Choosing Your Sound Door Wisely

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We manufacture our own line of interior doors online, known as IsoDoor and IsoDoor HD, entirely in-house here at IsoStore. These doors have been tested as inoperable slabs (puttied into a space the size of the slab), as fully operable individual doors, and in fully operable communicating assemblies. Testing took place over a period of 6 months in 2012 at three different accredited acoustic labs in the United States with the bulk of the testing at NWAA Labs in Washington.


With over 120 total lab testing hours and over 50 separate completed tests comparing styles of door slabs, door jambs, communicating assemblies, door handles, perimeter seal systems, door seal brands, etc., we know more about sound blocking doors than companies with limited/outdated testing or no testing at all. It places our company in a unique position. Our testing is recent, relevant to a wide range of assemblies, and extensive enough that we can specify a door or communicating assembly to meet or exceed requirements for any assembly. With this knowledge brings a level of experience that much of our competition online just does not have. We know the expected performance of nearly every common variable with acoustic doors. Unfortunately, a lack of experience or willingness to learn, many consumers are left to only compare stated ratings when choosing a soundproof door instead of comparing accurate lab tests.


Most would expect major acoustic door manufacturers to have extensive, current, and accurate testing. The truth is these companies are more likely to have minimal, outdated, and insanely inaccurate testing. Reason being is that they have been around since the early days of STC testing or even pre-date the creation of the STC standard. Several of their performance claims are based on testing done over 30 years ago when standards were not nearly as stringent as they are now and testing equipment was not nearly as accurate as it is now. Legally they can make these claims because they do have a test showing an STC result. It doesn’t matter that the same sound blocking interior door would rate 15 points lower in 2014 than it did in 1964.

To make matters worse, not only were these tests done during a time of weak standards and even weaker testing equipment, these tests were also often done in-house by the manufacturer and not a third party! Research these big companies and get as much information as possible about the testing. If the test was conducted by a third party at an accredited lab sometime after 2000, then consider the results accurate enough. If the testing was before 2000 or if the tests were not conducted by a third party, then the results cannot be considered accurate.


For years, one of our competitors has posted their door rating as “STC Ratings Up To 56”. They resell this door through several online dealers, most of them also make this terribly inaccurate claim, and no actual test exists showing this level of performance. One of their dealers we talked to has tried to get test results for years without success. Ask them for a test yourself and they will give you a number of excuses as to why they don’t have one, claim they will have testing soon, or claim testing was completed but needs to be redone because of limitations with the testing wall from the door rating ‘too high’ in comparison. The more likely answer is the testing is never going to happen, or possibly it has and the results were so much lower than the claimed ‘up to STC 56’ that they will never post the results and decided to opt for continuing as they were instead of fixing their door design to rate better.

We are confident this sound blocking interior door will not rate anywhere near STC 56. How do we know this? Their doors have several obvious limitations that would make it physically impossible to achieve this rating. Assuming their slab has no resonance issues, our experience and knowledge in door ratings would estimate the STC rating of this door to be, at best, an STC 44. That’s 12 points below the stated performance and the same as our 1-3/4″ thick IsoDoor weighing 240 pounds.


There are ways to legitimately manipulate the rating of a door in an acoustics lab that meets ASTM testing standards. Here are a few that we learned, but of course did not implement in our testing:

  • Including the door trim in the overall door dimension to increase the STC rating by 2-3 points. This is common practice with steel acoustic door manufacturers. They attach the steel trim to the jamb to manipulate the calculations of the test area. They will claim a surface area of something like 43″ x 87″ compared to our tested surface area of 37-1/2″ x 81-1/2″.
  • Testing doors in walls that are not significantly larger than the door itself. The larger the wall is, the more accurate the results for the performance of the door. Many doors are tested in walls that are 9′ tall and 12′ long, sometimes even smaller. This is a common wall size in the real world, but skews the results in a lab. Our doors were tested in a wall that is over 22′ tall and 35′ long.
  • Back filling space between the door jambs and framed opening with concrete. This is especially common with steel doors. Sure you can do this with our doors and increase the rating a few points, but people do not want to do this in real world installs. They use gasket material, putty, and acoustical sealant. So to keep it honest and accurate, we did too.


Our suggestion is to review recent test results from accredited acoustics labs as this is your best chance to objectively compare acoustic doors. Unfortunately, this is also where the waters muddy a bit. The expected consistency from lab to lab allows for a 6 point STC swing. An entirely legitimate test from Lab A may yield an STC 40 and at Lab B a solid STC 46. Miscalculations, different qualities in microphones, humidity, temperature, lab noise vs. outside noise, flanking, size of testing wall, and many other contributing factors can skew the results.

The result of this is a manufacturer sending a door specimen to five different labs fishing for whatever rating is the highest. Why not advertise the highest test result? It may be STC 40 at Lab A, but it was STC 46 at Lab B, so go with the higher number for marketing and access to additional markets that require at least STC 46 doors.


We chose to have our final testing conducted at NWAA Labs in Washington for several reasons:

  • New lab built entirely in the last few years with a quality design and incredible mass to limit flanking. Most labs have to estimate performance at several lower frequencies because their labs have significant flanking issues and hardware limitations.
  • Second quietest room in the world with a background noise level at -7 dB. This is because they built their acoustics lab inside of an abandoned nuclear facility with several 4′ thick concrete walls to pass before getting to their lab. This is compared to many accredited labs with background noise levels well over 20 dB.
  • Second loudest source room in an acoustics lab, behind only NASA, pumping out an impressive 135 dB.
  • Top of the line microphone and processing equipment for accurate results.
  • Huge testing wall to provide the most accurate results possible from 50 Hz up to 10,000 Hz without estimating performance at any frequency.
  • Ron Sauro, former NASA scientist and world renowned acoustical engineer, personally oversees each test. There are many talented engineers in this field, but this is the guy that writes the standards for the others.


Like all performance claims in the world of sound isolation, do your homework and educate yourself on choosing a soundproof door. Even legitimate manufactures have illegitimate testing and illegitimate manufacturers will claim whatever their customers let them get away with. Acoustical consultants and architects will see through the false claims, but then specify doors with outdated testing conducted in-house. So even the guys that are supposed to know what they are doing are making the same mistakes that the average consumer will make. This is frustrating for consumers and rightly so. Feel free to contact us with any questions about test results you have found online. We are happy to review and sift through the trash to help you out. If our acoustical doors are the best option for you then great, if not then we would at least prefer you purchase one that rates higher over one that just claims to rate higher!