This article is an excerpt from the following book: Home Recording Studio - Build It Like The Pros.
Simply put, where air goes—sound goes.
Some of the biggest problems I've seen over the years with sound transmissions from room to room have to do with a contractor's failure to pay attention to air paths created during construction between rooms. Contractors install walls designed (and tested) to provide STC ratings of 54, and when the job is finished, they have actual STC values as low as 34.
For example, think about the small cracks that exist at the bottom of walls formed between the bottom plate and the floor, which are generally ignored when finishing the rooms. A 1/16" crack located at the bottom of a wall 10' long is the equivalent of a hole through the wall 7 1/2" square in area. That's a hole in the wall 2" wide by 3" high. I am sure you can see why this would seriously reduce sound isolation room to room.
Similar weak spots exist with electrical boxes that penetrate the wall surface, the physical joints between the wallboard in the body of the wall in base sheets of drywall, as well as with the corners of the room where wallboards meet together. Yet these areas that I mention are the most commonly ignored items during construction, with no money allocated in the construction budget for proper treatment. It is important that your contractors (or you if you intend for this to be a DIY project) understand and pay close attention to duplicating the exact construction details I outline in this book. Otherwise, you may as well throw your money in the wind—it will be an easier way to lose money. After all, construction is a lot of hard work if you have nothing of value in hand when it's all done.
This is another area that acousticians pay close attention to—and most people, including the "experts" doing the actual work in the construction industry, are not even aware it exists. Sound traveling through the structure of the building itself follows what are called flanking paths. An example of this can be seen if you take a wood stud, place your ear to one end, and have someone tap lightly on the other end with a hammer. You will hear the sound quite clearly through the stud itself—much more loudly than you would by just listening to that same tap in the room. Sound transmissions travel very well through the structure of a building, so disconnecting your studio from the rest of the building is very important if you want to isolate yourself from people walking around above you, the TV playing, or if your wife is trying to sleep while you're recording your band. Other sources of flanking sounds can be water pipes, drainage and sewer pipes, and ductwork for HVAC systems. Anything that interconnects different parts of the building with your studio space can create pathways for sound to travel through.
If you are in the process of designing a new building, care should be taken to design isolation into the structure. If your studio will be within an existing building, you can minimize the effects of flanking paths by isolating your room from the existing structure.
I'm not going to bury you in math or confuse you with the complex analysis that an acoustic engineer performs when working on room designs. If that's what you're looking for, there are already some excellent books on the market. I have no interest in duplicating their work or competing with them. Instead, I'm going to show you the practical side of this industry—how to achieve what you want most and give you the ability to make your music in peace. In the process, I am also going to let you know where not to waste your time and money. Sometimes, you just can't get from A to B no matter how hard you may want to.
What To Avoid
It amazes me the number of times people have posted on sites that they have completed work on their studio and did not achieve the level of isolation they either expected or required. They ask how they can increase the level of isolation, to which I respond by asking them to explain exactly what they constructed, in as much detail as they can give. It usually takes me a while to get to the bottom of it, but I find that they had some "great ideas" to improve wall, floor, or ceiling assemblies. For example, they figured that they could gain more isolation by adding sheets of drywall within the cavity of a double wall assembly. Their logic was: "Heck, if two sheets on each side were good, then putting two sheets in the middle must be better." Unfortunately, reality is just the opposite. Not only is it important that you have the right materials, but it is equally important that you put them in the right place. We'll look at what those "right places" are in the next chapter. For more information, see Home Recording Studio - Build It Like The Pros.
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