Adding more layers of drywall can indeed improve soundproofing to some extent, even if it does not significantly increase the R-value (thermal insulation value). Soundproofing and thermal insulation are two different concepts, and the materials that work best for one might not be as effective for the other.
Soundproofing primarily involves reducing the transmission of sound waves through walls, floors, and ceilings, while thermal insulation aims to prevent the transfer of heat between different spaces.
Here's why adding more layers of drywall can enhance soundproofing without significantly affecting the R-value:
Mass and Damping: Sound waves can pass through materials, but the denser the material, the more difficult it is for sound to travel through it. By adding more layers of drywall, you increase the mass of the wall, which can help dampen sound vibrations and reduce sound transmission. The increased mass adds a barrier that makes it more challenging for sound to pass through.
Decoupling: Sound can also travel through structural connections. When you add multiple layers of drywall with appropriate insulation, you can create a decoupling effect. Decoupling involves isolating one side of the wall from the other, disrupting the path that sound waves would follow. This can further enhance soundproofing.
Resonance: Sound can create resonance in lightweight structures, causing them to vibrate and transmit more sound. Adding extra layers of drywall can help reduce resonance and improve soundproofing performance.
Sound Transmission Class (STC): The Sound Transmission Class is a rating system that measures how well a material or assembly can block sound. By adding more layers of drywall, you can increase the STC rating of the wall, leading to better soundproofing.
It's important to note that while additional layers of drywall can improve soundproofing, there are limitations to how much improvement you can achieve solely through this method. For more effective soundproofing, other techniques and materials can be incorporated, such as soundproofing insulation, resilient channels, acoustic caulking, or mass-loaded vinyl barriers.
On the other hand, increasing the R-value of the wall for thermal insulation generally requires different materials or approaches, such as adding insulation between the wall studs, using reflective barriers, or opting for materials with higher insulating properties.
In summary, adding more layers of drywall can contribute to better soundproofing by increasing mass and damping effects and potentially providing a level of decoupling. However, if you want to improve both soundproofing and thermal insulation, it's best to use appropriate materials and techniques for each purpose.