by Russ Berger

When people who work in existing broadcast and production facilities are surveyed about workplace comfort, their most prevalent complaints involve the heating, ventilating and air-conditioning (HVAC) systems. The problem they cite most frequently, aside from temperature control, have to do with excessive noise and vibration.

How can this happen so often? First, the requirements for an HVAC system in a technical facility are quite different from those in typical office spaces. Technical spaces have extensive heat loads and other unique characteristics associated with a high concentration of electronic equipment or production lighting. As a result, adequate cooling, airflow, humidity control and filtration are vital issues. Second, a majority of these technical spaces have acoustical requirements that are much more stringent than those for run-of-the-mill buildings. Combine these two features and it's no surprise that without a great deal of care, the results are often disastrous.

It would be nice if there were qualified engineers and acoustical consultants involved in the design of every construction project (we all have our little fantasies), so that a TV facility's management and staff would not have to understand the ways in which noise and vibration problems can arise from a poor HVAC design. Unfortunately, the reality is that architectural acoustics is one aspect of broadcast and production that is commonly misunderstood, misrepresented or misapplied - by broadcasters and by may design professionals.

This article discusses some problems with HVAC system noise and vibration that are commonly encountered in broadcast and production facilities. Sometimes just knowing the potential pitfalls is enough to avoid falling headlong into them.

Estimating Heat Loads

When designing an HVAC system, the first step is to accurately identify the heat loads generated by the occupants, equipment, lighting and surrounding environment. Although this isn't really an acoustical issue, it's an area that suffers from the "garbage in, garbage out" syndrome. If a project's mechanical and electrical engineers are given bad information about equipment loads by the broadcaster's engineers, consultants or systems integrators, the HVAC systems they design may never be able to provide an appropriate environment for equipment or operators. When HVAC systems don't work right, it can often be traced to a mismatch between capacity and actual load.

Unfortunately, it's difficult to obtain accurate information about the power requirements of broadcast equipment, much less the heat dissipation. Manufactures sometimes list peak power consumption, sometimes power consumption at idle and sometimes just the fuse rating or recommended circuiting requirements. Invariably, if they exist at all, the manufacturer's specs represent some sort of worst case, which for many equipment items (audio amplifiers or any machine with a tape transport motor, for example) bears little resemblance to their actual power consumption over time.

For someone trying to tabulate power consumption of dozens or hundreds of equipment items, it's tempting to guess high when the data you're collecting is ambiguous or undefined. This isn't much of a problem for the electrical systems, where the downside of excess capacity is only the cost of oversized circuits or transformers. For HVAC systems, however, it's sometimes worse to have too much capacity than to have too little.

Besides the increased noise and vibration that comes with oversized equipment, some HVAC system types - particularly less-expensive varieties - don't respond well to conditions outside their design range. If oversized, many systems will short-cycle, rapidly dropping the temperature in an occupied space over the span of a few minutes, then shutting off for a longer period while the humidity climbs and the air stagnates.

Compounding the problem is the diversity of room types that exist in a typical TV facility. Production control rooms and edit suites may have technical equipment that operates 24 hours a day, 365 days a year, but they undergo fairly drastic swings in their lighting loads and number of occupants over the course of a typical day. Technical operations centers or tape rooms, on the other hand, will have relatively constant equipment loads with few occupants. Furthermore, neither of these types of rooms are subject to the seasonal changes in HVAC system operation that are typical in the office areas. Technical spaces often require cooling year-round.

Studios and stages are yet another category. They are subject to enormous heat loads from production lighting instruments when they're in operation, but have little heat-generating equipment the rest of the time. In addition, the changing requirements of newer fluorescent-type production lighting and the next generation of cameras may combine for an order-of-magnitude difference in heat loads compared to today's totals.

TIP: Do your homework. When you're asked to estimate power consumption for your technical electronic equipment, the time spent getting accurate information can mean the difference between a successful facility and one that just never works right.

