# Sticky  Control Room Acoustics



## ejbragg

One of the most important (and popular) topics is how to setup the control room. There seems to be a stereotypic image planted in everyone’s mind about how this room should look. Obviously, there are some world class studios that have created some spaces for this purpose, and their shapes are very similar. But such spaces don’t necessarily have to fit the generic image that the industry has pushed forth. Imagination can play a powerful role in coming up with your own unique layout and look.

On the other hand, there are some concepts that need to be included in control room design that are not required stipulations for other rooms. The control room is, after all, the one room whereby we must judge all audio quality, and it usually contains our most expensive gear. And what good are these intricate tools if our environment does not allow us to hear what they're doing?

*Who Created the Science of the Control Room?*

If we are to understand the control room design, the following information is quite critical.

The early Roman empire was among the first civilization that took entertainment seriously. They built concert halls many times over, learning how to use reflection and room shape to project the sound out to the audience. Over the centuries, the science has become quite complex. Nevertheless, the ultimate goal is still simple – provide the audience with the best sound possible. The halls created that do this the best are arguably those supporting the symphonic music between the 18th century and today. The world has many concert halls that are considered quite excellent for audience listening, and rest assured, the construction of these buildings have been painstakingly designed. Many studies have been made over the years, testing the qualities of these rooms, in attempts to get to the bottom of what characteristics, exactly, objective and subjective, have made these halls so respected?

Perhaps the most famous concert hall in all the world, hailed by the most respected ears, is Concertgebouw, located in Amsterdam, Netherlands. This hall has been agreed to have perhaps the closest to the perfect listening environment of all. *Here is the most important information for control room designers: Most modern professional control rooms, whether they realize it or not, have their design based on this concert hall of the Netherlands!* All the ideas about initial time delay gaps, reflection free zones, absorption, diffusion, airspace….. the entire science stems from the study of these concert halls (the above-mentioned just happens to be the most famous) – and all we are doing in the process of designing these rooms are mimicking such concert halls, using “tricks of the trade” and modern acoustic technology to conform our space to sound like them, to the best of our efforts.

Note that this is not necessarily true of all control rooms. For example, those rooms for mixing sound for picture are often short and wide. Most are often set up predominantly for a mixture of speech and sound effects, and although can be used to mix music tracks, these rooms are not ideal for music mixing. They are however, often perfectly qualified for mixing music levels for the picture. Most musical content should be mixed in rooms with greater depth, which will soon be discussed. And this thread will lean toward tracking rooms designed for (strictly) music.

_Note: There are control rooms that are both wide and deep that are perfectly adequate for both music and sound for picture mixing._

The following concepts to be covered in this thread are:


Symmetry
Initial Time Delay Gap
Reflection Free Zone
Room Volume


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## ejbragg

*Most Basic Guideline: Symmetry*

First of all, we all have two ears. To us, the world is binaural. When something sounds louder or brighter in one ear than in the other, our brain tells us the sound is coming from somewhere to that side of our body. Of course, our brains are a lot more specific than that. But the point is that for starters, if we are to judge audio information critically, one need is a symmetrical room, with symmetrical installation of the sound sources and treatment.

An interesting thing about audio waves is that they can be split apart by objects in a room. However, they seldom “join forces” except maybe for an instant. They pass right through each other as the invisible objects they are. If sound from one source enters a room, heading in one direction, and a sound from another source heads in another, they pass each other by without the slightest noticeable influence. If your room is asymmetric, it is quite possible that room reflections will return to the listener quite out of balance. For example, if the rear of the room has a pocket on the right side for an entrance that is not copied on the left, the possibility opens up for different room modes, longer delays, different reflection paths, and who can predict what frequencies will strike which ear first? Although it may be possible to create an asymmetrical room that works for you, it would likely be very difficult getting the room balanced, and there may, in fact, always be errors that are not perceptible, affecting the engineer’s mixing ability.

There will, of course, always be objects in these rooms, in corners, on shelves, desks, chairs, instruments, racks, that cause perturbations in the wave fronts of the audio in the rooms; however, by far the strongest influence in room balance is in its shape, if the walls are solid.

In summary, no matter what layout or size a person chooses for their control room, symmetry is one of the basic needs.


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## ejbragg

*Initial Time Delay Gap*

Another fundamental concept for control room design has to do with your ear-brain connection. Studies have shown that any reflected signal that arrives at your ear within 1 millisecond of the original signal is almost indistinguishable. In fact, signals arriving slightly later, although becoming more noticeable, are still confusing to the ear. At approximately 20 ms, the two signals are able to be differentiated.

