Inside Audio: Dave Moulton

The Brave New World:Control Room Design

I promised last month I’d talk a little more about the design of control rooms, as part of our inquiry into the care and feeding of loudspeakers. This is a complex, expensive and difficult topic. It is central to audio performance because, as I have already noted, the loudspeaker system includes the room it is in.

For production work, if we haven’t got a decent room, by definition we can’t have decent audio monitoring. In practice, our primary strategy for dealing with this is simple denial. However, we can do better than that.

DESIGN NEEDS

There are two primary and one secondary production needs that have to be considered when we talk about working in control rooms. I call the two primary needs "looking back" and "looking ahead." The other, secondary need is for consistency between control rooms. For organizations that do a lot of production and need to manage multiple productions simultaneously, this need for consistency will be central. This of course includes many readers of TV Technology.

"Looking back" involves listening to the recording we just made, and evaluating how accurate that recording is, what flaws it contains and how it might be improved.

"Looking ahead" involves evaluating how the recording we have made will sound to our end users. It is a predictive task, and involves a lot of speculation, in the face of millions of end users with differing playback systems and environments.

As I’ve said before, we need to predict how well other people in other places listening over other loudspeakers in the future will think we’ve recorded a musical event.

Consistency between rooms reduces errors when producers and engineers must move quickly between different rooms and adapt to new spaces on a daily basis. It’s not much of a problem for most music production where we work in the same facility for weeks or months on end, but for broadcast it can be a big issue. The BBC, for instance, generates 100,000 hours of audio a year, about 30 hours of it live – there simply is no time for staff to get acclimated to different control rooms.

SCHOOLS OF THOUGHT

There are several schools of thought regarding control room design. Different opinions exist on room treatment, for the purpose of managing the early reflections and the decay of sound in the room. Loudspeaker response varies as a function of direction, so the early reflections and decay from the loudspeaker usually have substantially different spectral quality than the direct sound. This is quite important to the resulting sound quality.

The conceptually simplest but most expensive and difficult to build is the anechoic or near-anechoic room, wherein all possible room reflections are eliminated by absorption. Intuitively, such a design makes a lot of sense. We can really hear what’s coming out of the loudspeakers on-axis only. Further, all anechoic rooms, by definition, will sound the same.

Aside from expense, the main problem with such rooms is that they are inappropriate for the "listening ahead" task. I’ve done stereophonic critical listening, as a recording engineer, in an anechoic chamber, and I can tell you that it is extraordinarily difficult – probably impossible – to reliably predict how playback will sound in a conventional room from listening in an anechoic space.

EARLY-ANECHOIC ROOM

There is a design topology that has been around for about 20 years that I call the early-anechoic room. LEDE and RFZ designs are examples of this topology. The idea is that all early reflections for up to 20 ms should be either avoided or suppressed, but "reverberance," the reflected energy that decays after that, is permitted.

The reasoning is that these early reflections "confuse" listeners, and that the generalized room decay is fine. Such designs are still fairly expensive but not nearly as difficult or expensive to implement as anechoic or near-anechoic rooms.

BEST OF BOTH WORLDS

Early-anechoic rooms try to get the best of both worlds, with anechoic behavior for a brief period and then room "reverb" after that to make the space a little more suitable for the "listening ahead" task. If we have speakers with poor off-axis response, so that the early reflections really sound bad, such a design may be necessary. In fact, LEDE was originally developed precisely for this reason – to accommodate such a monitor that was really popular at the time.

My preferred approach is just the opposite of this, in what I think of as an early-reflection fast-decay design (and call, egotistically, a "Moulton Room").

We know from a lot of research and accumulated experience that humans integrate the early reflections (for up to 50 ms) with the direct sound. We also know that the sound decaying for the period from 70 ms to 150 ms really interferes with the clarity and intelligibility of the direct sound. The Moulton Room supports early reflections for up to 50 ms for that integration, and then begins absorption as completely as possible.

ON THE CHEAP

It turns out that this is comparatively cheap to do (in essence – make the wall behind the loudspeakers anechoic, leave the other walls alone). Further, the sound resembles the sound in end users’s rooms quite effectively for the first 50 ms, really improving the listening ahead performance, and not hurting the listening back performance at all.

The downside of this is that it requires speakers with good dispersion, and room-to-room compatibility requires reasonably similar dimensions.

CONTROL ROOM CRITERIA

With all this said, here are some sensible real-world criteria for control rooms. You might want to see how your control rooms stack up. Uh-oh!

• The noise floor of your control should be below NC-40 with all equipment and air-handling on.

• Your control room should have lateral symmetry to 2 inches (10 kHz), with the loudspeakers and the room sharing the same median plane. Surround rooms should have a viable median point as well, shared by all the speakers (time correction is OK).

• You will need to manage early reflections and decay time according to one of the schemes above. Your budget, plus your choice of studio monitor, probably determine which topology you choose. Naturally, I prefer and recommend my early-reflection fast-decay topology, on both cost and performance criteria. All the topologies, incidentally, call for absorbent ceilings.

• All these topologies manage low-frequency standing waves by absorption. I also suggest some care in choice of room dimensions, making sure that they are not near whole-number multiples. I suggest ratios such as approximately 1:1.4:1.7 for rectangular rooms.

Once you have such a room or rooms in place, you are in a position to start considering your array of studio loudspeaker monitors. Next month, we’ll talk about that.

Thanks for listening.

Dave Moulton has a new presentation on small room acoustics on his Web site. You can complain to him about anything at www.moultonlabs.com.