YDCG Design for Audio-Video Post

Post-production audio encompasses the recording and mixing of music, sound effects, and dialogue. The requirements for a good post room are not that different from the requirements of a good music control room. The primary difference is that a post room has the additional concern of relating the audio to the video. Studios for the recording of dialogue and sound effects require levels of performance beyond that of the typical music studio.

It is not uncommon to find one or more video monitors in a music control room, either as a part of a console automation system, for entertainment, or as an attempt to diversify into the post production field. But in the dedicated post room, the video is the focal point. A 25" monitor is typical, although larger size rear or front screen projection systems are often employed.

When dialogue or sound effects are being mixed with the picture, the phantom center image should coincide with the center of the picture. In order to be perfectly accurate, the characteristics of the audio monitors should be investigated prior to design and construction and provision for some adjustment upon installation might be wise. Certain monitors, such as the JBL 4430/4435, do not have vertically symmetric off axis response, which also should be taken into account. In most monitoring situations they should be mounted upside down.

A particular difficulty arises when either a window into an adjacent studio or a center audio monitor is brought into play. The traditional location for the window is between the audio monitors, basically centered at eye level and sized for maximum site lines. The preferred location for the audio monitors is at or just slightly above seated ear level, spaced 812' as room size and geometry dictate. To minimize neck ache, the optimum location of the video monitor would be centered, at or just slightly below seated eye level (see Tables 1 & 2).

In the other plane, there is a console of typically 3 1/24' in height plus whatever semi-permanent equipment, such as near field monitors, stacked on its meter bridge as a lower limit to sight and "hearing" lines. The upper limit is the ceiling height, which is often limited.

Starting with the simplest case of a video monitor with left and right audio monitors only, it should be placed so that its bottom (viewing area not cabinet) just clear any obstruction presented by the console.

The second case of a video monitor and a center audio monitor presents a serious conflict with the laws of physics. Unless we are using front screen projection with a properly perforated screen, a compromise must be made. Most engineers surveyed state a preference for the audio monitor below the video monitor for better-combined imaging.

Case three, the video monitor and window, again is a matter of trade-offs. For optimum viewing into the studio, the height of the window should be at least 6'. This places the bottom of the screen viewing area at an elevation approaching 7', good only for chiropractor pocketbooks. However, this might be an acceptable arrangement for large scoring sessions where the relationship to picture is less crucial.

The last case, the combination of a video monitor with center channel audio monitor and window, does not allow any reasonable compromise. The only possible solution is to position the video monitor to the right or left of the center audio monitor. This of course destroys the alignment of aural and visual images.

There is at least one alternative. The window and consequently the associated studio can be moved to one of the sides of the control room. This frees up the front wall for the video and audio monitors to vie for position, while removing a large reflective surface from the front of the room. An added benefit is that this configuration is faster and less expensive to construct.

(On the subject of monitoring, little interest as yet has been taken in permanently installing surround speakers. Part of the reason for this probably lies in the still fading memories of quad. As the market for surround sound video develops and standards are defined, the integration of surround speakers may become more important.)

In addition to music, post production can involve many hours of dialogue and sound effects recording, more commonly known as ADR and Foley. Recording voices down to the level of a whisper and the rustle of leaves requires an environment that is exceptionally quiet, free from extraneous rattles, resonances and reflections.

The first order of business in the design of an ADR or Foley studio, as with any studio, is to isolate the room from the outside world. There are numerous articles and books on the subject so we will dispense with the details here (see Recording Engineer/Producer August 1988). However, one important point must be made.

The conventional method of specifying the noise level in a room is the NC (Noise Criteria) curve. Introduced in 1957 B. D. (before disco) by Leo Beranek, the data on which the NC curves were based was gathered from surveys of office workers on the effects of environmental noise on their ability to perform their work and to communicate speech. The curves were developed for the purpose of providing criteria for reducing complaints to tolerable levels. At NC 20 and below, the typical goal for this type of use, the allowable noise level at 63 Hz can be as high as 50 dB SPL. For voice recording, this may not always present a serious problem if, for example, the use of a high pass filter is tolerable. But for Foley, when the sounds being recorded fall in this frequency range and are at or below the background level, a filter won’t help. If several channels are summed, the situation is even worse.

