noise barrier – megasorber 4-fold approach (abcd)

A noise barrier blocks sound. The most commonly used terms to describe the effectiveness of a noise barrier is:

sound transmission loss (STL)

A measurement of the reduction in sound level of a sound source as it passes through an acoustic barrier. It is the number of decibels that are stopped by the acoustical barrier or the wall and is measured at different frequencies.

sound transmission class (STC)

Sound Transmission Class is an integer rating of how well an acoustic barrier or a building partition attenuates airborne sound.

weighted sound reduction index (Rw)

A number is used to rate the effectiveness of an acoustic barrier attenuating airborne sound and is measured in decibels (dB). It also considers a correction factor for the human ear’s response.

what is the difference between STC and Rw?

STC covers 125 Hz to 4000 Hz, while Rw covers 100 Hz to 3150 Hz. Both cover 16 one-third octave bands. STC is commonly used in the USA, and the Rw is used outside the US.

what is Rw (C,Ctr)?

Rw (C; Ctr) = 33 (-2; -6) dB. Rw represents the weighted sound reduction index or a value that indicates how much dB is reduced when noise passes through an acoustic barrier. C and Ctr are correction factors, namely C for the medium frequency range (e.g. speech) and Ctr for the low-frequency range (e.g. traffic).

The higher STC (or Rw), the more air-borne noise is attenuated by an acoustic barrier or a building partition. STC rating goes from 25 – 65. The higher the rating, the less noise penetration. A rating of 25 is no soundproofing, and 65 is complete soundproofing.

how does transmission loss work?

STC (or Rw) ratings explained

Is it possible to have a thin building partition with high STC (or Rw)?

There are many ways to improve the sound transmission class of a partition, though the two most basic principles are adding mass and increasing the overall thickness.

With the latest material technology and smart engineering design, it is possible to design and build thin partitions with high STC (or Rw).

Mass law explained, and the importance of natural resonant frequency

The Mass Law governs the performance of barrier material. The Mass Law is divided into three regions, as shown in the diagram (right). In Region 2, the Sound Transmission Loss increases by 6dB(A) when doubling the weight of a noise barrier or building partition.

However, partition materials, such as plasterboards, fibre cement sheets, insulated panels, honeycomb panels, metal panels, etc., have their natural vibration frequency or critical frequency.

As shown in Region 3, at the critical frequency, the sound transmission loss of these partition materials decreased significantly, i.e., the materials have very little or non-capacity to attenuate any sound.

How to address this issue of critical frequency

The key is to introduce internal damping into the system, as shown in Region 3, with various damping levels.

This schematic drawing shows the performance of a stiff material, such as plasterboard, fibre cement sheets, and aluminium panels, with and without damping.

It is evident that the combination of adding mass and vibration damping optimized the STC (or Rw) rating for a thin partition wall.

Take the 90mm stud wall, for example. The addition of Megasorber B8, constrained with additional plasterboard, adding the mass as per Mass Law, and providing damping to address the critical frequency issue. As a result, it has a much higher STC (or (Rw) rating.

A typical stud wall, comprising two sheets of 13mm plasterboard on a 90mm stud wall with no insulation, provides an STC rating of 40.

If you add Megasorber B8 and the second layer of 13mm plasterboard to one side of the wall, the STC jumps to 47.

Adding the same treatment to the other side of the wall provides an STC of 51.

Furthermore, sound waves like to travel in the same media, i.e., the same material or homogeneous material. For example, the sound wave travels well with two layers of plasterboard. However, when a limp mass layer, such as Megasorber B8, with high internal loss, is sandwiched between the two layers of plasterboard, it causes the transmission pattern of the sound wave to breakdown completely, resulting in a much higher STC (or Rw) rating.

If we replace one of the layers of the plasterboard with MDF or plywood, it is expected that an even higher STC (Rw) rating can be achieved, due to the change of materials, causing much more difficulties for the sound wave to travel through.

As a result, for a thin 90mm stud wall, the STC (Rw) can be as high as 51.

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