About STC Ratings

Sound Transmission Class, or STC, was first used in 1961. It is one of the original ways to determine how much sound materials block, and is still considered to be one of the best ways to determine sound transmission loss. STC is an integer rating of how well building partitions reduce airborne sound. Simply put, this means that STC measures how much sound is lost when traveling from one side of a wall, floor, or ceiling, to the other side. There are three main terms to understand when understanding STC. The first is decibels. The decibel, or dB, is universally used to measure the intensity of a sound. The decibel is figured by determining how loud a sound is, in relation to how far away from your testing area it is. That determines the intensity. 0 dB is near-total silence, while sounds over 85 dB can cause hearing damage or loss. The second is frequency, measured in hertz (Hz). Frequency is the property of sound that determines pitch, such as the difference in a high pitch or a low pitch sound. The standard human range for hearing is between 20 and 20,000 Hz. Low pitch sounds include things like the lowest pedal on a pipe organ, bass drums, and large truck engines. High pitch sounds include such things as whistles or cymbals. The third term to know is Transmission Loss. Transmission Loss is the measurement of decibel difference on either side of a wall. An example would be if you were playing a drum set in room A, and there was a wall dividing you from room B. The dB in Room A could be 100 dB, and only 75 dB in Room B. Therefore, there is a transmission loss of 25 dB.

To simply: Decibels (dB) measures how loud a sound is. Frequency (measured in hertz (Hz) determines the pitch of a sound. Transmission Loss measures how much sound is blocked by a partition.

STC works by averaging the amount of noise stopped at 18 different common frequencies, measured in decibels. A curve is created using this test to determine the transmission loss at those 18 common points. The STC only tests frequencies between 125 and 4000Hz, so it cannot give you test data for extremely low or extremely high frequencies. What it can do is assure you of this scientific fact: the higher the STC rating, the more sound is stopped. Another important thing to note is that every partition has an STC rating, that provides some level of sound reduction. A standard wall assembly made of 5/8” drywall with standard 2”x4” studs has an STC rating of 33. This is where the STC rating can confuse some people. Just because you add, for example, a 1LB MLV to a wall does not mean you get to add together the STC ratings. The STC ratings use a logarithmic formula, which would mean that adding a 1 LB MLV with a 26 STC to a wall with a 33 STC would result in around a 41-42 STC. You can then increase this rating through various other methods, including adding another layer of drywall or simply using MLV on both sides of the wall. For example, taking a wall assembly that is constructed of 5/8” drywall, 2x4 studs, and 5/8” drywall, with no insulation, and adding a 1 LB MLV and another layer of 5/8” drywall would result in an even higher STC rating, around a 48-49 STC. Since it is a logarithmic scale, similar to the Richter Scale for earthquakes, a small change of 9 or 10 in the STC represents a much larger change in the amount of sound reduced. Think about this: changing an STC rating from a 28 to a 38 reduces 90% of the noise you experienced at the 28. A change of 28 to 43 brings with it a 95% reduction in noise. For such a small change, these are substantial differences in noise level.

The range shown in the chart to the right comes from the test results of our Noise Grabber™ brand MLV. As you can see, it has different degrees of sound blockage at different frequencies and is the most wide-ranging of all the MLV sizes. This is what makes it such a good all-around choice for a project.