Audio tests conducted at many famous concert halls share certain characteristics. One of these traits is the initial time delay gap of about 20 ms. From the center of the audience seating positions, the “best seats” were chosen to be centered with the stage, and several rows back. Test microphones were placed here. With an impulse sound source centered on stage, the mic picked up the original signal at the predicted time (distance / speed of sound); then silence entailed until about 20 ms later. (22 ms for the Concertbegouw.) These reflections came from side walls, ceiling, and / or the wall behind the stage (depending upon the specific hall and its layout). Where, exactly, this first delayed signal comes from is less important than the fact that it is there, and it has been deemed “pleasant to the ears”, and yet does not detract from the accurate judgment of the music.

Secondly, this 20 ms signal was shown to be somewhat diffused, due to the fact that it died relatively slowly (RT60 = 800 ms or so). If the sound was not diffused, the second attack would have arrived at the microphone more sharply and would have ended about as quickly as the original signal – more like an echo.

The third describable mannerism in the measurement is the evidence of the apparent silence between the two signals (within the 20 ms gap).

In a very general analysis of the concert hall acoustics, the original signal comes from the instruments on stage and strikes the audience. The same wave fronts rise to the ceiling, spread to the side walls, and move toward the back wall. Whether these surfaces are designed to absorb or reflect, the outcome is that at the audience’s listening positions, there remains no evidence of the original signal for 20 ms, followed by a diffused reflection of the sound, dying at a specific rate (varying between concert halls), then is gone: there is no returning sound from the rear wall. The sound is either absorbed by the rear wall or by the audience before it can make its way back to the “ideal” seating position.

A control room cannot practically provide as much space as these concert halls. Yet this audio effect _can_ be realized. The trick is in designing the walls, ceiling, or even the floor, for that matter, such that the first reflections to the control position arrive at approximately 20 ms or so after the original signal. Furthermore, this signal must be diffused, or to use a subjective estimation, the signal must be “softened”.

Doing some simple calculations, we can see that the nearest wall which offers direct reflection should be about 11 feet from the control position:

11 ft * 2 = 22 ft (the original signal travels past the engineer’s head, continues for 11 ft, strikes the wall, then continues back toward the engineer for an additional 11 ft.

22 ft / (1130 ft/sec) = 19.5 ms

It has been noted that as little as 16 ms and as much as 24 ms is acceptable, but 20 has been claimed as closest to ideal for the average ear. Using this little computation, it seems that a control room with a depth of less than 12 feet could not suffice as an ideal listening environment (that’s a 9 ft depth behind the listener with an allowance of 3 feet between the engineer and the front wall – barely enough space to fit studio monitors into… not to mention absorbent material which should be placed behind the monitors, etc.). On the other hand a room that is too large could possibly be rearranged to allow for this ideal environment by moving the engineer position about 11 feet or so from the rear wall.

The side walls of most control rooms have been shown with splayed walls. If these walls are closer than 9 ft from the control position, they should NOT have diffusers on them near the listener. This is because of the same reasons. The diffuser will likely throw some of the sound back toward the control position. If the walls are indeed splayed such that the room is more open toward the rear, sound directed from the source, striking those walls (if they are flat), will reflect toward the rear of the room. Diffusers on the rear wall(s) or on the side walls near the rear of the room, however, would not hinder. The idea is to create a *reflection free zone (RFZ)*, creating this initial time delay gap for the engineer.


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## ejbragg

*Reflection Free Zone*

This is the design of the area within the control room space which dictates the initial time delay gap between the original wave front and the first reflections for the engineer in the control position. Studio monitors will throw sound toward your work surface, your cabinets, the ceiling, even the front wall behind the monitors, which can actually be the one of the worst culprits of a bad listening environment due to the standing waves present.

Without trying to cover every detail, it is imperative to warn control room designers of these potential dangers. It is good to draw floor plans and even draw profile sketches of the control desk and layout, drawing all possible reflection lines between the monitors and the engineers. Non-splayed walls in a control room (such as a rectangular shaped room) can be worked around by using front corner absorbers, diffusers, splayed partitions, etc. Nevertheless, every signal path must be carefully considered. Many control rooms are also equipped with rack cabinets directly behind the seating position. Sound can strike the front surface, reflect to the floor, then …. Who knows? These are just some of the examples of what should be regarded.


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## ejbragg

*Room Volume*

One cold, hard fact about every listening environment is this: space is good. Room modes, near reflections, comb filtering problems…. all these plagues of a potential listening environment dwindle (or at least become easier to handle) as the space grows. Another writer simply commented, “Music needs breathing space.” This one of my favorite statements.

