When designing an HVAC system, noise should be one of the primary considerations. In many cases, HVAC systems have served the dual purpose of
temperature control and white noise. A certain amount of “white noise” is considered beneficial because its masking benefits help keep conversations
private, have a relaxation effect, and can even help with concentration. If a system becomes too noisy, however, it can prove disturbing to occupants
and—depending on the circumstances—may require repair. It is important to ensure that the fine line between acceptable white noise and
intrusive noise is maintained. The acceptable noise level for a system is based on Noise Criteria (NC) curves, which were established in 1957 to
represent generally acceptable levels of HVAC system noise based on a specific room’s purpose.

Terminology

To help ground a discussion of HVAC noise, here is a description of basic acoustic terms.

• Noise Criteria (NC) curve—Published data on the acceptable sound-pressure level for rooms, based on their intended use. This
  data has historically been used to determine acceptable levels of background HVAC noise.
• Airborne noise paths—These paths are the same as airflow paths and are easy to identify: if air comes out of it, so will
  noise. Common airborne noise paths include where noise exits the duct at diffusers or through the walls of the duct, and places where sound energy is
  transmitted through the common walls of adjacent spaces.
• Structure-borne noise paths—This type of noise is found by following the vibrational energy of equipment as it transmits
  into a space through the framework. This type of noise path can be more challenging to follow since all possible vibration paths need to be
  considered—including vibration energy transmitted through the floor, ceiling, ducts, and common walls.
• Airborne/structure-borne interaction—These paths can be the most complicated to determine because you must consider how
  vibrational energy impacts the surrounding air, and the effect that airborne noise has on the structures nearby. For instance, vibrating equipment can
  cause the surface area of a structure—such as a duct wall—to act as a loudspeaker. Alternately, loud, low-frequency sound waves can cause
  vibration in lightweight structures such as ceilings.
• Sound absorbers—These reduce the amount of reverberant energy in an area by absorbing some of the energy in sound waves as
  those waves come into contact with the surface of the sound absorber.
• Noise barriers—These are designed to reduce the transmission of noise from one area to another. They are rated in Sound
  Transmission Class (STC) value. The STC value indicates the change in noise levels, in a laboratory setting, when you move from one side of a barrier
  to the other.

Proper Equipment

In HVAC system design, proper equipment selection, placement, and installation are key. In addition, products such as duct silencers, duct liners,
vibration isolation mounts and pads, and acoustical lagging materials can be used to reduce the noise in a system. Duct silencers and liners are sound
absorbers that are placed within a duct to absorb some of the sound energy as the noise transmits down the duct. Vibration isolation mounts and pads
reduce the transmission of vibration from an HVAC unit to its surrounding structure, reducing the sound energy transferred through vibration.
Acoustical lagging, which includes a heavy, mass-loaded vinyl, increases the STC value of the walls of the duct to significantly reduce break-in and
break-out noise.

Design Tips

Designing HVAC systems with proper acoustics can seem like a daunting task, but with appropriate equipment selection, placement, and installation,
it can be achieved. In addition to selecting equipment with lower noise ratings, consider proper isolation mounts, ducts (including duct stiffness),
fan size of diffusers and grills, and the use of variable speed drives over variable vane controls in variable air volume (VAV) systems. It is also
useful to remember that not all spaces will have the same NC curve, which can be used to your advantage. A mechanical room, for example, should not be
placed near a noise-sensitive space with a low NC curve, but it may be appropriate in spaces where the NC curve is higher. When required, make sure to
place vibration isolation equipment and silencers as close to the noise source as possible, and avoid 90-degree elbows or sharp turns when designing
the duct layout, as this can increase noise. In addition, avoid placing any duct turns near fan inlets or outlets, and remember that duct branches
should be at least 3 dimensions away from any noise source. When installing the duct work, be sure to use silencers, liners, and lagging materials in
appropriate locations to reduce the noise traveling down and breaking through the walls of the duct. Make sure that roof top units (RTU) will be
mounted on stiff roof sections and run RTU ductwork along the roof top before it enters the building. In rooms located directly below RTU, consider
using a noise barrier on the back side of traditional ceiling tiles to reduce noise in that space.

Here are 5 steps to follow when determining if a space will be acoustically correct.

1. Determine the NC curve for the space.

   When working on a project, architects and engineers should
   refer to published NC curves. For example, in a private hospital room, the NC curve would be 25; while in a restaurant, NC curve of 45 would be
   acceptable. The appropriate noise levels will change depending on the purpose of the room.

2. Identify each sound path that affects the area being analyzed.

   It is imperative to take all
   potential noise paths into consideration. For example, noise from a HVAC system can follow 3 types of paths to enter an occupied space: airborne noise
   paths, structure-borne noise paths, and airborne/structure-borne interaction paths.

3. Evaluate each path for individual noise contributors.

   What are the noise sources that will enter a
   space through the noise paths identified? For example, if there is a diffuser in the space being evaluated, what are the noise sources that will
   travel through the ductwork and exit at that diffuser? Some of the noise sources will be obvious, such as the noise generated from a RTU that enters
   the ductwork. However, some noise may be contributed along the noise path from sharp transitions and elbows, as well as any noise generated by the
   duct itself, such as the popping and banging sound caused by expansion and contraction of the duct.

4. Total the sound levels from all noise paths and sources to determine the overall sound level in the
   area being analyzed.

   At a specific location, the ambient sound level will be a combination of all noise levels from all sources along all paths to
   that location. There are algorithms available that can help you accurately calculate the effect of each contributing noise source.

5. Compare!

   Does the ambient sound level calculated in step 4 exceed the design level determined in
   step 1? If so, steps need to be taken to minimize the noise levels. Once the acoustical design criteria is achieved, everyone involved in the project
   must work together to make sure the HVAC system works as designed.

HVAC systems make our home and work environments comfortable, but the noise they generate contributes to the satisfaction—or
dissatisfaction—of the occupants. As with any design element, it is most cost effective to design an acoustically correct HVAC system in the
initial planning stage, rather than trying to correct the problem later in the project. While it may be a challenge, with thought and planning, it is
possible to design a HVAC system with minimal noise so the affected spaces are acoustically beneficial.