YDCG Design Considerations for Recording and Production Facilities

THE GRAND DESIGN: DESIGN CONSIDERATIONS FOR RECORDING AND PRODUCTION FACILITIES

The evolution of recording technology during the past decade has modified the design criteria for the current generation of recording and production facilities. Digital recording technology and enhanced analog recording processes have expanded the available dynamic range commonly attainable. Consequently, the challenge for reducing the acoustic noise floor in the recording environment is raised. The addition of the 20-40 Hz octave to the reproduction chain has placed further demands on the combined electronic and acoustic monitoring system.

Concurrently, the cooling requirements of high density digital electronics has initiated the trend to moving these devices out of the easy reach of the control room into dedicated, central equipment rooms, not unlike the typical video facility or the "lathe room" of our industry's infancy. The additional benefit of this topology is that during this transition period, analog and digital reel to reel, hard disk, ADAT, and other storage formats can be allocated to multiple control rooms more easily.

All recording and production facilities have the common requirements for sound isolation from the outside world and sound containment within the assigned space. For the single room facility, location is the determining factor for sound isolation. Proximity to neighbors, transportation and other fixed noise sources will dictate the amount of isolation required. In choosing a site, be sure to evaluate all possibilities. Truck traffic, railroad and manufacturing plant schedules may be quite different on weekends than on weekdays. Air traffic patterns can be altered due to weather.

In a multiroom facility, the unavoidable proximity to high level, low frequency sources demand a level of isolation that in most instances exceeds those of any potential external sources. The degree of sound isolation attained is dependent upon three factors: 1.) The mass of the enclosing walls, ceiling and floor, 2.) Airtight sealing of all penetrations for doors, windows, cables, and air ducts, 3.) Physical separation from the enclosing structure. (1)

Once external noise is under control, internally generated noise sources such as lights, dimmers and fans as well as noise sources necessarily introduced into the room such as air-conditioning and cable penetrations must be dealt with. Transformers, ballasts and dimmers can be remotely located. When budget permits, DC lights and dimmers eliminate this potential noise source as well as significantly reducing the introduction of hum into electronic circuits.

After doors and windows, air-conditioning ductwork represents the most significant penetration of an enclosure. Careful sealing and structural decoupling of these penetrations are crucial.(2)(3)

Large cable ducts and conduits provide very efficient transmission paths for sound. These must be sealed airtight, yet must be available for future changes. A good solution, especially when cable trays must be used, are products such as Crouse Hinds' "Thru-Wall Barrier." These devices consist of mounting frames of various sizes and elastomeric sealing blocks that form a tight seal around cables and conduits.

Air conditioning noise, especially low frequency noise, presents one of the most difficult challenges to providing a low noise floor. Conventional techniques such as fibrous duct lining and passive silencers can be effective above 250 Hz. In lower octave bands, duct lining, as conventionally employed, is less effective. Passive silencers of sufficient length place severe static pressure restrictions on the air handling units.

A recent study has suggested that placing the duct lining in a manner that the fibers are normal to the duct axis can nearly double the attenuation at low frequencies over that of the conventional orientation. (4) Active noise control systems are quickly becoming a viable solution to solving low frequency attenuation problems.

First patented in 1934, active noise control systems consist of a microphone that detects the noise as it propagates down the duct. A DSP controller processes this signal, determines a canceling waveform and introduces this signal through a loudspeaker. A second microphone located just beyond the speaker provides error correction feedback. Attenuation is 12-20 dB between 40 Hz and 160 Hz. (5)

Since costs typically increase geometrically with lower noise levels, it is important to determine the specific requirements for each situation. A common, albeit crude, guideline for determining this is the Noise Criterion Curves.(6)(7) Traditionally, recording and broadcast studios were recommended to follow the NC 15-20 curves while television studios the NC 25 curve. Recent studies have questioned the appropriateness of these recommendations considering the recent advances in recording technology. (8)

Introduced in 1957 by Leo Beranek, the NC curves are based upon a survey of office workers with the concern of the effects of noise on productivity and speech communication. Although less than ideal for determining noise audibility in critical recording and production environments, most published data is based upon this system. It is also a system that some architects and construction professionals understand.

After external and internal noise sources are under control, the next step is to sculpture the "sound" of the room. This entails the control of the build up and decay of the sound field in the room.

