Building Control Room For Foley – Part 2

April 20, 2021| Yuri Pridachin

This is the second part of Foley First series on the construction of a control room. In the first part, I shared my thoughts on the general ideas and goals in building the studio and its soundproofing.

If you missed it, you can find it here: Building control room for Foley. Part I

In this part, I will discuss acoustic design and talk a little about soundproof doors.

So make some coffee and get comfortable. Let’s get going…

ACOUSTIC DESIGN

PROJECT OVERVIEW

With a big enough room to experiment some, I wanted to make the room acoustically neutral with a significant amount of diffusion zones, and, of course, I wanted to place massive bass traps.

By the way, because I had a sore lack of time for the whole project, I spent no more than 10 hours on designing acoustics for this room in total, but I had an exact idea of what I wanted to get in the end. Therefore, I could make any needed adjustments right on the spot. Decisions on rebuilding some things were made on the spur of the moment, but promptly and efficiently, so they did not extend the project’s deadline.

When building the first room in my living apartments, I paid too much attention to every minute detail and clearly suffered from perfectionism. Fortunately, this disease quickly disappeared after I built the third room because I realized that excessive scrupulousness is not worth the effort and, most importantly, does not provide any serious benefits in the results.

I wasn’t pursuing saving as much usable space as I could — I didn’t calculate every centimeter to preserve the maximum usable space — but in the end, I hoped to get an area of at least 22 sq meters of usable space after the entire acoustic design was installed. The only critical thing for me was the ceiling height, so that a minimum of absorption or scattering overhead is all I had to deal with in the future.

FRONT ZONE: MAXIMUM ABSORPTION OF LOWS

In general, I liked the idea of integrating hanging bass traps into the front zone right away. And, I must admit, I was curious how they would fight against modes in a square room. I did not have previous experience with this kind of design because almost always, my rooms’ sizes prevented hanging bass traps placement. In the past, I had to use an absorption in early reflections, not exceeding 150 mm with a slight offset from the wall, because of the size of my rooms.

Before building the false wall frame in the front zone, I reserved about 55 cm of space for hanging traps. In the corners of the room, where low-frequency resonances are mainly concentrated, between the point of contact with the false wall frame, which is at an angle, and the corner of the soundproofing wall, the length reached 80 cm.

It was clear that their efficiency at low-mids should be very high since each trap’s distance to the wall varied, making the maximum absorption coefficient uniform at different frequencies in this range.

Made of 50×100 mm timber, the false wall frame itself, which hid the bass traps, stood at an angle to the soundproofing box. There was no scientific calculation of the angle at which the wall should be located and how this, in a practical sense, would affect the absorption coefficient.

It was only necessary that I had a general understanding that I wanted to get symmetrical but not parallel walls with effective absorption in the entire spectrum and partial scattering at highs by installing wood slats.

Hanger bass traps

The bass traps themselves are also very uncomplicated structures, which, I am sure, everyone can build. I did not use plywood for the core to avoid excessive weight, pressure on the frame of the false wall, and the increase in cost. Instead, I took 12 mm gypsum board as a base. In this case, the only important thing was that the attachment points for hanging the trap were reinforced with something to avoid breaking the gypsum board.

Hanging traps consist of an absorber and a solid mass. In my case, the construction looked like this:

50 mm Rockwool – 12 mm gypsum board – 50 mm Rockwool

All the traps were 220 centimeters tall, and their widths varied. Over the entire surface area, the mineral wool was covered with windproof material tightened with tape to guarantee that the mineral wool would not shed over time or drop dust.

In my version, the hanging fasteners of the traps are made from rope, and the places where the rope goes through the gypsum board are reinforced with wooden slats so that the rope itself does not break the gypsum board over time.

I did not mount the traps on the concrete ceiling slab because I did not want to drill holes and complicate the process.

As you can see in the photo, the traps hang on transverse beams supported by two pieces of wood attached to the inside of the frame of the false wall and to the soundproof wall. Thus, the traps can be moved or corrected, and their angle changed.

While the height of the traps was consistently 220 cm, the traps’ width varied from 400 mm to 750 mm, depending on their location, but the entire front wall of the studio was covered with 500 mm pieces.

The traps’ angle on one side of the centerline on the front wall mirrors the angle on the other side of the centerline on the front wall. In the scheme, you can see the trap placement arrangement behind the false front wall, where the distance between the elements is 170 mm, so they overlap a bit.

In general, such a hanging scheme, without using chains and fasteners to the concrete ceiling, seemed to me to be easy to install and reliable. By the way, my construction guy, who completely built the whole room, likes to test the elements’ reliability by hanging on them himself! So the traps calmly withstood a mass of 80 kg.

Front-side wall additional absorption

After the bass traps were installed, the front wall frame was filled with 100 mm medium density Rockwool (to add extra absorption) and then covered with windproof material. All the electrical and HVAC had already been extended out into the room. I did not plan to place any other things, like an LCD or speakers, on the false wall frame, so no additional reinforcement was made.

At this point, I left the front wall to put the finishing touches on later.

