The mass law and sound insulation for single panels

Mass Law

Mass is crucial in sound transmission. Heavier panels generally offer better sound insulation. The effectiveness of a panel is measured by its Transmission Loss (TL). For a single panel a doubling of mass increases the performance of the partition by 6 dB. It is also helpful to understand that this effect also applies to frequency. As the frequency doubles, the acoustic performance also increases by 6 dB.

Some examples:

• If we have a block wall that is 200 mm thick, and increase its thickness to 200 mm, then we improve its acoustic performance by 6 dB.
• It we have a similar wall that is 150 mm thick, then the acoustic performance at 2000 Hz will be 6 dB better than the acoustic performance at 1000 Hz.

Although the mass law is a useful as a basic guide to acoustic performance, it is not as simple as that. Even for simple single layer constructions, there are other factors that affect the results.

Critical Frequency and Bending Waves

At a certain “critical frequency,” the performance of a single layer is reduced due to to bending waves. Heavier, flexible materials like lead have higher critical frequencies, while stiffer materials like concrete have lower ones.

Damping and Edge Effects

Vibration damping at the edge of the partition can affect its performance. This occurs due to energy loss where the partition meets other elements of the construction. The extent to which this occurs depends on the mass of the partition itself as well as the surrounding structure.

Multiple Layers

The most common way to improve the acoustic performance is to include multiple layers. Additional layers of different material types can improve the sound insulation above that which is predicted by the mass law. This is one of the reasons that cavity wall partitions and independent timber stud walls are so effective.

If you would like our help in designing an acoustic partition for your project, please don’t hesitate to contact us for further information.