News

Without a Net
October 01, 2004

By Staff
Appeared in Broadcast Engineering

On-air live audience television is becoming a benchmark of the broadcasting industry, and it is attracting increasing numbers of viewers worldwide. The design of mechanical and electrical systems for live audience television studios presents unique challenges. Systems must not only meet stringent technical criteria for spatial function, noise level, cooling, lighting flexibility, power quality, emergency power, system redundancy, and isolation grounding, they must also be designed to provide comfort and life safety for a large group of people.

Facilities that are used for on-air live audience television broadcasting typically consist of a studio ranging in size from 2,500-5,000 sq ft, control room, audio room, video room, green rooms, machine rooms and other support areas and amenities. The mechanical engineer must design a system that is capable of providing 24-hour cooling for technical equipment, accommodating a significant lighting load, and providing a comfortable environment for an audience ranging from 50-200 persons, while meeting noise criteria (NC) of 25 or lower.

This combination of design challenges requires an HVAC system designed to deliver air to the studio at a low velocity and distribute it using convection and diffusion principles, with mechanical and electrical rooms located as far as practical from the studio and other sound-sensitive areas to avoid noise and/or vibration transfer.

Minimizing Noise and Vibration
Defining and selecting the right type of HVAC system is a crucial step toward meeting these requirements. A fan coil system, which cools spaces by blowing air over coils that circulate chilled water, is not a good choice for an on-air live audience studio. Some manufacturers’ claims to the contrary, no one can guarantee that this type of system will meet stringent noise criteria; they also generate a lot of vibration. Since this equipment must also be placed as close to the space as possible, an unacceptable transfer of noise and vibration is likely.

Packaged air handling units or air conditioning units are a good choice. All of the components of air handling units or air conditioning units should be self-contained to include components such as fans, motors, chilled water coils or direct expansion (DX) coils with compressors and, in some cases, sound silencers. This type of system typically meets studio noise criteria; if not, the system can be upgraded from a packaged unit to a custom or semi-custom manufactured unit.

The units should be mounted on the rooftop or located indoors in a dedicated mechanical room. For indoor installation, the HVAC units must be mounted on isolators in an acoustically lined mechanical room located as far away from the studio as practical, at least 100-300 ft away. The use of variable frequency drives and direct digital controls (DDC) also manages the transfer of sound and vibration.

The studio HVAC load is sized based on lighting load, number of occupants and, in some cases, envelope load, which consists of roof, walls and glazing. Lighting load can be as much as 35-75 watts per sq ft, yet the mechanical system should be designed for an average operating condition of 25-50 watts per sq ft for practicality, diversity and economic reasons. For example, for a studio of approximately 4,000-5,000 sq ft and a live audience of about 75-125 persons, the size of an HVAC unit can range from 60-75 tons.

A live audience studio with occupancy of 50-200 persons also increases the fresh air requirement, typically 15-20 cfm per person is provided based on ASHRAE Standard 62-2001 (“Ventilation for Acceptable Indoor Air Quality”); thus, in terms of cooling capacity, the size of the air handling unit must be increased. This is another good reason to locate the air handler on the rooftop if possible. If the air handler is installed within the building, shafts to the rooftop must be large enough to deliver the required volume of fresh air as well as relief/exhaust air. In an existing building, new shafts will have to be provided to accommodate these requirements.

Air Distribution Versus Structural Limitations
Air distribution presents structural challenges, and developing a solution requires ingenuity on the part of the mechanical engineer and close coordination with the architect, structural engineer and acoustical consultant. On-air live audience studios typically require a minimum floor-to-ceiling height of 18-22 ft to accommodate the lighting grid, and the height requirement increases in proportion to the studio’s area. As a result, typically there is little space available above the lighting grid for ductwork due to the structural support and seismic restraints system that is required for the lighting grid.
Large ductwork is required in order to slow down the speed of the air to the low velocity (600 ft per min) required to meet noise criteria. To achieve the acoustical requirement for NC 20, for example, the exit velocity of the air conditioning system must be 300 ft per min or less and, as a result, the ductwork size increases significantly. Noise can also be reduced using silencers and acoustical lining in the ductwork. Silencers must be located outside the studio in order to provide the proper sound reduction due to HVAC.

In some applications, air velocity is further reduced at the diffuser, typically from 600 ft per min to 100-150 ft per min. A custom made plaque diffuser is the most practical solution for live audience applications because it achieves both thermal comfort and acoustical requirements and it is cost-effective. Another type is a tapered linear diffuser, which is also a custom-made solution; however, it is heavier and costs more than a plaque diffuser. A third type is a fully perforated duct/diffuser. This type is custom made and only recommended for studios with height over 24 feet. Whenever possible, the diffusers are located so that they direct airflow toward the faces of audience members, which provides the most comfort because they feel coolness but no breeze or drafts.