The human ear is less sensitive to low frequency sound. Most microphones are designed to be not as selective. In passing through the electronic chain, any rumble picked up can increase distortion or even cause amplifier clipping.

Wall, floor and ceiling transmission loss at low frequencies becomes much more complex to predict. The transmission loss is no longer governed by simply the mass, but such additional factors as damping, stiffness and panel dimensions come into play. Very little test data exists for frequencies below 125 Hz since the most testing is targeted for use in the residential and office construction markets.

Materials such as concrete and concrete block perform well in the low frequency range by virtue of their mass and stiffness. However, at 70140 lbs./cubic foot, a 6" thick 12' high wall will weigh in at 430860 lbs./linear foot. This is fine on grade, but in a high-rise office building may present some structural challenges. The wisest and often the most economical means of ensuring good isolation always is to locate a noise sensitive areas away from noise producing areas.

This common sense approach applies in particular to HVAC (heating, ventilation, and air-conditioning) equipment. The mechanical portion of the system, compressors, chillers and fans, must be installed in a manner that does not introduce either airborne or structure borne noise and vibration. Then the air must be brought into the room without bringing along any of the noise or vibration generated by the equipment. Usually this is accomplished with some form of ductwork.

Independent duct systems for each room are a must. Whether sheet metal, rigid fiberglass, or flexible, the ductwork should be routed and isolated so that it does not pick up any noise along its path or couple sound from one room to another. And the ductwork should not produce any noise of its own.

Sheet metal ducts, although having higher transmission loss that rigid fiberglass or flexible duct can generate popping noises when it is pressurized or depressurized. It is also more susceptible to producing aerodynamically generated noise. Round or flat oval sheet metal duct minimizes both of these problems in situations where sheet metal duct must be used.

Flexible duct has very low transmission loss, can generate crackling noises as it expands and contracts, and if not installed properly can add airflow noise. On the other hand it is very economical and does not transmit vibration as well as the other types.

To introduce the air into the room without adding noise requires a low outlet velocity of approximately 250 ft./min. A ton of refrigeration, a carryover from the days when ice was used to cool, i8 equivalent to 12,000 Btu/hr. or about 400 cubic feet per min. Consequently, a low velocity requires a large outlet area since CFM= Area x Velocity. Dampers should be avoided as they reduce the net area and therefore can only increase noise.

Air distribution should utilize the techniques employed for clean rooms or surgical suites. In these types of rooms, the air is distributed over the entire ceiling. Diffusers, if they must be used for aesthetic reasons or physical constraints, must be sized so as not to restrict airflow. A good rule of thumb is that 1 square foot of grille or diffuser has approximately 65% open area.

Once externally generated noise is under control, attention should be given to internal sources. Any device with a fan is a prime candidate for exile to a machine room. Devices which must remain in the room should be treated on an individual basis to minimize their noise contribution. Light fixtures, air grilles and registers, and console and rack panels can be set into vibration at particular frequencies. These offenders can be easily identified with a sweep oscillator and damped with neoprene or foam.

In the studio, script stands or tables should also be investigated for resonance and covered with an absorbent material such as heavy felt or carpet. Chairs should be selected for quietness as well as comfort. Foley prop storage cabinets should be built so as not to rattle or resonate and prevent any items stored from doing so as well. Once again, the best solution is to remove potential problem items from the room if they are not required.

Unlike music recording studios where room character" is usually desirable, for ADR and Foley, the room should be as transparent as possible. This usually translates into dead". Unlike many film Foley stages, Foley studios for video are very small, often doubling as ADR studios. Because of their typically small volume, traditional reverberation time calculations are not meaningful. However, any reflected energy should be diffuse and of essentially uniform frequency content.

Again, it is the low frequencies that cause difficulty. Most "acoustical" materials such as Sonex, 703 or 705 insulation and carpet become less effective to some degree below about 250-500 Hz. Resonant absorbers such as panel absorbers or Helmholtz resonators are effective at low frequencies but also re-radiate a portion of the energy that is not absorbed. The effectiveness of porous absorbers are dependent upon the thickness of the material or the distance they are spaced from a rigid wall. Space permitting, they are the better choice.

