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How to Measure STC Ratings (ASTC Measurements)

by Melinda Miller on July 12, 2012

image of how to measure STC ratings - acoustical engineerWhen we are testing a noise issue between units in a multi-unit housing complex or between closed office spaces, I often get the question “Can we just turn on some loud music in one room, and listen in the next room?” Good question. Although this can help confirm that there is an issue with inadequate sound isolation, it doesn’t exactly give us enough information to help provide a solution.

To ascertain the level of sound isolation of an existing assembly, there are two approaches. If the original drawings are available, we can look up the designed assembly and determine the intended sound isolation by comparing the assembly to existing lab tested assemblies. These lab tested assemblies provide a single number rating called the Sound Transmission Class (STC) which tells us how much sound the assembly is supposed to block. The higher the STC rating, the better the sound isolation. Although the STC of the designed assembly provides a good starting point for investigating the problem, existing buildings aren’t always constructed exactly as designed. Because of this, a field test will provide more accurate information on how the assembly performs.

A field test provides the Apparent Sound Transmission Class (ASTC) which is typically 5-8 points lower than the STC rating. Testing according to recognized standards is the best methodology to make sure that the test is done in a repeatable and reliable way. This involves using an amplified speaker to produce broadband noise in one room (source) according to specific internationally recognized standard, usually ASTM E336. The sound level is then measured in the source room and the adjacent room (receiver). We then determine the reverberation time in the receiver which allows us to calculate the sound absorption and along with the source and receiver measurements, the resulting ASTC of the assembly. If the field tested assembly is significantly lower than expected, we know that it wasn’t constructed as designed. If the assembly tests as we expect it, we know that the assembly wasn’t designed to provide enough sound isolation for the current use of the space. Regardless of the results, in an existing building, an ASTC test is a useful tool for diagnosing a noise transmission issue between spaces.

At Acoustics By Design, we can help you measure or predict the STC rating of a an assembly. Our engineers want you to get it right from the beginning, so your building has the right amount of sound isolation.

Melinda Miller

Melinda Miller

Melinda Miller, PE, LEED AP BD+C, EDAC, INCE. Bd.Cert., has been working in acoustical engineering since 2001. Her expertise includes diagnosing and preventing noise problems, as well as designing acoustically optimized environments using 3D computer modeling, and evidence-based design practices. Melinda earned her Bachelor's Degree in Mechanical Engineering from the University of Idaho in 1998, and Master's from the University of Illinois, Chicago, in 2003. Melinda is the Director of Acoustics for Acoustics By Design, and the Senior Acoustical Consultant in Portland, Oregon.

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rizgar January 23, 2014 at 1:22 pm

dear all
i am interested in the meethods to reduce noise transmission between the rooms especially in student dormiteries or muli story buildings.

best regards

Brian Atkinson Brian Atkinson January 24, 2014 at 11:57 am

Reducing noise between spaces, or noise isolation, is usually done through a combination of building techniques and materials employed during construction, and architectural finishes used later.
While this is of course not an exhaustive answer to your question, I hope it can steer you in the right direction.

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