HVAC Equipment Location

There is no substitute for keeping equipment that generates noise and vibration as far away as practical from acoustically sensitive spaces. If an air handler is too close to a studio, obtaining adequate sound attenuation through the duct system can quickly become a losing battle. And the risk of excessive noise or vibration via every other potential path is greatly magnified.

Perhaps due to their superficial resemblance to warehouse space, TV studios may appear to inexperienced mechanical engineers to be perfect candidates for rooftop units located directly overhead. This is almost always a big mistake, because exposing a studio to noise and vibration by poking a hole in its roof and placing rotating machinery there makes it virtually impossible to achieve industry-standard background noise levels. Standard building mechanical systems are almost always inadequate to handle the specialized needs of technical facilities. In evaluating an existing building to house technical spaces, it is essential to consider the existing mechanical systems, as well as the space needed for supplementary systems.

TIP: Think ahead. Whether in a new facility or a renovation, knowing where HVAC equipment will be located relative to the acoustically sensitive spaces can keep you from having to face intractable problems later on.

Duct Silencers

You would think that any device called a sound attenuator, silencer or sound trap would be invariably beneficial to HVAC noise control. Sound attenuators are an effective means of reducing broadband noise as it travels down a duct system, and have the advantage of predictable performance. Like any other tool, however, these products work properly only when they're used correctly.

Duct silencers operate by restricting the airflow through a system of baffles exposing as much of the air stream as possible to sound-absorptive filler materials and/or resonant cavities. As a result, they have "self-noise" characteristics, meaning that they generate noise themselves due to turbulence in the airflow through their internal baffles. Silencers should be located far enough upstream of any acoustically sensitive space to ensure that the noise they generate is adequately attenuated before it reaches the occupied room.

Another frequent misuse of sound attenuators involves their placement in the duct system. If a silencer is located within a mechanical room, noise may enter the system through the sheet metal duct after the silencer. If a silencer is located away from the mechanical room walls, noise may escape the system through the sheet metal duct before it is attenuated by the silencer. Ideally, a sound attenuator should be located within or immediately adjacent to the mechanical room's walls.

TIP: If you're using sound attenuators, make sure they're located where they will perform the job that's intended.

Duct-Borne Noise

The duct work that connects a fan or air handler to a room is a contained system that will also connect the equipment noise and vibration to the room unless adequate precautions are taken to attenuate the noise before it gets there. Without internal sound-absorptive duct liner or prefabricated sound attenuators, noise travels effectively down the duct system right along with the conditioned air.

Acoustical crosstalk is a similar problem that occurs when two spaces are connected by a common duct system with inadequate internal sound attenuation. Noise from one space enters the duct through the supply-air diffusers or return-air grilles and travels through the duct to a similar opening in another room.

Although noise control issues through the supply air duct system are routinely considered in HVAC design, inexperienced mechanical engineers and contractors often forget that the return-air path is an equally important contributor to noise problems. In fact, because return-air systems sometimes employ common plenums above corridor ceilings, there may be less duct work in the return-air path to attenuate the noise, and the transfer of return air from one space to another may be a significant breach of the sound isolation between them.

TIP: Make sure that all duct systems are laid out to prevent crosstalk and to attenuate the fan noise.

Velocity Noise

As conditioned air travels from a fan to an occupied room, it is subjected to acceleration, deceleration, changes in direction, division and a variety of surfaces and obstacles. Each of these effects disturbs the uniformity of the airflow and causes turbulence, which in turn creates noise.

It is essential to limit the velocity of the airflow through all duct work systems in order to keep it from generating excessive noise. This is particularly true at the final branch ducts and the neck of the supply diffusers and return grilles where this regenerated noise is exposed directly into the occupied spaces.

TIP: Keep airflow velocities low throughout the duct systems serving acoustically sensitive spaces.

