In the first article of the Respect your Room series, we examined how broadband (mid-high frequency) absorbers can help reign in early reflections and flutter echoes. Today, we will take a look at how to address the low frequency issues with your room and some of the mechanisms used to treat them.
Resonant Frequencies or Standing Waves
Between any two parallel surfaces, there exists a frequency who’s wave length is equal to the distance between the two surfaces. These two surfaces will act as a resonator for this frequency (and it’s subsequent harmonic frequencies), reinforcing that frequency both in amplitude (volume) and time (sustain).
To experience this phenomenon, take another trip back to your shower. Once inside your shower, sing a slowly ascending pitch starting from low to high. Eventually, you will pass through a note that causes your shower to “sing” the note back to you. That pitch is a resonant frequency for those two walls of your shower!
For further understanding of this concept, a quick video search for standing waves will offer a nice visual representation of what’s happening. It’s a cool trick for a science project, but it is a big problem for your mixing environment.
How It Effects Your Sound
A standard room has three sets of parallel surfaces which means that there will be three main standing waves or room modes. When these particular frequencies reflect off the walls, they interact with the continuing sound emanating from your monitors and either combine constructively, creating a peak, or destructively creating a null or in some cases, even complete cancelation.
As the waves are ‘standing’, these peaks and nulls will occur in different locations in the room. For instance, before I treated my room, I had a low frequency null zone right at the mixing position. This caused my mixes to always come out too bass heavy because I couldn’t hear the bass frequencies properly and I was overcompensating with levels and EQ trying to hear the bass as I thought it should sound in my room.
What Not To Do
Continuing with the example of my room, I first thought that maybe I just needed a sub or better speakers that could produce more bass.
It is important to remember that the cause of the problem is not the sound producing mechanism, but the room itself, so adding a sub or an EQ to boost the low frequencies would only make the problem worse. The only thing that can be done is to adress these frequencies with bass traps. Using the same materials as our broadband absorbers, we can build and install some bass traps at a very reasonable cost. There are many different designs for bass traps, but for this article, we’re going to look at the more simple and easy to instal designs.
Our broadband absorbers worked by using a porous material to absorb and dissipate the acoustic energy. Bass trap are going to function in a similar way, but on much longer wave lengths, so they are going to need a greater volume and density to have an effect.
In the last article, we talked about the different types of rigid fiberglass. Owens Corning fiberglass comes in two densities (703 [less] and 705 [more]) as well as FRK (foil backed) and regular (no foil). For bass traps we want to use the 705-FRK model, and if you can afford it, the thicker, the better (2-4 inches).
Similar to the broadband panels, we are going to cut the 4′x10′ sheet into 2′x4′ panels. This will give you a total of 5 panels per sheet (not to shabby). But unlike the broadband panels, instead of placing them flat against the wall, we need to add an air gap between the panel and the wall, and the bigger the air gap, the more effective it will be at lower frequencies.
Bass frequencies tend to build up in corners and you can hear this by playing some music and walking around your room. It also happens that corners are the easiest and most space effective way to increase the air gap of your bass trap. Here is a picture of a corner bass trap from a top view perspective:
(From Ethan Winer)
The first two corners I would address are the vertical ones behind your desk. However, bass frequencies will build up in any corner so if you can, I’d try and add these types of corner traps to any and all available corners (including the wall/ceiling joints).
When using the FRK fiberglass, the FRK will act as a membrane making the trap even more effective. Facing the foil side out (into the room) will increase the volume of the air gap, but will also act as a reflective surface. So depending on the live-ness of your room, you may want to face the foil side in.
The diagram above shows the most affordable method for installing these traps. Simply wrap your panels in burlap, screw some 1″x2″ wood stripping onto the walls for an anchor, and then use screws with fender washers to mount the fiberglass to the wood striping.
To add thickness to your panel, you can add some fiberglass or mineral wool batting (the fluffy kind) behind the rigid fiberglass panel. To do this cleanly, you may need to add a frame to your panels. You can use 1″x4″ wood to construct a 4″ deep frame. On the front side of the frame, attach some 1″x2″ wood stripping to the interior edge. This will serve as a lip barrier to keep the FRK pannel in the frame. (It is also easier to mount the burlap to this lip, rather than wrapping the FRK panel.
Once the burlap is stapled in and the FRK panel is in place, add the R18 fiberglass batting behind the FRK panel. This will probably be taller than the 4″ deep frame, especially if you used a 2″ FRK. Luckily, the batting is fluffy and can be compressed. You can use some of the left over 1″x2″ stripping to make lateral support bars to hold in the R18 batting. I’ve also seen designs that used chicken wire to hold in the batting.
Now you have a thicker and denser total volume. You may need to use some corner brackets to mount these panels to the wall as they will be significantly heavier.
Tri-corners, the room corners made by the joints of 3 walls, cause an interesting issue structurally/ aesthetically speaking. If you are trying to put corner traps on all 3 of the corners, you’re going to end up with a strange gap. The easiest fix for this is to make a cubic piece to go right up in the corner. You can make the ’3 sides of a cube’ piece using extra rigid fiber glass. You could also make it by cutting the fiberglass batting into 2′x2′ square pieces and stacking them to form a cube. In either case, the finished product should be arranged something like this:
Here is a tutorial for making really nicely framed corner traps. It could be done a little easier in my opinion. NOTE* In this tutorial he used a few layers of batting. I’d still recommend the FRK/batting combo as the FRK ads a membrane effect into the equation.
The RealTraps website has some tips on how to mount panels in ceiling corners.
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