Noise and Vibration Control
For HVAC and Piping Systems

By James F. Yerges, PE, PhD, and John R. Yerges

When the editors at HPAC approached us about writing an article dealing with noise and vibration control for building mechanical systems, we explained that this is not a new subject and that it has been dealt with ad nauseam in literature. The editors countered by asking if we still see system designers make mistakes. That placed us in the position of having to admit that we are making a very good living (thank you!) fixing the same problems over and over again. We agreed that if they would let us recite a litany of things-that-everybody-ought-to-know-better-than-but-do-anyway, we would do it. Here goes.

Prepare a Complete Design

The design of ducted HVAC systems must address six distinct but related issues - airborne equipment noise, equipment vibration, ductborne fan noise, duct breakout noise, flow generated noise, and ductborne crosstalk. Each and every one of these issues must be addressed, or the design will fail.

Control of airborne equipment noise reminds us of an oft-quoted platitude, usually attributed to Conrad Hilton. It states that there are 10 requirements for a successful hotel project. The first five are location, location, location, location, and location. The same incontrovertible axiom applies to locating mechanical equipment rooms. Do not place a mechanical penthouse on a lightweight bar joist roof over the top floor corner office of the chief executive officer. Do not place a major fan room up against the back of the audience chamber of an auditorium. Do not place the mechanical equipment room on grade, in a basement, or in a sub-basement.

Use non-critical spaces to buffer critical spaces from equipment room noise. These buffer spaces include corridors, elevator cores, toilet rooms, storage rooms, telephone equipment rooms, etc.

Acoustics does not respect gravity. Equipment noise travels up, down, and sideways. This means that equipment room walls must be selected as a function of the equipment housed therein and the noise sensitivity of the adjacencies. Unfortunately, standard spec building steel stud and dry wall partition constructions usually demonstrate very poor low frequency sound insulation performance. (Have you hugged a mason today?) Equipment room floors and ceilings must also provide adequate sound insulation to protect adjacent spaces. Unfortunately, modern lightweight floor constructions typically consist of a fluted metal pan with a nominal 5 inch concrete topping, which is often not adequate.

Defy alleged floor space constraints, and do not place air handling unit cabinets or built-up air handler enclosures or transformer cabinets close to walls or ceilings. There is a phenomenon called "close coupling", in which a small air space will conduct cabinet vibratory motion to the wall or ceiling. A space of approximately 3 ft. usually suffices. Provide a nominal 4 in. concrete housekeeping pad beneath equipment cabinets to minimize the effects of close coupling to the floor.

Like airborne equipment noise, the principal strategy for controlling equipment vibration is location. Put it on grade, in the basement, or in the sub-basement. (Most pumps need isolators even in these desirable locations.) Do not place equipment on limber long-span floors or roofs. The structure must be a least 10 times as stiff as the equipment vibration isolators.

Speaking of vibration isolators, specify isolators for equal static deflection. If the weight at each of the four corners of a machine base is significantly different, then four different isolators with four different stiffness are required. (This subtlety is apparently lost on most OEM purchasing agents.) Specify isolators with adequate static deflection and flex connections of adequate length. Use thrust restraint isolators when necessary. Do not use housed isolators or thin pads to restrict vibratory motion.

Resiliently mount coolant lines between compressors and remote condenser and evaporator coils. Resiliently mount large transformers. Glass fiber pads 1 or 2 in. thick are usually sufficient. Resiliently mount large transformer conduits, at least within the electrical room. Hydraulic elevator pumps should be enclosed in absorptive housings with built-in vibration isolation and pulsation dampers in the hydraulic lines.

The best way to control ductborne fan noise is to not create it. Select quiet fans based on sound power data. Do not buy noisy fans and try to "fix" them. Provide good fan outlet conditions in accordance with ASHRAE guidelines. Do not turn the air in "the wrong direction," or the ducts will rumble. Turning vanes are not a panacea. In VAV systems, do not use inlet vanes to modulate air flow, particularly if the fan motors are 10 hp or greater. Inlet vanes cause rumble. Provide variable frequency drives.

As a result of decades of "value engineering," the sheet metal in SMACNA ducts is sized just barely heavy enough to prevent the ductwork from exploding. As a result, fan noise will break out or escape though the duct walls. Provide in-duct silencers or long runs of internally lined rectangular duct to reduce fan noise prior to allowing a duct to run above the acoustical tile ceiling of an occupied space. The acoustical tile ceiling provides almost no attenuation of low-frequency noise. (Use 1 in. thick duct liner instead of 1/2 in., which is too thin to be useful.)

Like fan noise, the best way to control air flow noise is to not create it. It's best to design low-pressure, low-velocity systems. From a noise control standpoint, 1000 fpm is low velocity, 2000 fpm is medium velocity, and 3000 fpm is high velocity. Design proper duct fittings for smooth flow and gradual velocity changes.

Incidentally, returns are noisy, too. Sound does "swim upstream."

Do not use opposed blade dampers in noise sensitive applications. Locate the dampers well upstream of the outlet within the lined ductwork. Of equal importance, size the ductwork correctly to minimize the requirement for dampering.

Large static pressure drops (typically greater than 1in.) across VAV valves are noisy. If possible, design to avoid them. If necessary, provide lined ductwork and/or silencers downstream.

