Why Does the Battery Cover Vibrates When the Device Volume Is Increased in A9/ A5 2020?

Situation: You turn up the volume of your phone and noticed the battery cover vibrating.

Applicable to: A9 2020, A5 2020

Please be assured that this is normal because the mobile phone has adopted an additional speaker for stereo playback mode. Compared to the conventional single speaker solution, dual stereo speakers create a strong vibration. Since the internal airflow of the whole device is large, it causes the battery cover to vibrate.