4. Insulation materials 

 Gas-tight solid materials, for both airborne sound insulation effect, sound insulation effect is good or bad depends on the most fundamental point is that the quality of material per unit area. 

 See Figure 7, an area of ??very large compartment, its mass per unit area as ms, when the wave normal incidence from the left to inspire a whole compartment for vibration, the vibration again 
 Right acoustic space radiation. Per unit area of ??consideration, the sound transmission to the right space to the compartment with the incident sound energy on the ratio of said transmission coefficient τ. Define an infinite interval 
 Transmission loss of the material layer (also known as transmission loss)