The sound field can be controlled by absorption with porous materials, diaphragms and resonators, reflection management and diffusion. All of these techniques are to varying degrees non-linear. Porous absorbers become ineffective between 250-500 Hz, depending upon their thickness. Diaphragmatic and resonant absorbers can be extremely frequency sensitive. Reflective surfaces become absorptive, usually with increasing frequency and at some point can become diaphragmatic as well. Diffusive surfaces not only vary with frequency but also can be dependent upon orientation. They also usually become absorptive and diaphragmatic at certain frequencies.

At frequencies below 250 Hz, conventional techniques require a depth of porous material or a cavity equal to 1/4 the wavelength of the lowest frequency to be absorbed. At 40 Hz this is approximately 2.13 m (7 feet). At 20 Hz it is over 4.27 m (14 feet). This eats up much real estate but the improvement in low frequency transient response is amazing. Recent work at the U. S. Army Construction Engineering Research Lab has developed low frequency absorbers with normal incidence absorption coefficients approaching 1 with a thickness less than 10% of a wavelength. (10)

In control rooms, another common goal is monitoring linearity. The best way to accomplish this is to start with a linear monitor, with dispersion selected to complement the room geometry and use. In most cases, some degree of high frequency rolloff is preferred to a ruler flat response to beyond 20kHz.

At least in the near term, a console of some substantial size is an essential fixture of every control room. Audiophile monitors intended for floor placement are not compatible.

In the cost effective 90's, recording and production environments must be more flexible than ever to serve an expanding client base. The most prevalent is expansion into audio-for-video.

In the studio, this poses the conflicting requirement for an acoustically dead space to accommodate ADR and foley work verses a more moderate space for most music recording. Budget permitting, both uses can be satisfied by providing some degree of variability, using heavy drapes or more sophisticated methods.

In the control room, the physical conflicts between a dedicated center audio monitor for surround mixing, a video monitor at something less than neck straining height and a panoramic window into the studio presents a dilemma. Dolby recommends that the center monitor match the left and right and be placed at the same height for optimum imaging.

Some compromise must be made depending upon the relative importance of each element to the user of the facility. One alternative that has worked in several situations is to place the viewing window to one side.

Two surround speakers are typical, mounted behind and .6 to .9 m (2 to 3 feet) above ear level. Their bandwidth does not have to exceed 100-7,000 Hz. The goal is dispersed sound so they should not be aimed directly at the mixing position.

The trend to a central equipment room as opposed to dedicated equipment in each control room can become a challenge in a multi-room facility, especially when space is restricted. The criteria are that client and talent access the studio and control room independently of engineers' access to the machine room. And there is still a desire for the engineer to have at least visual contact with reel to reel machines.

From a technical power and signal interfacing standpoint, a single room facility is relatively straightforward. However, all technical systems take on an added dimension of complexity when designing the multiroom facility. Inattention to detail can result in facilities where individual rooms function internally, but interface between more than one room may be erratic, or even impossible. This would severely limit the usefulness of the facility.

Paramount among considerations is a well planned and scrupulously executed grounding scheme. For AC power, 2 grounding concepts predominate: 1) a true "star ground" with every outlet having a separate ground wire pulled back to the central technical ground point, and 2) a devolved ground, in which each major room has a central ground point, and these points connect to the central technical ground for the entire facility. The true star ground is by far the more effective method. The third pin ground wires from all outlets should be brought to a bus bar connected to the central technical ground point. All receptacles must be the isolated ground type, usually identifiable by their orange color or green triangle. All technical power should be completely shielded in steel conduit and raceways. Romex should never be used. However, conduits must be connected to ground and must not be mechanically connected to conduits for any other power system, or to anything else metallic such as water or sprinkler systems. Technical power should never be used for any other function such as copiers, kitchen equipment, air-conditioning, or any non-production equipment. Only when all these conditions exist can an interface system be developed which is consistent and trouble free.

All equipment should have only one connection to the ground system. The most convenient ground connection is the third wire, usually green, in the AC power cord. According to the National Electrical Code (NEC) and other codes this wire must be used for safety. Some prefer to use a separate ground wire to each piece of equipment. This is not usually necessary and is very inconvenient in cases where equipment has to move from room to room.