Meanwhile, we had not forgotten about the ventilation system. Plastic air ducts were installed inside the room. We changed aluminum ducts to rectangular plastic HVAC ducts because there was very little space left for venting into the room after installing the traps in the sidewall. Also, there is less resonance from plastic than from aluminum.

It would be incredibly annoying to lay ductwork under the Armstrong ceiling, which was already filled with mineral wool, so it was good that was not actually necessary. Doing so would have required us to raise the ductwork to a sufficient height, which neither space, time, nor budget would allow. Not seeing much sense in doing that, I brought the inlet and outlet ductwork through one of the sidewalls, placing both ducts at a sufficient distance for normal air recirculation.

Realizing that I would have to find the best listening position later, during the first tests to find the best low-end frequency response in the room, I put away the inflow as far as possible from the center of the room. It is difficult to predict for sure where the mixer’s chair may be, and I did not want to get under a direct air stream, even if it’s warm.

BACK ZONE: ABSORB LOWS, DIFFUSE MIDS AND HIGHS

More than once I have designed the back wall of a room to work as a low-frequency trap and at the same time diffuses the middle and high frequencies. In our small control room, where the acoustic design is partly based on the Live end/Dead end principle, the back wall works as a huge bass trap with a skyline diffuser embedded in a 55 cm space. I should say that this room sounds much wider and richer than 16 sq meters should. And that is thanks to the back zone, which works more for sound diffusion than absorption on mids and highs.

Therefore, I didn’t try to discover the continents again, so to speak, but decided to follow the proven way that has worked for me in the past, albeit bigger.

Skylines

The room’s size and the listener’s potential distance to the walls allowed me to use skyline diffusers. I think these elements of acoustic design are pretty laborious to build. In all my years, I have made about 5 sq meters of them, and that’s it… I’m done!

Moreover, the first time I made them for myself, perfectionism took a huge toll. I was very scrupulous about how exactly the columns fit together and how accurate the height of each was. But this is wood, and it tends to deform. Therefore, my advice to anyone who wants to try himself in this: Down with perfectionism; it will work anyway!

It’s still labor-intensive, though, so this time, I hired a couple of brave guys who helped me do all this meticulous work. Many thanks to them!

For a calculator, I always use this one: http://www.mh-audio.nl/Acoustics/DiffusorCalculator.asp

My design consisted of a matrix of 4×2 sections of diffusers with dimensions of 550×550 mm each, with the maximum height of the column 240 mm. The column width at the bar is 40 mm, which in theory gives us a diffusion in the range from 700 to 4300 Hz. Why is the column width 40 mm and not 35 or 45? Pretty simple. I can find wood with dimensions of 40×40 mm or 50×50 mm as the most common in our market. If I choose a smaller wood size than the ready-made dimensions, I face an even more labor-intensive building process for the diffuser. If the wood is larger, though, the efficiency drops significantly at highs. Therefore, a 40×40 mm column size, in my case, is the best solution.

As a result, the sections tightened together to form one large diffuser with dimensions of 220×110 cm — a massive piece that works well and looks pretty damn cool.

Bass «closet»

The back wall bass trap frame, which is also a niche for installing diffusers, was made of regular 50×100 mm wood. The electricity and heating pipes were brought out, and the heating radiator was mounted on the back wall, but eventually moved to the opposite wall. More on that later.

The dimensions of the bass closet were 385 x 240 x 70 cm. The entire space is filled with Rockwool from top to bottom. The bass closet is also covered by windproofing material to prevent harmful particles from entering the room.

Thus, most of the room’s walls work as low-frequency absorbers, which is a big plus in a square room.

CEILING: CLOUD PANEL

For low ceilings, a cloud panel seems to be the best solution since it is easy to design and install and, most importantly, can adjust its height and angle, which allows fine-tuning absorption in the low-mid range.

Personally, I am strongly against installing the cloud panel parallel to the floor and ceiling, as that will not fully take advantage of its effectiveness. The fact that the distance from the absorber to a hard surface affects the absorption coefficient at various frequencies makes it a very versatile, broadband, and effective tool for absorbing in the early reflections zone. And it is customizable.

The dimensions of my cloud panel were 190x130x14 cm. A 140×20 mm board was used as the base for the frame. Medium-density 100 mm Rockwool absorber was put inside. The panel was hung using metal hooks, dowels drilled into the ceiling slab, and a chain. I used a chain instead of a rope because that later made it possible to easily and quickly change the panel’s height and angle to become as efficient as possible in the range of 120-250 Hz.

I deliberately did not put an absorber with a larger thickness in the panel, knowing that I still had enough space between the panel and the ceiling to experiment further to increase the Rockwool’s thickness to find the best result. If I found that 100 mm is not enough at a specific angle and distance to the floor, and that the panel works more efficiently with a thickness of 150 or 200 mm, then I could easily add the missing thickness on top of the panel and hide it under the fabric. Or if there was time, rebuild it.

FINISHING FRAMING

The last thing that I had to do after all the basic structure was assembled was to cover the walls with decorative fabric and fix the wood slats on top of that.