F. Alton Everest once declared that 1500 cubic feet is approximately the smallest size for a decent listening room. It can be shown that a larger room volume allows for longer distances between reflections, allowing for more time between them. This, in turn, necessitates a lower number of reflections to acquire the sought reverberation decay time (often an RT between 200ms to 600 ms for a control room – depending upon its size and application).

If, for example, a control room design goal is to have a 400 ms decay rate, this might be virtually impossible with the small room without using extremely hard surfaces, which will support the resonances we’ve tried to destroy using modal and wide frequency room treatment.

Another science of the early theaters which is still used today is the concept of room shape. If a room has a form which is small at the source, but expands as the wave front travels, this allows the strongest, clearest reception throughout such a room. In fact, it is ideal for the audio wave, because it follows the natural progression of sound during its voyage through space. This is another concept reflected in control room layout, and likely responsible for the similarity between different rooms in this field.

With all that being said, there are certainly more ways to build such rooms. Being aware of these principles should help prevent pitfalls in acoustics. But as hinted earlier, the sky’s the limit when it comes to what shapes you can model your control room after. Remember that there is no such thing as _THE_ right way to build! So use your imagination and show us your results! :sn:


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## mhtplsh

Short, & simple, but very nice


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## stephendbarnes

After reading this, is it possible to create a reasonable mixing room (for music) in a 10X13 room?


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## electrocamel

Very interesting topic, as I'm fighting with my home studio's control room.
I find that my room is much smaller than what you recommended, but alas, I can not push the walls ...
I'm currently trying to calibrate my good old vintage JBL100 monitors, I do not have too much reflections problem, since I covered my walls with foam, and the floor with a carpet.

To precise my setup, the control room dimensions are 6 feet wide, 8.6 feet long and 6.8 feet high, for a volume of about 32.5 cubic foot.
And the distance between my ears and my monitors is 1.26 feet
Despite this, I create descent mixes...

Regards


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## rocksure

The control room in my home studio is far from the ideal size and shape. However, because I have worked for many hours in it, I have learnt basically how things should sound in the particular room in order to create good mixes. However, once my mixes are done, for the most important ones I always take them to and listen in a larger studio control room located in another studio that I have access too and check them there too. THat studio also has different speakers, so I can hear how mixes sound on those also.


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## rocksure

I must say that this topic, and the simple but clear obsevations and advice that has been given here is very helpful and gives plenty of room for thought.


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## bobbysdad

Just think of a Control Room being just like a 3 dimensional Pool Table and you git it licked! :T


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## bobbysdad

stephendbarnes said:


> After reading this, is it possible to create a reasonable mixing room (for music) in a 10X13 room?


Absolutely "yes".
In my day, I knew a lot of great sound guys that could produce an awesome sound using not much more than a pair of headphones!
Even in a small room a decent home stereo could be made to sound great.
Well the same principal applies.

Good luck.


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## Capt. Ahab

Another excellent topic and one I'm going to be dealing with possibly in two rooms of vastly different size very soon. Thanks for the info!


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## jef252

Yeh, just follow the 38% rule and you should be fine with like 9 2x4x4 703 panels and bass trapping.


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## DanTheMan

Great topic! Symmetry should be the first thing. Then the equilateral triangle. I was surprised that on a recording forum just a few months ago, no one had even thought of that. The education of recording 'professionals' can be fairly low about these matters.

If you have a large room, symmetrical reflections, equilateral triangle(though I don't necessarily agree with the rationale, but these things have been in place for a long time), and a solidly EQed modal region at the listening position. you've pretty much got it licked. The most important thing though is actually good monitors(if you buy into all that research).

I've got a boatload of blog posts about all these subjects. Ultimately, REW, a tape measure, and a measurement mic is indispensable during set up.

Dan


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## AudiocRaver

Anyone done any control room setup or acoustical treatment recently?


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## AudiocRaver

What kind of RT60 would you aim for in a mixing room or home/hobby mixing room?


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## STMY

As to symmetry: It is simply impossible to create a 100% symmetric working environment; e.g what about your mixing desk? Entry door, wall angles, equipment stacks, etcetera.

It is possible however, to position the main monitors in such a way that the number of reflections form left speaker to left ear, equals that of right speaker to right ear, while making use of the room's specific assymetric aspects for delegated reflection.


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## AudiocRaver

STMY said:


> As to symmetry: It is simply impossible to create a 100% symmetric working environment; e.g what about your mixing desk? Entry door, wall angles, equipment stacks, etcetera.
> 
> It is possible however, to position the main monitors in such a way that the number of reflections form left speaker to left ear, equals that of right speaker to right ear, while making use of the room's specific assymetric aspects for delegated reflection.


For sure! That sounds like a well-thought-out, creative approach.


