The speed of sound varies depending on the medium through which it travels. In general, sound travels fastest through solids, slower through liquids, and slowest through gases like air. While it’s true that sound travels more slowly through air compared to solids, the ability of a material, such as a wall, to block sound is not solely determined by the speed of sound within that material. The key factors in sound insulation are related to how sound waves interact with and pass through the material:

1. Density and Elasticity: The speed of sound in a material depends on the material’s density and elasticity. Solids, being denser and more elastic than gases like air, allow sound waves to propagate more quickly. In contrast, air has a lower density and lower elasticity, which results in a slower speed of sound. However, this doesn’t mean that air is inherently better at blocking sound.
2. Sound Attenuation: The effectiveness of a material in blocking or attenuating sound is influenced by its ability to absorb, reflect, and dissipate sound energy. Materials like walls are designed to be relatively dense and thick, which helps to absorb and scatter sound waves.
3. Obstruction of Path: When sound waves encounter a solid material like a wall, they may be partially reflected, absorbed, or transmitted. The thickness, density, and mass of the wall can obstruct the path of sound waves, preventing them from easily passing through to the other side. This is why solid materials like walls can act as barriers to sound.
4. Soundproofing Design: Effective soundproofing involves multiple layers and materials that are specifically designed to block, absorb, and dampen sound. Walls in buildings are constructed with layers of materials, such as drywall, insulation, and structural elements, to minimize the transmission of sound from one side to another.
5. Air Gaps: Some soundproofing techniques also incorporate air gaps or layers of air between solid materials, as air can serve as an additional barrier to sound transmission. The combination of air and solid materials can improve sound insulation.

In summary, while the speed of sound is indeed slower in air compared to solids, the ability of a wall or other solid material to block sound is primarily due to its physical properties, density, thickness, and design. Walls are constructed to impede the passage of sound waves, making them effective barriers for reducing the transmission of sound from one area to another.