Sound Transmission Control and the Design of Wall Sections
Quick post here… I am looking into the issues surrounding the design of wall sections for sound transmission control. Here are the things I am considering in the design:
- Sound frequency is measured in Hertz (Hz). Hertz is defined as one cycle per second, and is typically applied to Sine waves.
- Sound is a compressive wave that is sent through a media, whether it is air (as we usually experience it), water or any other solid material.
- The control of sound therefore must happen as a means of dispelling the sound wave, which happens through the process of refraction.
- Absorption of the sound wave could also be considered as a type of refraction, through which the sound propagates through a given medium, but at a lesser intensity.
- First thing you should take into consideration in the design of a wall section is the expected sound levels of the rooms that you are blocking between. Since the spaces under consideration are normal office spaces, we will assume a sound rating level of 60dBa for a moderately loud office. The dB (decibel) level is a measurement that gives an indication of the loudness of the sound.
- The ultimate capability of the wall or any media to block sounds is its ability to handle a number of frequencies (through dispelling each sufficiently) within the expected range of frequencies that will be encountered.
- From the early 1960’s, the range of sounds was determined according to what has become to be known as STC… the Sound Transmission Class.
- STC is determined through testing various assemblies of materials and comparing the reading of the sounds from one side of the material to the other side for a range of frequencies.
- The range of frequencies tested for STC ratings are from 125 Hz (Hertz) to 4000 Hz.
- Most complaints are generated due to sounds that are below the 125 Hz frequency level.
- The STC rating of each of the various frequencies are graphed and a curve is developed from the various points. The resulting curve is called the STC rating.
- A higher STC rating is usually better than a low one.
- The STC rating doesn’t correspond to the number of decibels the wall could stop.
- STC numbers can’t be added together, since they are a representative of a logarithmic indication.
- For an office application, an STC of 45 or over should be sufficient for most typically encountered situations.
- “Green Glue ” is an adhesive that is used in many sound control applications.
- If a wall occurs along a curve, then Gyp. board that is flexible is needed. However, flexible gyp. board comes in 1/4 inches thick sections and has a lower STC rating than 5/8 inch thick drywall.
- I should not expect to get more of an STC rating than about 30 for a non-insulated wall with 1/2″ drywall.
- With a curving wall section that has an extended portion, the studs will need to be staggered to increase the sound refraction capabilities.
- To proceed, I called the USG to determine what they would recommend to get both the curve as well as the STC rating I am wanting to achieve. I will update this post as I get more information.
For this information, I consulted the following references: