But I don't know what's an effective way to soundproof it.
As much as possible I don't wanna spend much money that could reach $20 and above.
I don't have that much budget
Best Answer
Suspend a heavy rolled-up quilt (get it from a flea market or discount re-seller) above the door. When you are inside, unfurl the quilt so it covers the entire door and frame. probably your best option for the money you are willing to spend.
Ridged foam insulation might work here. It comes in large pieces and is lightweight enough that it won't take many fasteners to hold it up (so you'll have less nail holes to fill when you leave). The downside of course is that it isn't very pretty.
These products are typically used to weather seal an exterior door from the elements, but should work just as well in your situation. They should all be available at your local home improvement store.
Best Answer
Suspend a heavy rolled-up quilt (get it from a flea market or discount re-seller) above the door. When you are inside, unfurl the quilt so it covers the entire door and frame. probably your best option for the money you are willing to spend.