In early 2013, poindexters was contacted by a newly constructed low-income housing complex that was having some complaints about sound transmitting through to adjacent apartments. They had taken steps to minimize sound transmission during construction, but they were afraid they weren't hitting the numbers originally projected. We were asked to come test a number of partitions to determine the "Apparent Sound Transmission Class" value (ASTC) of the walls. This is a real-world test that is conducted in accordance with specific standards, and the result is a single value that describes the efficiency of the partition at blocking sound. The higher the number, the less sound is able to transfer through the partition.
We came in with objective, scientific testing tools and began testing different partitions around the complex. We tested party walls between apartments, as well as floor/ceiling assemblies. A total of 15 assemblies were tested and then analyzed. The report found that many of the partitions had around a 40-45 ASTC rating. The specification for construction was an STC - sound transmission class - value of 50. The problem with that is a real-world ASTC rating is often lower than a laboratory STC rating because of construction limitations known as flanking. Flanking is when sound travels around or through weak points in a structure. For instance, cutting a hole in drywall for an electrical outlet is such a path.
The results being less then expected was not surprising to us since there was a number of flanking paths, and it turns out that the construction methods varied quite a bit from the laboratory tests. For example, the lab tests they were using for their base-line partition assembly was a 2x4 wood stud wall with studs every 24 inches. In the real world, many of the walls had studs every 16 inches. In addition, a standard 8 foot tall wall has only about 4-5 screws per stud. However, the walls of the apartment complex were "shear" walls so the extra support came from an additional 20 screws per stud! The screws couple the drywall to the studs allowing more sound vibrations to transfer through the wall - decreasing the ASTC of the wall.
The lesson learned from this testing project: stopping sound transmission is not something you want to aim low at, rather, aim high and you'll end up hitting the results you were shooting for. There are 3 "grades" we use for ASTC numbers on party walls of dwellings:
Grade 3 - ASTC 48 Loud speech not audible, loud noises above the level of loud speech such as a loud stereo would be heard, may cause annoyment.
Grade 2 - ASTC 52 Very loud sounds such as shouting or coughing, some musical instruments, a loud stereo or surround sound system may be heard, bass energy perhaps that generated by a subwoofer may be the most bothersome frequency range with the greatest ability to excite and breach a wall system built to STC 50+.
Grade 1 - ASTC 60+ Considered excellent sound proofing between walls, most sounds with the exception of loud low bass (subwoofer) frequencies or loud percussive instruments such as a drum set would be contained within the sending room.