Short runs of ductwork can cross-connect rooms, like a speaking tube. Provide adequate runs of internally lined ductwork, with elbows, to minimize ductborne crosstalk. In critical applications such as high school music suites, in-duct silencers may be necessary. Dumping return air into a ceiling plenum above a common corridor can cause crosstalk unless adequate return ductwork or silencers are provided. Peruse the references, then adopt a standard detail for sleeving and packing duct and pipe penetrations through sound rated walls.

In critical applications, sound can actually break into a duct in a noisy space (like a band room) and then break out of the duct in a quieter space (like a choral room). Provide completely separate runs, in-duct silencers, or double-wall high-TL ductwork.

Rooftop Units

Packaged rooftop air handling units cause more than their share of noise problems. (The problems are value engineered into the units.) Locate RTUs with extreme care over toilet rooms, storage rooms, or other non-critical spaces. Do not place rooftop units on limber, long-span roofs. If the roof is not stiff at the mounting location, provide a structural steel frame to transfer the weight to bearing walls or columns.

OEM internal isolation of the fan and motor is often inadequate. It is much safer to support the entire unit on properly selected isolators. If a vibration isolation curb is provided, it should be the type that permits visual inspection of the springs.

Provide roof openings only for the supply and return ducts - not one large opening for the entire unit. (Efforts to in-fill a large roof curb opening with drywall are usually futile.)

Beware the fan discharge condition in a rooftop unit. Downblast configurations are a very bad thing. Horizontal discharge into a lined plenum section is better. Horizontal discharge into internally lined and externally insulated ductwork is best.

If the rooftop unit has compressors and/or an economizer section, its noise may antagonize the neighbors. Obtain noise emission data and compare it to applicable local ordinances.

Little Fans in Occupied Areas

Fans belong in the mechanical room, not in occupied spaces. Fan powered VAVs, for example, are noisy. Locate them over corridors or other non-critical spaces. Noise emitted from the opening to the plenum, ductborne supply and return noise, cabinet radiation, and break-out noise must all be considered because the acoustical tile ceiling is not much help. Fan-coils and unit ventilators are noisy. If the design does not allow for ductwork to attenuate noise, there may be no remedy. Simply do not put fan-powered devices in quiet spaces such as church sanctuaries or courtrooms. Propeller fan unit heaters are noisy. They may work in a factory, but do not specify them in a location such as an auditorium stage house, even if they seem to offer an attractive solution to occasional load requirements.

Big Buildings, Big Noise Problems

Buildings of monumental scale cause monumental noise problems. Institutional buildings such as hospitals and universities may have steam pressure-reducing valve stations. These stations require silencers, pipe lagging, and isolated hangers. Large chillers may cause noise complaints at properties hundreds of feet away. Noise control enclosures for this type of equipment are expensive - too expensive for retrofit. The same thing is true of emergency generators, particularly if cogeneration is contemplated. Even the intake and exhaust air louvers in the mechanical room of a large building can cause property line noise complaints.

Consultant Contracts

Gone are the days when the architect was the project architect, and the consulting engineer was his confidant and advisor. Today, the architect views himself/herself as the project manager who parcels out little packages of responsibility in the form of "deliverables." The consultant receives a request for proposal consisting of one 8 1/2 by 11in. checklist stipulating the CAD format and the word processor format and dictating a lump-sum fee with 16 percent payable after design development, 42 percent payable after construction documents, and 42 percent payable after completion. "Coordination" consists of a set of 50 percent completion drawings that actually get printed about two weeks before the 100 percent completion date. Basically, nobody is driving the bus.

It often appears that the only consultant working hand-in-glove with the architect during the critical design development phase, when all of the important decisions are made, is the structural engineer. We recommend that consultants try to get their contracts rewritten to move more of their fee (and more of their work) into design development. We recommend that consultants be more forceful in directing the location of their equipment and more tenacious in fighting for adequate mechanical room space and more volume for ductwork.

Whenever possible, the consultant should try to get to the structural engineer early in the process, when he or she can serve as an ally, not an antagonist. Concrete is pretty cheap. There is no particular reason that the floor of a mechanical room needs to be a lightweight 5 in. composite slab on limber, long-span bar joists.

We suggest that consulting engineers use the acoustical consultant as an ally and a stalking horse. The acoustician gets paid to be Chicken Little and frighten the architect with admonitions that the sky will fall if the mechanical room walls consist of 3-5/8 in. metal studs and drywall.

Tedious Anecdote

A venerable yarn depicts an eager young county agricultural agent enthusiastically lecturing a dour old farmer about exciting new no-till cultivation, soil conservation, and run-off control techniques. The old farmer stopped him short, declaring, "Sonny, I already know 10 times as much about farmin'' as what I'm usin'' now." As engineers, we tend to fall into the same just-grind-it-out-and-get-it-done routine as the old farmer. Experienced consulting engineers already know a lot more about noise and vibration control than they demonstrate in many of their designs.

As acoustical consultants, we absolutely dread assignments in which we must perform post mortems on failed projects and quote the ASHRAE Handbook, chapter and verse, as to why this simply should not have been attempted. Unfortunately, we get too many of these assignments. Read the ASHRAE Handbook.