Of course, It’s good to see some expensive, fine wood in an audio sound studio, but in our market, wood of the thickness that I wanted to use (20 mm) was different widths than I needed, and I would have to wait a very long time before the right stuff would be available. So, I bought a simple piece of inexpensive wood from the store and finished it with 2 coats of varnish.

There is no scientific explanation for why I used slats of the width I chose or why the distance between them is as it is. I knew for sure that I wanted to cover about 70-75% of the area, which will scatter the high frequencies. And it was technically crucial that the slats were not dead fixed so that I could easily remove them in the process of fine-tuning the studio if I felt that the scattering area was too large (or I could have removed only those that seemed superfluous to me — in particular, in the axis of the speaker in the early reflections zone).

Now, I can affirm that it is very comfortable to work in a room with angled wood slats. It is commonly believed that the presence of abundant diffusion working in mids and highs affects the room frequency response in these ranges. What I’ve found, though: It doesn’t affect at all! And also, it doesn’t affect decay time. During the room’s fine-tuning, I covered all critical areas in the early reflections zone with an absorber and did not see any changes in the graphs. Have I heard the difference during playback? Maybe.

But definitely, wood slats and skyline diffusers, give significant liveness in the Foley control room, and it is much more comfortable to work in such a room for a long time rather than in a more damped one.

I know that this design looks like a Helmholtz resonator, but I initially violated all the technology not to get it. In my case, the slats on the walls are nothing more than just a reflector.

All walls were covered with gabardine. Here, it is very cheap, and it is an acoustically transparent enough fabric. Most likely, specifically-designed acoustic fabric will be more effective, but come on! Who will hear this besides your wallet?

Regarding the floor, the changes were minimal. We simply laid laminate flooring through the backing onto the already existing tile.

ADDITIONAL ABSORPTION AND DIFFUSION

Okay. The walls are ready; the ceiling and the floor are ready as well. The only zone left untouched by the decoration is the zone between the sidewalls and the back wall, which is mated with the doorways. The original idea was to make a Polycylindrical diffuser filled with an absorber in order to get a low-frequency absorber and a high-frequency diffuser in one piece, but something went wrong with my plan. I decided to build a structure that worked around 50 percent as an absorber in a wide range and around the same proportion scattered mids and highs not in a critical zone. Also, this construction would be pretty effective on low-mids due to the varying distance from the absorber to the hard wall.

An easy-to-manufacture frame was filled inside with 50 mm Rockwool and covered with gabardine. On top of the fabric, I attached 40×20 mm slats with an equal distance from each other.

DOORS

I should have discussed doors in the first article focusing partly on soundproofing instead of including it in the acoustic design part, but I decided to move consistently in the chronology of the captured photos.

The first time I was seriously puzzled by soundproofing doors back in 2014, when I built my first room in my living appartments. At that time, while working for hire in various studios in the city, I noticed that the doors in these studios were, for the most part, the weakest link since they did not have sufficient thickness and good sealing. But there were exceptions when, in serious companies, fire doors with a good sound insulation coefficient were installed in the studio.

The original area of the room was about 21 sq meters. That is, it was definitely clear that the installation of a fire door in such a place was excluded, so I spent a long time exploring this issue in search of an optimally effective and inexpensive alternative solution.

I can’t remember who I copied on the door construction technology (possibly Phillip Newell, but I could be wrong) which was subsequently installed in my first workroom. This door certainly had a very high sound soundproofing coefficient. Due to the large mass, the thickness of the door leaf, and the presence of sound-absorbing material inside, I do not remember that this door let in any more noise than the soundproofing walls did. On the other hand, though, its mass, fastening complexity, and difficult construction made it not the most practical and durable in daily use. Therefore, by the time I started on the second room construction in 2015, I was already considering alternatives.

I invited one of the city’s experienced acoustic engineers for a cup of coffee, and he told me about two-chamber uPVC doors made of glass. We visited one of the studios in which he did the acoustic design. I realized that this was exactly what I was looking for, especially since the cost of making such doors to order in Russia is around $200, which is much cheaper than massive fire doors.

Since then, I have only used these doors in all of my Foley studios and control rooms. They are convenient, efficient, and unpretentious. And, as a bonus, I get transparent glass over the door leaf’s entire width, which makes staying in a windowless studio more comfortable. At least you don’t feel like you’re locked in a dead bunker.

Considering the door’s low cost, building double frame walls with a small tambour and installing two doors is a particularly viable option for a low budget. As a result, this design gives an extremely high soundproofing coefficient.

The doors that I still use are uPVC doors, with two chambers formed by three sheets of glass. The vacuum chambers’ depths are 10 mm and 16 mm, and the thickness of the glass is 6 mm, 4 mm, and 4 mm.

It is important that the clamping element that pulls the door leaf to the door frame is located on all sides of the door, not only from the top, bottom, or side. Thus, the door through the seals is well attracted to the door frame, not allowing sound to come through the crack in the door.

TO BE CONTINUED…

In the third part of article I will do the first measurement and will fine-tune the control room’s acoustics in a few steps.


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