The above represents an overhead air distribution system, which is the most widely utilized distribution system in studio applications. Another distribution method is to deliver air through a raised floor plenum. With a low supply system, the return is located high for proper air circulation and effective cooling/comfort. In both methods, overhead and under floor, return air has to be sized at low velocity not exceeding 300 ft per min. Also, the placement of return air is crucial. With overhead distribution the return has to be located at a low level. With under floor distribution, the return must be located at a high level.

Meeting Variable Loads
In a typical studio, constant air volume control can be effective. However, in the on-air live audience studio, the load generated by the audience can vary from day to day, show to show, and even during the course of the broadcast. Therefore, use of a dedicated air handling unit with variable air volume (VAV) control is recommended for both comfort and energy conservation. The VAV control allows the air volume to be adjusted during a show to maintain adequate cooling, typically about 68 degrees F or lower in some cases. Moreover, the system can be cost-effectively designed with a single air handling unit serving the studio and controlled by DDC.

To reduce the risk of heat-associated dimmer failure, the mechanical system must be designed to keep the dimmer room at a constant temperature around the clock (24/7).

Finally, air distribution is critical in technical equipment rooms: cold air outlets must be located in the front of or above the racks and return at the back of the racks to carry the heat to the return and maintain the equipment at the proper temperature for trouble-free operation.

Some on-air live audience broadcasting facilities include a number of additional support spaces. Such spaces include dressing rooms, green rooms, and a living space for hosts and other talent. Some on-air facilities they also include a radio broadcast station that may be used simultaneously or during separate hours.

Heightened Electrical Requirements
The typical requirements involved in design of the electrical system for a television studio are heightened when designing for an on-air live audience studio. The electrical engineer must provide a power system that provides maximum flexibility for the user. The lighting load is typically defined by a studio lighting designer and, in some cases, set designers.

The electrical system must have the capacity for lighting loads of as much as 35-75 watts per sq ft. Similarly, power supply must be designed for maximum flexibility to meet both permanent and temporary equipment loads as well as occasional special needs. Dimmer panels/boards are an integral part of the studio lighting design, and these can be programmed to bring designated stage lights to partial or full power as needed by the show.

Life safety is a key issue in a live audience studio. Emergency lighting can be provided using fluorescent house lights mounted above the pipe grid. The emergency fixtures serving the studio are usually put on a shunt-trip relay to allow for total blackout during shows if needed; however, in the event of a real power outage, the emergency lights will come on. Emergency lights must be powered by a battery pack or, preferably, by a generator.

Technical Power is Essential
It is essential to provide technical (“clean”) power to the dimmer room, control room, equipment room and miscellaneous outlets in the studios. The typical solution involves the installation of isolation transformers with isolation ground. Technical ground is provided for such facilities and consists of ground bus bar at each technical room and copper conductors all daisy chained together and tied into the building main service electrical ground.

A cost-effective electrical service consists of 480 V, 3-phase, 4-wire service delivered to the facility, where it is stepped down via transformers to 277 V or 120 V, for example. A filter can be added to further reduce the possibility of interference/harmonics in the technical equipment. Moreover, the power and AV cabling/signal should be on separate distribution systems to prevent distortion. Because the transformers and dimmers are noisy, separate rooms must be provided to house this equipment and should be located at least 100-150 ft away from the studio. Alternatively, to reduce the length and associated cost of cable runs, the transformer and/or dimmer room can be lined with acoustical insulation and the transformer should be placed on vibration isolators.

The electrical system must also be designed to allow maximum flexibility in configuration of the various types of equipment in the control room, equipment room, video room, etc. A raised floor allows power to be run to multiple connections—both outlets and hardwired connections—under the floor. Power should be provided at 20-30 watts per square ft; in some cases, higher watts per square foot would be required to meet equipment loads.

Additional outlets, say four to five per wall, may be provided to allow for flexibility. Electrical panels are located typically within the space that they serve, either in the control room, equipment room, machine room, video room, etc. This will also allow for expansion and redistribution of the power supply.

Reliable Back-up Power
Reliable back-up power is crucially important for live audience on-air broadcasting facilities. A back-up generator will maintain power to the lighting, cooling system, servers and routers. However, generators require up to eight seconds to start up; for some applications, that is acceptable. If the time lag is unacceptable, an uninterrupted power supply (UPS) should be provided for critical equipment—that is, servers, router hubs, and a few light fixtures—to allow continuous broadcasting.

On-air live audience television broadcasting is like a high-wire act without a net. With no room for error, the medium creates unique opportunities and challenges for television directors, producers, talent, and facility designers alike. With careful planning and skillful design, the mechanical and electrical systems will play their crucial roles in supporting successful live broadcasting today and well into in the future.

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Charbel Farah, PE is Associate Partner and Hisham Barakat, PE Senior Vice President in the Los Angeles office of Syska Hennessy Group, a consulting, engineering, technology and construction firm that provides technical solutions in such areas as building system design, facilities management, energy management, technology consulting/engineering, and turnkey design/build services.