Why can I hear my neighbour even though my walls are soundproof?

Acoustic consultants avoid the word soundproof because it’s almost impossible for a real-world wall to be soundproof.

Instead, acoustic designers seek to achieve the appropriate acoustic performance for a wall, based on local codes, the expected noise generated on one side of the wall, and the expected sensitivity on the other side of the wall.

Hearing your neighbour could mean that your wall is achieving less sound insulation than it should or your neighbour is making more noise than the wall was designed to block.