Volume Dampers

An important feature in the proper design of a duct work system is the ability to control the amount of air that flows through each segment of the duct to ensure that the volume of air supplied to each space is tailored to its conditioning needs and that each supply diffuser in a given room is balanced with the others. To accomplish this, volume dampers are needed to limit the amount of air that is allowed down the duct path. Unfortunately, dampers accomplish their volume control by pinching down the air stream, increasing the pressure and consequently the noise wherever they occur.

For office spaces, ceiling supply diffusers routinely are installed with face dampers, which are volume control dampers located right at the inlet to the diffuser. The airflow noise created by the face dampers is essentially exposed directly into the room. In acoustically sensitive spaces, even if face dampers are left wide open they can generate audible noise.

TIP: Don't use face dampers on the diffusers to adjust the air volume. Adjust the volume upstream using opposed-blade-type dampers.

Vibration Isolation

Airborne noise that radiates directly from HVAC equipment is only one part of the story. Rotating or motor-driven machinery also generates vibration energy that can travel through a building's structure and radiate from the walls, floors and ceilings in the form of airborne noise. It is essential to control vibration at its source, because once it's allowed to transmit into the building structure, vibration from HVAC equipment is widespread and extremely difficult to contain.

Vibration is best controlled by decoupling the vibrating equipment from the surrounding structure. This can involve spring mounts, elastomeric mounts, inertial bases, floating floors and/or structural isolation joints.

Vibration isolators must be matched to the load they carry. It's fairly intuitive that a spring that is fully compressed doesn't offer any isolation from the supporting structure. What may not be so obvious is that an uncompressed spring is just as ineffective. If the weight of the equipment doesn't deflect the spring, it means that the spring is stiff enough that the vibration can transmit directly through its coils. Oversizing a spring's capacity is just as detrimental as leaving it out entirely.

TIP: Use properly sized isolation mounts to keep vibration at the HVAC equipment from being transmitted into thebuilding structure.

Flanking Paths

When isolating HVAC equipment or piping or duct work from the building structure, it's important to verify that there are no flanking paths or other means for their vibration to be transmitted into the surrounding construction. For example, it is futile to mount a fan unit on springs unless there are flexible canvas or neoprene connectors at the supply and return ducts that attach to the fan. Otherwise, the vibration will travel through the duct to the first place it attaches to the building. Similarly, any piping and conduit that are connected to vibrating equipment must also be isolated with flexible connections.

TIP: Remember that noise and vibration don't always follow the most obvious path in getting from the source to the place you don't want it to go. Don't allow rigid connections to defeat the operation of vibration isolators.

Penetrations

One of the most commonplace problems caused by HVAC systems has nothing to do with noise and vibration generated by the equipment or duct work. As it is distributed throughout the technical spaces, duct work inevitably penetrates the wall, ceilings and floors that are responsible for a room's sound isolation capabilities. If these penetrations are not adequately treated, they allow sound leaks that can render these acoustical barriers completely ineffectual.

First, the penetrating duct work should be supported independently from the partition. If a duct rests on the wall it penetrates, any vibration within the duct can be transmitted into the wall itself, which can then provide a large radiating surface to turn the vibration into airborne noise. Second, the duct should be resiliently isolated from the surrounding construction as it passes through the partition, to avoid any contact that might transmit the vibration into the wall. Third, the area surrounding the penetration should be sealed airtight, using materials that won't allow noise from an adjacent space to leak through the gap.

TIP: Make sure all penetrations through sound isolation walls and ceilings are sealed resiliently and airtight.

The Best Defense

When it comes to HVAC noise and vibration control, even with the best of intentions, there are hundreds of ways to make acoustical blunders that can render a technical space virtually unusable. There is no substitute for getting qualified help for the mechanical and acoustical design of HVAC systems in a technical facility.

If you're cognizant of the general mechanisms behind typical acoustical problems, however, it's much more likely that you'll be able to avoid at least the most common ones.