What is posted under the door with good sound insulation effect?

If the gap of the door is relatively large, it can be sealed with password tape or good quality sealant. If the requirements are not good, it can be filled with foaming agent.

There are other methods you can use:

First, fill the gap with cement mortar or water lime and engine oil lime;

Second, fill the gap with good quality glass glue;

3, filled with gypsum, and then painted with a paint brush;

Fourth, small gaps can be directly painted with epoxy paint.