Although we have placed a heavy emphasis on the acoustic aspects of facility design, other areas deserve careful attention as well. Good lighting, for example, depends upon two key points: fixture layout to minimize shadows and fixture and lamps selection to minimize glare.

The light source should not be behind the observer (engineer, producer, and talent) but should be located slightly to the left or right so that the light comes over the observer's shoulder. To prevent glare, the light source should not be visible. The lamp should be screened by some type of baffle. The lamp itself should be selected for good beam control. As well as being energy efficient, the MR 16 lamp is particularly useful in this application.

Low voltage fixtures with integral transformers and the standard autotransformer dimmer commonly found in studios must be used with caution. All power transformers radiate a certain amount of hum. They should be kept away from low level signals especially audio signal transformers found in microphone preamps and tape machines.

Autotransformer dimmers are a common fixture in most studio applications because they present far lower potential for the radio frequency interference inherent in electronic dimming systems. Unfortunately, they must be banished from ADR and Foley studios because they can generate audible noise from the vibration of the coil and subsequent resonance of their enclosure or adjacent wall. One solution is to remote them with motorized controls, assuming the motor noise can be sufficiently isolated from the studio. The simple solution of placing them in an adjacent room is not always permissible under various building codes.

One of the most often used pieces of equipment, the seating, is seldom given careful consideration in selection, other than perhaps color coordination. Much research has been done over the past several years into what constitutes good seating design. It has been found that the seat tilt and cushion density are even more important than the design of the back cushion in providing long term comfort.

We have already discussed the importance of quiet air conditioning and the need for proper temperature control is obvious. However the V in HVAC is not always given proper emphasis.

The precautions necessary to insuring a quiet environment also preclude the introduction of natural ventilation from open windows, for example. Sound tight means air tight as well. Any ventilation must be supplied mechanically, most often in conjunction with the cooling system.

In the control room, by removing the electronic equipment with noise generating fans, we have also eliminated a primary source of heat. And the energy efficient low voltage lighting we are using to reduced glare, as well as meet energy regulations, has reduced that source of heat by as much as 50%. Each watt of equipment and lighting removed from the room reduces the cooling load by 3.412 Btuh. In the studio, one or two actors generate little heat, about 420 Btuh each.

Consequently, less cooling is required in the studio or control room. A byproduct of the reduction in supply air required is usually the reduction in ventilation. Variable air volume (VAV) systems are particularly at fault since they regulate the amount of cooling by reducing the volume of air entering the room.

The American Society of Heat, Refrigeration and Air Conditioning Engineers (ASHRAE) publishes standards which are the criteria for some building codes. These standards are based upon acceptable levels of carbon dioxide and odor. Carbon dioxide levels, rather than oxygen levels, determine human respiration rates.

The currently applied standard requires a minimum of 5 cubic feet per minute per person of outside air for general offices where smoking is not permitted, although 10 CFM/person is a commonly used design rule. The standard is currently undergoing revision and the minimum is expected to be raised to the rate of 12 to 15 CFM/person. In buildings in which equipment or tobacco smoke generates additional indoor pollution, the minimum ranges from 20 to 35 cubic feet per minute per person.

The dilemma lies in the fact that outside air is usually at the wrong temperature and brings along with it numerous, small pollutants. The ASHRAE Fundamentals Handbook indicates that 99% of the particles in a typical atmospheric sample are less than 1 micron in size. And, of course, outside air also can bring along our least favorite pollutant, outside noise.

All these factors force us to rely on recirculation of existing indoor air to maintain proper air change rates, about 7 to 10 times per hour. Unfortunately, indoor air is also potentially laden with pollutants. (see sidebar) Although some pollutants, such as asbestos, require specific control measures, many can be removed with a properly designed, installed and maintained filtration system.

Because of the small particle size involved, electronic air cleaners or high performance dry filters are required, often in conjunction conventional fibrous media filters. Electronic air cleaners should be used with the precaution that can generate random clicking noises. Regular cleaning or replacement of the filter media is essential.

The goals for the design of a good postproduction facility are not unique. Good isolation, interior acoustic control, good HVAC, lighting, seating and video monitor (television) placement can apply in some degree to any environment. However, these are some of the areas that deserve special attention and continued research.

