Noise Sources
Mechanical system noise can be a problem in any occupied building.  Control of the mechanical system noise is most efficiently achieved if they are evaluated during the early design phases.  Consideration of noise and vibration control strategies early in the design phase of a building facility may alleviate potential problems much more cost effectively than retrofitting systems after construction.

Typical mechanical system noise is made up of a variety of noise components.  Figure 1 shows an illustration of how each of these noise sources could be generated.  The sound generated by the fan will travel along the ductwork both upstream and downstream of the fan.  Noise can travel equally well upstream and downstream because the velocity of sound is much greater than the velocity of air in ducts.

1. Ductborne noise propagates along the ductwork, follows all transitions and takeoffs, and ultimately exits at the diffuser or grille, thus, impacting the space being served.

2.  Radiated equipment noise transmits through the wall into the adjacent space or, in the case of roof-top equipment, to the environment.
3.  Break-in noise is radiated equipment noise that enters the ductwork and propagates down the duct system.

4.  Break-out noise also propagates along the ductwork, however, transmits through the wall of the duct, thus impacting the adjacent space.

5.  Ductborne noise enters the room through the supply diffusers.  Air noise from the diffusers and from transitions can cause additional noise in the receiving room.

Crosstalk occurs when noise from a space, e.g. talking, music, radiated noise, etc., enters the ductwork, propagates along the duct work, and ultimately impacts an adjacent space.  A common example occurs in residential homes when on the third floor, you can hear the television on the first floor by listening at the supply diffuser or return grille.  Crosstalk is typically composed of ductborne noise, break-in noise, and break-out noise.

Radiated noise is the free-field noise radiated in any direction from powered equipment such as fans, pumps, chillers, generators, compressors, etc.

Design Criteria
Many rating systems have been developed to help establish acceptable noise exposure levels in occupied buildings.  The rating systems most commonly used to describe mechanical system noise are the Noise Criteria (NC) and Room Criteria (RC) rating systems.  The RC rating system has become the preferred rating system by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE), and is based on the level of the mechanical system background noise in octave band frequencies.  The RC rating system employs two descriptors; a number descriptor which represents the speech interference level of the spectrum, and a letter descriptor which represents the subjective quality of the sound to a typical listener (N=Neutral, R=Rumbly, H=Hissy, T=Tonal, RV=Perceptible Noise-Induced Vibration). The RC rating system also considers low-frequency induced vibrations in the building construction.  The RC curves are widely used to evaluate existing mechanical systems and to establish design goals for new mechanical systems.  The optimal RC rating design goal for mechanical system background noise depends on the intended uses of the occupied building space.  Room Criteria design goals are typically stated in ranges, as shown in Table 1.  For many situations, such as offices and conference rooms, the lower RC limit is as important for speech privacy as the upper RC limit is for speech intelligibility or noise masking.  The RC ranges shown in Table 1 are recommended for the various different spaces.

Table 1    Design Guidelines for Acceptable HVAC Background Noise Levels in Unoccupied Rooms.

Intended Use       RC(N)Rating
Office building
Executive office    25-35
Conference room    25-35
Open-plan offices    30-40
Lobbies    40-45

Church, mosque, synagogue
General assembly    25-35

Schools
Classrooms >750ft2    <40
Classrooms >750ft2    <40
Large lecture halls    <35

Residences, apartments, condominium    25-35

Hospitals and Clinics
Private rooms    25-35
Wards    30-40
Operating rooms    25-35
Corridors/public    30-40

Performing Arts Spaces
Drama theaters    <25
Concert/recital halls    <25
Music studio    <25
Music rooms    <35

Libraries    30-40

Indoor Stadiums, Gymnasiums
Gymnasium        40-50   
Natatorium        40-50
Large seating space  45-55