement should probably be discarded.
Signal to noise issues are typically resolved by removing sources of noise or increasing the volume of the audio system. In rare situations the microphone may be faulty.