When you close a door and sound becomes quieter, it is not because sound travels slower in air compared to solids. Instead, it is primarily due to the concept of sound insulation and the phenomenon of sound waves spreading out and losing energy as they encounter obstacles.
When sound travels through air, it propagates as a series of compressions and rarefactions, forming sound waves. These waves spread out in all directions from their source. When a door is open, sound waves can freely propagate into other areas, including adjoining rooms or spaces. However, when the door is closed, it creates a barrier that obstructs the path of the sound waves.
Closing the door effectively introduces an additional solid obstacle (the door itself) in the path of the sound waves. While sound does travel faster through solids compared to air, it also encounters greater resistance and loss of energy when transmitted through solid objects. This is due to the inherent properties of the materials, such as their density and elasticity, which can absorb and dampen sound vibrations.
As the sound waves encounter the solid surface of the door, some of their energy is transferred to the door material through vibrations. These vibrations are then partially absorbed and dissipated within the door material, resulting in a reduction in the intensity of the sound wave. The door acts as a barrier that hinders the transmission of sound waves, preventing them from reaching the other side with the same strength as when the door was open.
In summary, even though sound can travel faster in solids, the closing of a door creates a physical barrier that causes sound waves to encounter resistance, leading to the loss of energy and a subsequent decrease in the perceived volume of the sound.