Improving The Sound Of Your Room - Part 3
These days it seems that nearly everyone has some sort of a recording studio in their home, but that doesn’t mean the space you use actually sounds good. In this series, we’ll look at how to improve your listening environment for a lot less money than you’d think was possible. In Part 3, we’ll look at the appropriate measures needed to isolate your room.
Also available in this series:
- Improving The Sound Of Your Room - Part 1
- Improving The Sound Of Your Room - Part 2
- Improving The Sound Of Your Room - Part 3
Measuring Sound Transmission
Before we get into how isolation is accomplished, we need to take a small detour into the world of sound transmission measurement to help understand why some techniques work better than others.
All building materials have what’s known as an STC rating, which stands for Sound Transmission Class and is the measurement of a material or a partition’s ability to block sound over a range of 16 different frequencies from 125Hz to 4kHz. Just to give you some examples of how materials effect sound transmission, check out Figure 1. The higher the STC rating, the more isolation it provides at some, but not all, frequencies.
As you can see from the chart, the more mass a material or structure has, the more the isolation is increased. The only problem is that mass costs money and there’s no way around it.
Transmission Loss In The Real World
When a material, wall or partition blocks the sound, it’s said to have some “transmission loss.” What this really amounts to is how many decibels of sound the partition stops. The STC numbers correlate closely to decibels, which can be seen clearly in Figure 2.
Let’s take a look at how STC relates to what we can really hear.
So what do both of these charts mean in the real world?
- Loud speech can be understood through a wall rated at STC 30, but probably not one that’s STC 60. You still might at least be aware of the loud speech with a wall that’s STC 50 however (that’s pretty much the standard for most office buildings), so you can count on music being heard on the other side.
- When soundproofing rooms, the STC of your doors and windows need to be equal to or greater than the STC of your walls in order to maximize the rating. It’s the old “weakest link in the chain” scenario.
- By just structurally decoupling drywall panels from each other (using steel studs, a staggered-stud wall, double wall stud, or resilient channels), you can produce an STC rating as high as 63 (for a double stud wall) that will result in fairly effective low-frequency loss. This figure, when compared to a normal wall with an STC of 33, will make most frequencies inaudible, making the room sound 88 percent quieter.
- Music requires the highest STC ratings. In practical terms, a 55 STC rating will prevent a resident living in a multi-family home from being bothered at all by their neighbor’s loud music, but they’ll still hear a loud band playing.
- Homes usually require 50-80 STC for any meaningful isolation from loud music.
STC is highly dependent upon the way a wall or partition is constructed. As a result, the rating, and therefore the isolation, can be increased in three ways:
- Adding mass
- Adding or increasing an air space
- Adding absorption material like fiberglass inside the partition (wall)
All three of these items can work together. Let see how.
The Studio Walls
Regardless of whether you’re building a new wall or just trying to beef up the isolation in a structure that’s already built, the principles are the same. Adding mass means adding more weight to the walls, ceiling, doors or windows in order to raise the STC. For example, if you look at Figure 3a, you can see that a basic stud wall with 2 x 4 studs and ½ inch gypsum drywall sheets like what you find in just about any house, garage or office has an STC of only 33, which is very easy to hear through.
If we were to add some simple R-13 batt fiberglass insulation in between the studs, the STC raises to 36 as in Figure 3b, which helps the isolation just enough to notice, but not near enough to provide any meaningful isolation.
It’s easy to think that if one wall works well, two walls will solve everything. Unfortunately, it just doesn’t work that way. If you add an identical insulated wall it raises the STC up to 40, which you can hear still but doesn’t make a huge difference (see Figure 3c). If we were to remove the interior plasterboard sheet, however, the STC jumps up to 57 (see Figure 3d). The airspace made a huge difference. Now if we add an additional plasterboard sheet to the outer sheet of each wall, the STC increases to an impressive 63 (see Figure 3e)! Now we’re beginning to hear some significant isolation.
How Can We Improve on That?
We can see the improvement with simple design techniques, but with some small additional tweaks we can raise the STC even further. We can:
- Add some thicker fiberglass like R-19 (anything thicker will be more expensive and not that much more effective). The insulation’s paper backing is attached to the studs with a staple gun before you put the sheetrock on, facing the air space. Don’t let the insulation touch. The airspace is important.
- Use 5/8 inch drywall on the outside wall instead of ½ inch. Surprisingly enough, having two different widths of drywall (½ inch and 5/8 inch) can be an advantage in lowering the resonant frequency of the wall, thereby lowering any vibration. It’s also possible to use MDF, Wonderwall or HardieBacker (cement backing board as used in most showers), which are denser than drywall and maybe even a little cheaper.
- Increase the airspace between the wall from 1 inch to 2 inches. The bigger the airspace, the more effective it is, although you loose floor space as a result.
Remember, the idea is that mass + air + mass = good isolation.
Studio isolation is an "all-or-nothing" proposition in that you have to treat all of the walls, the floor, the ceiling, the doors, and the windows for it to be effective. You can’t successfully isolate a room just by putting up another sheet of drywall on one wall any more than you can build an aquarium by putting only one sheet of glass in the frame!
That being said, there are some general rules of thumb for controlling noise between spaces:
- A wall must extend to the structural deck in order to achieve optimal isolation. Walls extending only to a dropped ceiling will result in inadequate isolation.
- Sound will always travel through the weakest structural elements, which are usually doors, windows, HVAC openings and electrical outlets.
- When the mass of a wall or ceiling is doubled, the isolation quality (or STC rating) increases by approximately 6 dB, which is clearly noticeable.
- Installing insulation within a wall or floor/ceiling cavity will improve the STC rating by 4 to 6 dB, which is clearly noticeable.
- Usually specialty insulations don’t perform any better than standard batt insulation. This isn’t true when treating the acoustics of a room, only for isolation.
- Metal studs perform better than wood studs. Staggering the studs or using dual studs can provide a substantial increase in isolation.
- Increasing the air space in a wall or window assembly will improve isolation.
As you can see, nothing is cheap or easy when it comes to isolation. Until we come up with some sort of sonic invisibility cloak, it’s still going to cost some money for a somewhat major rebuild. And remember, just doing a rebuild isn’t that easy either. You have to consider local building codes, get permission from landlords, or consider what the construction might do to the resale value of your house if you own it. Luckily that’s not the case with changing the acoustics of your room, as you’ll see in later editions of the series.
All that said, the main culprits when it comes to isolation are windows and doors. Even if you don’t change anything else, you can still make a difference in your isolation by just paying attention to them alone. That’s what we’ll look at in Part 4.
Some of the above material comes from my book The Studio Builder’s Handbook (written with Dennis Moody). You can read excerpts at bobbyowsinski.com.