When you close a door and sound gets quieter, it's not because sound travels slower in air compared to solids. The decrease in sound intensity is primarily due to the process of sound transmission and the properties of the materials involved. Here's a breakdown of what happens:
Sound Transmission: Sound travels through different media by creating a series of compressions and rarefactions in the particles of that medium. In the case of air, these particles are more spread out compared to solids, which makes sound propagation less efficient.
Sound Reflection: When sound encounters a surface, part of it gets reflected while the rest is transmitted or absorbed. This behavior depends on the nature of the surface and the frequency of the sound.
Reflective Surfaces: Smooth, hard surfaces, like walls or doors, tend to reflect sound waves. When you close a door, it creates a barrier that reflects a significant portion of the sound back into the room where it originated. This reduces the sound intensity in the adjacent space.
Absorptive Surfaces: Materials with porous or soft surfaces, such as carpets, curtains, or foam, tend to absorb sound waves rather than reflecting them. These materials dissipate the sound energy as heat, reducing its intensity.
Sound Leakage: While closing a door helps in reducing the sound intensity, it doesn't entirely eliminate sound transmission. Some sound can still leak through gaps around the door, such as the space between the door and the floor or the door and its frame. These gaps allow sound waves to escape, albeit at a reduced intensity compared to when the door is open wide.
In summary, the decrease in sound intensity when you close a door is primarily due to the reflection of sound waves from the door's surface and the reduction of sound leakage through the gaps. While sound does travel faster in solids, the nature of sound propagation and transmission in air and solid materials is different, resulting in different perceived sound levels.