When sound travels through different mediums, its speed can vary based on the properties of those mediums. While it is true that sound generally travels faster through solids than through air, the phenomenon you mentioned can be explained by the concept of sound transmission and the nature of doors as barriers to sound.
When a door is closed, it creates a physical barrier between two rooms, typically consisting of a solid material like wood or metal. While sound can travel more efficiently through solids, the transmission of sound from one room to another is not solely dependent on the speed of sound in the materials involved. Other factors come into play, such as the density, thickness, and composition of the door.
When sound encounters an obstacle like a closed door, several things happen. First, the solid material of the door absorbs and reflects some of the sound energy. This absorption occurs because the sound waves cause the door material to vibrate, converting some of the sound energy into heat energy. As a result, the sound energy is diminished.
Additionally, the closed door acts as a barrier that restricts the direct transmission of sound waves. While sound can pass through solid materials more effectively than through air, it still encounters resistance. The sound waves need to traverse the door's surface, which can attenuate the sound as it interacts with the material. The thickness and density of the door further impede the passage of sound waves, reducing their intensity on the other side.
Furthermore, air gaps or cracks around the door can contribute to sound leakage. If the door doesn't fit tightly or has gaps, sound waves can escape through those openings, leading to decreased sound insulation.
Overall, while sound may propagate faster through solids, the factors mentioned above, such as absorption, reflection, and the door's barrier effect, significantly affect the sound transmission between two rooms. As a result, closing a door between two rooms can make it harder to hear a sound through it, even though sound generally travels faster through solids than through air.