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## KlankOntwerp

I'm currently rebuilding my small studio. The room behind me is not symmetrical (the angles are not 90 degrees; the left side is somewhat longer than the right side). I chose this as the rear wall because the front wall is symmetrical (except for a window on the right side). My plan is to make the rear wall act as a diffuser, at the same time compensating for the difference in length. The window is asymmetrically placed too, and I am really contemplating what to do with it. A little bit of daylight would be nice... So I was thinking about lexan in combination with absorption/diffusion right behind the speaker (on the lexan). Or else I will use the window as an extra bass trap (the window is about 20 cm deeper than the wall). Do all of you work without sunlight?


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## AudiocRaver

KlankOntwerp said:


> I'm currently rebuilding my small studio. The room behind me is not symmetrical (the angles are not 90 degrees; the left side is somewhat longer than the right side). I chose this as the rear wall because the front wall is symmetrical (except for a window on the right side). My plan is to make the rear wall act as a diffuser, at the same time compensating for the difference in length. The window is asymmetrically placed too, and I am really contemplating what to do with it. A little bit of daylight would be nice... So I was thinking about lexan in combination with absorption/diffusion right behind the speaker (on the lexan). Or else I will use the window as an extra bass trap (the window is about 20 cm deeper than the wall). Do all of you work without sunlight?


Welcome to Home Theater Shack!

Daylight is good, but windows seem to get sacrificed for sound quickly if there is an auditory benefit.

Asymmetry is always a challenge, the right absorptive treatments minimize the downside.

A diagram and pics would be nice, maybe a few REW measurement files.:bigsmile:


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## KlankOntwerp

I have not used REW yet, only Fuzzmeasure. I know the problems and I am currently making a few boxes that I will place throughout the room and fix. Those boxes are made of wood and filled with rockwool. They will be 10 cm deep, 61 cm wide and 122 cm long and will include two layers of rockwool fixed to the backside. the front panel will resonate freely. I calculated the density of the front plate and together with the depth and rockwool know the resonance frequency is around 90 Hz, close to one of the worse standing waves in my room (45 Hz is another but I have to see how I can get rid of it without sacrificing space - I was thinking of placing some of the panels in the corners, maybe I'll get some double action that way, I have to see).

This is a rough sketch of my studio:



This is a waterfall spectral delay graph made with an ECM8000 at the sweet spot. 



Note that the sketchup picture is not my real setup. My speakers are setup on stands, I only use a small table and sit at 38%. I have placed my speakers in the known 60 degree angles and the tweeters are never symmetrical in distances from floor/walls/ceiling (all distances are non-dividable). My sweet spot has soundproofing at the top and left and right wall, though this is only for frequencies above 300 Hz of course. Behind my sweet spot I have carpet on the floor (tightly woven, short hair) to control the chatter there. I understand my room is a bit dry but I prefer a bit dry above chatter (echoing of high frequencies). The room is (too) small anyway.

I am thinking about building a new table for my laptop and mixing table that will function as a bass trap. That way I can build a bass trap that is deeper than 20 cm without sacrificing space in the room. I understand it will be less functional than bass traps in the corners but I think it could help.

I have Adam A7X monitors as L & R and for surround I use three Yamaha HS50M. I understand they sound different (a little harsher) but I rarely have to mix surround and when I do it's for theater which will always be sound checked on locations. The surround speakers are placed in a circle around the sweet spot at 0 degrees and 110 degrees respectively. I do not have a sub (yet) although I use this in live theater (5.1) so I am still thinking of buying one. But... Because the studio is so small I wonder if this would make any sense because the sound waves do not fit in the studio. The studio walls (except the window side) are drywall though, so maybe they can propagate through the rest of the house and end up back in my studio though :rofl:

Making sense or not: Now I cannot really mixdown the .1 so I probably need a sub (or better: an LFE) anyway, just to check the mix. I have not made any choice yet what kind of sub this would be because the standard Adam (and a lot of other studio monitor subs) seem to have quite a rolloff below 45 Hz which makes absolutely no sense to me in a .1 mix (yes I would like to go deeper please!).

So there are all my ponderings for now.


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## starryi

built a small studio , using a x-32 mixer. Need to measure and set-up the room acoustics.


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## juliahe

Short, & simple, but very nice


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## STMY

If you have an unsymmetrical room, you may(read: must) deviate from the standard angles. Use our ears to find the optimum positions and angles for the stereo image only. Start from the 60 degrees. Move them inward and outward one at a time.

Use a laser on axis of the speaker and think about the reflection from the surface that is hits.

if you think you found an optimum, than remeasure using your ECM8000 and study the results. Repeat this until you think it can't get better and then position the surround units while taking the same approach.

Once you're done with that procedure, start installing absorbers designed on the basis of your last results.


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