When you close a door between two rooms, it becomes harder to hear a sound through it because the door acts as a barrier that obstructs the transmission of sound waves. While it's true that sound generally travels faster through solids compared to air, the key factor here is the obstacle itself, rather than the speed of sound in different mediums.
When sound travels through air, it propagates as longitudinal waves, causing the air particles to vibrate and transfer the sound energy. When a sound wave encounters an obstacle like a closed door, several things happen:
Reflection: Some portion of the sound wave is reflected back towards the source. This reflection can cause echoes or reverberations, but it does not contribute to the sound being transmitted through the door.
Absorption: A portion of the sound wave is absorbed by the material of the door. The door material can dissipate some of the sound energy as heat, reducing the overall sound intensity.
Transmission: The remaining portion of the sound wave can be transmitted through the door if the door is not completely airtight or if it has openings like gaps or cracks. However, this transmission is often significantly reduced compared to direct transmission through the air.
The ability of a material, such as wood or metal, to block or dampen sound waves is determined by its density, thickness, and other acoustic properties. Solid materials, like a wooden or metal door, are denser than air and can effectively block a significant portion of sound waves. As a result, when a door is closed, it acts as a physical barrier that hinders the transmission of sound, making it harder to hear sounds from the other side of the door.