Final answer:
Difficulty hearing clapping behind a stone wall occurs because sound waves are reflected or absorbed by the wall, reducing the sound intensity that reaches the listener on the other side.
Step-by-step explanation:
When you stand behind a massive stone wall and have trouble hearing a person clapping her hands on the other side, it's because the sound waves emitted by the hand clapping cannot effectively travel through the dense material of the wall. Sound waves are types of pressure waves that require a medium to travel, such as air, water or solids. When they encounter a barrier as dense and thick as a stone wall, most of the sound energy is reflected or absorbed, making it difficult for the waves to reach the other side and thus making the clapping sound quieter or inaudible to a listener standing behind the wall. The wall acts as a shield, preventing the sound intensity from being strong enough beyond it to be easily heard. This is different from an echo, where sound waves bounce back from a large reflecting surface and can be heard.