The mass law describes the relation between sound frequency, mass per unit area, and sound insulation efficacy. In other words, a material with a higher density is less likely to vibrate when in contact with sound waves and will transmit less noise through the substrate.
The sound transmission loss of a floor or wall differs depending on the structure’s method of assembly and the physical properties of the material, such as stiffness and mass. For example, in a double-layer wall consisting of wallboard on metal or wood framing, the sound transmission class (STC) can be critically affected by the following factors:
- Depth of air spaces
- The absence or presence of sound-absorbing material
- The extent of mechanical decoupling between the layers
Noise can be either blocked or absorbed, and mass law and sound transmission loss focus on blocking noise. When attempting to prevent noise transmission between rooms, the mass and weight of materials between them must be able to disrupt the transmission of sound waves through the structure. When comparing a 4 inch concrete wall of an industrial building to a standard 2×4 stud wall of a residential property, the wall with a higher mass will transmit less noise and naturally be quieter.
Learn more about the factors influencing sound transmission loss and the industrial applications of these acoustic concepts.
Factors Affecting Sound Transmission Loss
While residential applications vary considerably from industrial applications, the same logic applies. The most important physical property that controls airborne sound transmission through a structure or assembly is the mass per unit area of the component layers. The mass law equation states that when the mass per unit area of a single-layer wall or frequency doubles, the transmission loss will increase by approximately 6 decibels.
Since sound is directional in nature, we only need to strategically place noise barrier products in specific areas that are directly in the line of the noise. For example, in an excavator cab, we would only put a barrier on the wall between the engine and the cab, not around the entire space. This results in effective noise reduction as well as minimized material costs.
Mass-Law & Sound Transmission Loss: Industrial Applications
There is a misconception that doubling the mass of a material from 1 lb/sq ft to 2 lbs/sq ft pounds will result in two times the noise reduction. Mass law states otherwise.
At X frequency, a material performs at Y, resulting in an average STC rate. STC can help you compare different materials and products. Most typical 1 lb/sq ft materials have a 26 to 27 STC rating. According to mass law, if the material doubles in mass, its STC rating only increases by 6, meaning that 26 STC would only go up to 32 STC, resulting in only a 6 dB reduction in noise.
Cost vs. Performance Balance
While doubling materials can cost twice as much, it does not double the performance. However, adding a thin foam layer(decoupler) between the barrier layer and enclosure wall,will create an air gap between the materials. Separating the two structures increases performance according to the double wall effect, in which an air cavity exists between the barrier and the enclosure wall and will increase acoustic insulation properties.
Learn More About Mass Law With Technicon Acoustics
As a leading supplier for OEMs throughout North America, Technicon Acoustics specializes in developing noise control products for a wide variety of industries.
Contact us to learn more about our acoustic barriers, including floor mat barriers and mass-loaded vinyl materials, which offer exceptional noise absorption and sound transmission loss. You can also request a quote for your specific application.