Asbestos: This particulate can be found in about half of the office buildings constructed between 1958 and 1970. Over 3,000 products contain asbestos fibers. Over the years, the asbestos fibers, which are nonconductive, noncombustible, and chemical resistant, can come loose and circulate in ventilation systems. If its fibers are inhaled, asbestos can cause a debilitating and ultimately fatal lung disease called asbestosis. It can also cause lung cancer, cancer of the chest lining and other forms of cancer.

Carbon monoxide This gas combined with hemoglobin in blood inhibits its oxygen carrying capabilities. Longterm, low dose exposure to carbon monoxide can result in headaches, dizziness, decreased hearing, visual disturbances, personality changes, seizures, psychosis, palpitation of the heart, loss of appetite, nausea and vomiting. The biggest source of carbon monoxide is outside ventilation air introduced through improperly placed intake vents.

Fiberglass can produce hives, scratchy throat and severe rashes. When inhaled into the lungs, it becomes permanently lodged there.

Formaldehyde This pungent gas is used in as many as 3,000 different building materials such as fabrics, furniture, carpets, plywood, particle board, urea-formaldehyde foam insulation, caulking and glue. Ailments attributable to formaldehyde include burning eyes, coughing, breathing difficulties, nausea and dizziness.

Ozone A colorless gas with an odor, ozone is produced by photocopiers. It is a severe irritant to the lungs, nose and throat.

Polychlorinated Biphenyl’s (PCBs) PCBs are organic compounds formerly used in electrical transformers, fluorescent light ballasts, waterproof adhesives, various plastics and carbonless paper. PCBs can cause irritation to eyes, skin, nose and throat.

Radon A natural radioactive gas created by the decay of radium, which occurs naturally in soil, rock, groundwater and concrete. Decaying radon emits alpha particles linked to lung cancer.

Tobacco smoke Tobacco smoke contains nearly 3,000 compounds including ammonia, benzene, formaldehyde, propane, hydrogen sulfide and methane as well as carbon monoxide. A room in which tobacco smoke is moderately heavy will have a particle concentration of about 30 million particles per cubic foot.

Vinyl chloride This synthetic compound is found in pipes, lighting fixtures, weather stripping, upholstery, wall coverings, electric wiring, laminates and synthetic carpeting. Vinyl chloride is a carcinogen that emits vapors as it deteriorates which have been linked to chronic bronchitis, ulcers, allergic dermatitis and bone disorders.

Screen Size (Diagonal)	Minimum Viewing Distance	Maximum Viewing Distance
9"	3'9"	7'6"
12"	4'0"	10'0"
15"	5'0"	12'6"
19"	6'3"	15'9"
23"	7'9"	19'3"
25"	8'3"	20'9"
31"	10'3"	25'9"
35"	11'9"	29'3"
45"	15'0"	37'5"

Seat Height A	5th	50th	95th
Adult Male	15.5"	17.3"	19.3"
Adult Female	14.0"	15.7"	17.5"

Seated Eye Height B
Adult Male	28.7"	31.3"	33.5"
Adult Female	27.4"	29.3"	31.0"

Blankenship, Jeff, HVAC for Audio Facilities, Recording Engineer/Producer, August, 1988

Gilford, C. L. S., The Acoustic Design of Talk Studios and Listening Rooms, Journal of the Audio Engineering Society, 27:1/2 (Jan./Feb. 1979)

Gizzi, Vin, Acoustic Design: Noise Control, Recording Engineer/Producer, August, 1988

Schindler, Thomas A. Acoustical Design for the Technical Building at Skywalker Ranch: Part 2 Mechanical and Electrical

Schwind, David R., Acoustical Design for the Technical Building at Skywalker Ranch: Part 1 Sound Isolation and Room Acoustics," SMPTE reprint No. 12981

Wadsworth, Raymond H. Basics of Audio and Visual Systems Design, Howard W. Sams & Co., Inc., Indianapolis, 1983

Yaniv, Simeone L. and Flynn, Daniel R., Noise Criteria for Buildings: A Critical Review, NBS special publication 499, U. S. Department of Commerce, National Bureau of Standards, Washington, D. C.