What are sound tests and how are they conducted?
Lab tests are conducted per ASTM E90 standards. This is a laboratory measurement of airborne sound transmission loss in building partitions. The end result is an STC and OITC rating that can be used, to some extent, to compare the performance of two similar type products. STC numbers are for 125 hertz or higher and OITC includes 80 and 100 hertz as well. The numbers are calculated in a different way, but overall meant to create an average expected performance for each rating. Most of our customers should be more concerned with performance between about 50 hertz and 1,000 hertz, which largely would be outside the scope of STC ratings. For the labe test, they will test a solid wall as a control, then cut an opening for the door and test the wall again with a door installed as operable or inoperable. Using the results of the full wall test compared to the door test, and using the size of the door specimen as an important part of the equation, an STC and OITC number will be determined. This rating can then be used as the official rating for that door assembly forever, with no expiration date, even when requirements change and testing equipment becomes more accurate. Not likely a huge difference for assemblies tested 2000 to now, but a significant asterisk to lab results for assemblies tested pre-2000.
Where did we test?
We have results from testing at NWAA Labs and Intertek. Two of the most respected acoustics labs in the country. We chose these labs for their reputation so we can know where we truly stand, making our results much more likely to be replicated in the field.
What did we test?
We tested various slab builds to determine the best lineup of models for our customers. We went a step beyond that and tested various seal design, door handles, and communicating assemblies. We couldn't think of anything else to test, so it had to stop somewhere. Ultimately, we tested in 6 different sessions from 3 labs, finishing with more than 100 completed tests.
Are all tests the same?
Common to the industry is manufacturers sending slabs to one lab, test it, then pack it up and ship it to the next lab. Repeat until they have visited every lab and promote the STC rating that was the highest. There's a plus/minus difference on STC of about 5, so the range can be significant. Meaning an STC 40 door at Lab A is an STC 45 at Lab B and an STC 35 at Lab C. The size of the room, standing room noise, quality of the equipment, humidity, and knowledge of the acoustical engineer overseeing the test are the largest variables. Many tests will be riddled with asterisks meaning they simply applied an educated guess to that frequency based on the successful results of other frequencies. Where a test was done is just as important than whether it was tested at all. ASTM E90 tests are not apples to apples, neither is it a standard that you should rely soley on when specifying products for your project or choosing a door assembly for your project.
Do all manufacturers test their doors?
The larger companies have all of course tested their assemblies thoroughly. Their testing generally isn't recent, most pre-dating the year 2000. Some testing we see was done over 50 years ago and still used today as if it is relevant. Other 'manufacturers' you might find online tout high STC ratings and actually have never tested their doors. The industry is not well monitored and really any claim can be made without much oversight or follow up to prove efficacy of door assemblies.
What rating do I need for my assembly?
STC ratings are essentially an average over 16 frequencies, so the answer isn't that straight forward. Many of our inquiries are for doors with STC ratings that match the STC ratings of the walls they have planned. Requests for STC 50, STC 60, so forth, although the door assembly STC rating does not need to match the wall exactly. If a true STC 55 or higher is required, then it must be accomplished with a communicating door assembly. Our standard practice is to match our doors with projects using this basic logic:
- Standard wall, specify the IsoDoor V.
- Standard wall with MLV/Green Glue Compound, specify the IsoDoor V or IsoDoor SD.
- Staggered stud wall with MLV/QuietRock/Green Glue Compound/resilient channel, specify the IsoDoor SD or IsoDoor HD.
- Double stud wall or wall decoupled with resilient clips, specify the IsoDoor HD or IsoDoor HDLF.
- Doublt stud wall or wall decoupled with resilient clips to isolate a home theater or recording studio, specify a communicating assembly that fits with the wall thickness.
Should I look at something other than door tests when selecting doors?
If you can't trust the testing, or find the test results provided are limited, look at the door specifications. How thick is the door, how much does it weigh, and what are the jambs made of. A door that weighs about 10 pounds per square foot will likely perform in the range of another door of the same weight. A door could have significant resonance issues, causing it to perform lower than expected, but the weigh is generally a good indicator of performance. From there, look at the seal system. If it's one heavy surface mounted seal, then performance will be limited to around 42 STC. If there are multiple seals with air space between them, then you can expect performance into the higher 40's for STC. Then look at the jamb/frame. If it is made of wood, it will top out at STC 49. If it is made with steel and back filled with concrete, then it could push into the low to mid 50's STC.
Apples to apples though, two doors that weigh about the same, use the same jamb/frame material, are tested to show they do not have significant resonance issues, and use a similar seal system, are likely two doors that perform about the same.