Making Your Home Control Room The Best That It Can Be

Most of the articles we read on control room and studio acoustic design seem to center around materials we might use to treat wall surfaces, and some tips about using those materials for fun and profit. In this article, I’d like to take a different approach, and present what I believe are some basic principles of control room design. You’ll find that these are actually pretty straightforward, and you can apply them almost anywhere, including your own home studio. And where you can’t apply them successfully, you’ll hopefully come to understand a little better what sort of problem you have, and how to cope with it more effectively.

I first came up with this list of principles while preparing for participation on a panel on “Small Room Acoustics” for an Audio Engineering Society convention. It occurred to me that we don’t usually think about what it is that we want to get out of our control rooms, even though that is what is really important.

Anyway, I’ve got seven principles that need to be addressed in the construction of ANY control room, including your bedroom, basement, minivan or closet. If you systematically work through these, and do your best to optimize your particular setup with the resources you have at hand, you can actually get surprising decent monitoring quality, often for very reasonable cost. You’ll see.

The seven issues you need to deal with are:

• noise floor,
• room symmetry,
• early reflections,
• decay time in the room,
• standing waves,
• loudspeaker placement and
• loudspeaker behavior.

We’ll deal with each of these, briefly.

Noise Floor

Noise floor can be defined here as the level of noise in the control room when it is “at idle,” which is to say nobody is talking, no music is playing, all the equipment and air conditioning is turned on, etc. This is a pretty important issue, because it pretty much defines the audible dynamic range of music that you can hear in your particular control room. Naturally, you’ll want to make your noise floor as low as possible, so you can hear as wide a dynamic range as possible. Right?

There are two parts to dealing with reducing such a noise floor. First, you’ve got to shut out noises from elsewhere in the building or outside. Then you’ve got to reduce noise from machinery in the room.

Room isolation (shutting out the noise from elsewhere) can be difficult and expensive. However, in your bedroom control room, you can usually make big initial improvements for very low cost simply by “weatherproofing” all of the doors and windows, as it is through these passages that the bulk of the sound leakage occurs. Certainly, that’s where to start, and see how far you can get.

A big problem here has to do with air conditioners. Aside from the noise that window air conditioners make when they’re running, they also allow sound to fairly freely pass through them, severely compromising isolation. Solutions to this vary widely, including using a “split” unit (that isolates the compressor outdoors in a separate unit and puts a separate heat exchanger on the inside wall), the construction of baffles around the air conditioning unit to reduce the noise of both the unit and from outside, and installing central AC with oversize ducts and baffles, etc.

You are gonna have to suss out what works for you in your particular case. Just keep in mind that it can be really hard to get it really quiet while keeping it really cool for really cheap! Also, keep in mind that the very simple option of going without AC is usually a pretty unpleasant way to go. You can try it, of course, but be prepared to change your mind after your first 12-hour session.

Meanwhile, machine noise is insidious. Manufacturers are generally indifferent to the noise their equipment makes. The only solutions (aside from not buying equipment that makes noise – a radical thought, eh?) are to isolate the machinery in remote “machine rooms” (a closet?) or to put them in “iso” boxes (containers that hold in the noise). Iso boxes can be homemade, and you can get away with cheesy and cheap (how do you spell “cardboard?”), although you may have to make some provision for cooling air flow. Remote machine rooms seem very elegant (and at their best, they really are), but they can bring on infuriating computer problems (when you have to trundle from keyboard/monitor to CPU, 20 feet back and forth and down on your knees in the back of the closet, ten times in quick succession!). In the case of air conditioner fan noise, time sharing may be the best solution – you run the AC at all times EXCEPT when mixing. That’s what I do – with a handy remote control I got.

So what’s a reasonable noise floor? I say you’ve got to get below 40 dBA SPL with all the machinery on. That’s not easy to do to do, but it’s worth trying for. Assuming you can hear audio down to 10 dB below the noise floor, that 40 dBA noise floor gives you a signal-to-noise ratio of something like 60 dB from the standard calibrated mix level used by the film industry (85 dBC SPL from one speaker, as heard at the mix position). This spec will also yield an overall acoustical dynamic range of approximately 75 dB. Doesn’t sound like much, I know, but you’ll be surprised by how good it actually is, when you get there!

Symmetry

Room symmetry is hardly ever mentioned, yet it turns out to be critically important for the reliable monitoring of stereo images. Without getting into “why” this should be so, I suggest to you that it is usually the single most effective thing you can do to make your control room start sounding really good and turning out decent mixes.

What you want here is “lateral” symmetry, which is to say that both sides of the control room are identical in shape and materials, that there is a “median plane” down the middle of the room, and that the two sides of the room constitute “mirror images” of each other. Needless to say, the loudspeakers need to also use this median plane (see below).

Unfortunately, such symmetry by necessity includes furnishings, windows, etc. The good news is that most rooms are rectangular and rectangular rooms start out life being symmetrical by definition, at least until you start adding doors, windows, closets and fireplaces. I generally recommend that you set up the median plane along the long axis of a rectangular room. You’ll have to fiddle with furnishings, etc., and it can be a pain to deal with doors and windows, but you can usually obtain decent symmetry with a little effort in any rectangular room. Again, the solutions here are common-sense ones, and you simply are going to have to both figure out and then negotiate how to get your layout symmetrical (as in “The bed goes where? Hey listen up, Billy Bob Rock Star, if the bed goes there guess where I go!? I don’t care what Dave said!!”)

Early Reflections

Early reflections are the group of reflections of a sound that arrive at the mixer’s ears within 50 ms. after the direct sound (from the monitor loudspeaker) arrives. Without explaining why, I believe the behavior of these early reflections is critically important.

Based on research that I and others have been doing, I now believe that the best possible array of early reflections are broadband untreated reflections from the side walls, and no reflections (especially no high frequency reflections) from behind the loudspeakers (the front wall) or overhead.

This, it turns out, is actually fairly easy to deal with. Put lots of absorbent material on the front wall, and on the ceiling as well. Use a carpet with padding on the floor. See below for more.

Decay Time

Decay time is the amount of time it takes for sound to die away in a room. In a small room, it is usually pretty short, and so it is generally not a problem. Nonetheless, you’d like to help the decay along (it’s called “tightening” the room), and you’d like to encourage the sound to decay equally quickly in ALL audible frequency ranges.

I like to make the front wall absorbent at ALL frequencies (this can require some moderate carpentry, building a flexural absorber for low frequencies and covering it with foam or fiberglass for high frequencies). Also, make at least the front third of the ceiling absorbent down to 500 Hz. (4 inches of fiberglass covered with fabric or any 4” foam panels will do fine). This usually gets the decay time down to under 150 ms. and with other absorption in the room, it is possible to get the decay time under 100 ms., which will yield really good results.

Interestingly, if you do it right, the room will still sound pretty natural, and not particularly dead, absorbent or weird. To give you an idea of how I approach this for clients, I’ve included drawings of a generic “monitor shell” I often use. Use it for inspiration!

Standing Waves

Standing waves are the long low frequency waves supported by the room dimensions. They add a resonant signature to the room, and are generally problematical at low frequencies. At mid and high frequencies, standing waves are everywhere in great profusion, and they don’t matter for our purposes here.

Happily, standing waves are dependent on the room walls being reflective at low frequencies. IF you make the front wall absorptive at low frequencies, you will pretty much take care of any standing waves that would be a problem. Further, openings like doors and windows can make excellent bass traps, if isolation is not an issue – for instance, I use a big atrium window as a bass trap in my room, and it works quite well.

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