Obviously, if one leaves the AC cord ground wire intact to avoid ground loops, any other signal connection between equipment must not complete a ground connection. In other words, all signal shields must connect at one end only. Various methods exist for achieving this goal, including complex schemes involving shields cut just about everywhere and bussed grounds on patchbays. Most of these ground schemes can be made to work as long as the cardinal rule of grounding is applied: Do it consistently. However, any scheme involving ground bussing can create different ground "nodes", with varying impedance to true technical ground. In a multi-room installation, this can create certain patches or equipment configurations that never seem to work totally hum and buzz free.

The simplest shielding method is to carry all shields through any interconnects and patchbay normals, and lift the shield at one end. The most prevalent choice is to connect any shield at its source and lift it at its destination. This means right at the equipment and not at an intermediate connector or patchbay. Most equipment has some type of captive cabling allowing easy disconnect and establishing a connector "standard" such as XLR to "Elco" for multitrack machines. Thus, all input ground lifting is done in this equipment specific cable and not in any truck or tie lines. This greatly simplifies installation as all bulk wiring is done in one simple way, with all shields intact.

Evaluation of an existing room or proposed environment

Before buying or leasing any facility, the prospective owner should minimize his investment risk by conducting an initial walk-through, preferably together with the architect, designer, and contractor. The following are some of the items that should be assessed:

Control Room Shell of 55-85 square meters (600-900 square feet)

Studio Shell as required (55 cubic meters / 2,000 cubic feet minimum)

Support Space as required

Room for expansion

Clear Height of 4.5 m (15 ft.) minimum

Column spacing 7.5 m (25 ft..) or greater

Floor loading capability of 730-975 kg/sq. m (150-200 lbs./sq. ft.)

Existing mechanical system capacity (heating, ventilation, air-conditioning)

Electrical service-ease of upgrade - power conditioning required

Structural system and exterior wall composition

Ability of roof to accept additional loads for isolation construction and HVAC

Roof Condition

Air traffic paths

Railroads

Automobile, truck, motorcycle

Road condition (potholes)

Soil type

Proximity to TV & radio transmitters

On the basis of this on-site research and other information, the prospective owner should be able to determine the suitability of the existing structure. Some idea of the extent of changes required for transition from the existing structure to the new use should also be developed. Assess the scope of these changes as they affect both the interior and exterior of the building. Examples of some specific considerations would include zoning requirements, building codes, required upgrades to an older building, disabled access, additional parking, earthquake or wind reinforcement, and fire codes.

Some general contractors offer preconstruction services, such as value engineering, that can save time and money, and minimize surprises and headaches later on in the project.

References
1. Retinger, Michael, "Handbook of Architectural Acoustics and Noise Control, (TAB Books, Blue Ridge Summit, PA, 1988.

2. Jones, Robert S., "Noise and Vibration Control In Buildings", (McGraw Hill, New York, 1984)

3. Waropay, Vincent M and Roller, H. Stanley, "Design Aid for Office Acoustics", Form & Function, United States Gypsum Company.

4. Wassilieff, Con, "Inverse Anisotropy Ductlinings", Internoise 91, pp. 87-90.

5. Goodman, S., Burlage, K., Dineen, S., Austin, S., and Wise, S., "Using Active Noise Control for Recording Studio HVAC System Silencing", presented at the 93rd Convention of the Audio Engineering Society, San Francisco, 1992, Preprint 3376 (E-4).

6. Acustica, Vol. 14, pg. 33, Fig. 14, 1964.

7. Gizzy, Vin, "Acoustic Design, Noise Control," Recording Engineer/Producer, August, 1988.

8. Cohen, Elizabeth A. and Fielder, Louis D., "Determining Noise Criteria for Recording Environments", Journal of the Audio Engineering Society, Volume 40, No.5, pp. 384-401 (1992 May).

9. Ballou, Glenn M., Editor, "Handbook for Sound Engineers", (SAMS, a Division of Macmillan Computer Publishing, Carmel, IN, 1991).

10. Jinkyo, Lee and G. W. Swenson, Jr. "Compact Sound-Absorbing Structures for Low Frequencies," "Proceedings of the 1991 National Conference on Noise Control Engineering," D. A. Quinlan and M. G. Prasad, Eds. (Noise Control Foundation, New York, NY, 1991)

11. Westerink, J. H. D. M. and Roufs, J. A. J., "Subjective Image Quality as a Function of Viewing Distance, Resolution, and Picture Size, "Journal of the Society of Motion Picture and Television Engineers, 98:113-119, Feb. 1989.

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