What Is Transmission Loss In A Wall?

If you asked most people they would tell you that adding acoustical material to a room will help reduce noise emanating in an adjacent spaces. This is not true. Careful planning of the construction will deliver proper acoustic isolation for a space. A typical wall, solid with no leaks, will stop 40 to 50 db of sound at the middle frequencies. This is equal to 99.99% to 99.999% of the sound! With a good design and careful construction practices a figure of 60 db can be achieved.

However, the success of this type of wall construction has many other factors that will limit its performance such as sound leakage around the wall itself, doors, and windows. A small leak of just 1 square inch can have a major effect on the walls performance. High frequency sound is easier to block the low Bass frequencies. The major factor that blocks low frequency sound is mass or weight and isolation. As an example, a material by itself may have a loss of 25db at a certain frequency, by simply doubling the thickness of the material an additional loss of approximately 5db should be achieved. If these material were separated by an air gap the additional loss would be in the 20db range. An additional loss will be gained if the cavity between the two materials is filled with a sound deadening material. Light Gauge steel studs are better for isolation than wooden studs. A double wall construction with two sets of studs is better for isolation than a single wall. Some types of wall construction are rated by there ability to block speech. This rating is call Speech Transmission Class and is the STC value of a wall. The higher the STC value the more isolation, however a higher STC figure may not indicate better performance at the lower frequencies and may actually have a poorer result.

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