Loudspeaker-to-microphone feedback causes echoes in audio/video conferences. A microphone typically has a background-noise level estimator, which provides noise-level estimates for the purpose of setting parameters within acoustic echo cancelers (AEC), noise reducers, comfort noise generators (CNG), etc. Output from a noisy loudspeaker may be picked up by the microphone, adding to background noise already present at the microphone. Loudspeaker noise, when picked up by the microphone, causes a misestimation of background noise, which in turn leads to incorrect parameter setting. This can lead to divergence of adaptive AEC circuitry, audible echoes, howling, etc.
Schuldt, Christian and Lundberg, Robin, "Noise and howling mitigation in audio/video conferencing", Technical Disclosure Commons, (November 07, 2017)