Noise Control in Air Handling
Units & Ventilation Systems
While the design usually caters for the least amount of noise from the AHU and ducting systems, changes in the operating profile as well as improper maintenance cause the noise levels to increase beyond design levels. There is, thus, a need for the O&M teams to firstly understand that noise in the workspace from the AHU/Ventilation system is a problem and then plan mitigation strategies to minimize the noise so as to have a healthy and productive workspace…
- Aneesh Kadyan
Modern offices, with no or minimal partitions on the floor plate allow for greater amounts of light to perpetrate the workstations as well as gives the workforce a sense of “openness”. Added benefits include lower cost to build and increased engagement of the people on the work floor. One drawback of the open plan office design that is seen in mostly all new spaces is the increased noise levels on the work floor. This noise can originate from the chatter of the teams working on the floor, or an employee speaking loudly on his or her mobile. An often ignored source of noise is that generated from moving machinery in an office environment, most prominently the air handling unit and also from the air movement in the ventilation system. The background noise increases due to the AHU noise or the ducting system generated nosie and has a negative impact on the occupants in terms of their productivity and long term health.
Most people associate Indoor Environment Quality (IEQ) with thermal comforts, air quality and to an extent the ambient light levels. A key component of the quality of the work environment is the noise levels that the occupants are exposed to on a regular basis. The effect of high work place noise levels is not as visible as for example, low light or foul air and hence are not addressed with the impotence that is needed.
Impact of AHU Noise
Many organizations are now using video conferencing (VC) for meeting across cities or offices. This saves the organization cost of travel of the employees as well as time while allowing for face to face interaction which is needed in such meetings. During a VC session at a large IT services firm with an important client, the attendees were hearing a “humming” sound which was impacting the quality of the interaction as the members could not hear the discussions clearly. After multiple checks by the audio Visual maintenance team, the VC system was found to be working properly. Further, investigations pinned the noise to be coming from the AHU room that was adjacent to the VC room. Low frequency noise due to vibrations of the AHU were generating the noise which was transmitted through the structure and was getting picked up by the sensitive VC system microphones. The impact – an important client meeting to close out a large value contract was postponed and there was also reputational damage to the firm in front of its customers.
In another incident, a throbbing sound was heard in the washroom for a few days. The maintenance team attributed it to the AHU placed adjacent to the washroom but did not carry out immediate repairs as temperature in the workspace was being maintained and hence did not see the ‘noise’ as an immediate problem. Within two days, part of the false ceiling of a section of the work space near the washroom fell and when the incident was investigated, it was seen that the supporting structure of the ceiling tiles had got dislodged due to the vibrations of the AHU that was adjacent to the washroom. While poor quality of installation the false ceiling was a major cause of the incident, a contributing factor was the vibrations of the AHU, the telltale sign of which was the noise that was heard in the washroom but ignored by the maintenance team.
Sources of Noise in AHUs & Ventilation System
Different workspaces have different noise level requirements – libraries, conference and meeting rooms will have a much lower ambient noise rating whereas offices will have a more tolerant noise criterion. The noise level is typically measured using the decibel scale (dB). However, for ventilation systems, the following are the noise measurement approaches typically followed in the industry:
• Noise Criteria (NC) which is the most popular way to assess noise in workspaces and is expressed by a number e.g. NC 40.
• Room Criteria (RC) which is an enhancement of the NC method and assess the impact on speech of the background noise.
• Quality Assessment Index (QAI) is a guide to support the RC approach and takes into consideration the user’s reaction to changes in the background noise. A QAI rating of less than 5dB is acceptable and within 5 – 10 dB is in the marginal band of acceptability.
Noise from ventilation systems and AHU’s can be classified based on their frequency which in turn is a function of the component of the system that is generating the Nosie. The table shows the relationship between the type of noise and the equipment’s generating the noise1.
Minimizing AHU & Ventilations System Noise during Operations
To be able to reduce the noise due to the AHU and ventilation system components it is important to understand the mode of transmission of noise in the workspace. Typically, noise is generated by the fans and fan coil units, grilles/diffusers and Variable Air Volume (VAV) components of a HVAC system. A target ambient noise level using the NC/RC criteria is established based on the type of work space and is factored in the design of the of the ventilation system and selection of the components. A well balanced system will easily achieve the design noise levels as the quality of system components and ducting are of a very advanced and high level these days. While the system works as per design and desired noise levels are achieved in the early stages of the system operations, the common observation is that the system deviates after a short time (usually 1 – 2 years), mainly due to the changes in the internal layout of the work space (cabins converted to work desks, more staff than planned for etc.). Poor maintenance is also a contributing factor for the high noise levels once the building has become operational. Some of the common causes of increased noise levels and their remedy are listed below:
AHU noise: This is the most common reasons for increase in the ambient noise in offices. The characteristics of AHU noise is a throbbing noise or an in some cases a humming sound. Physical vibrations that get transmitted through the structure also are a common problem in AHU operations and lead to both noise and structural failures. The key contributors to noise from AHUs are:
• Improper vibration isolation dampers which could be due to a design fault or poor maintenance of the isolators that are installed.
• Imbalance in the AHU fan leading to cyclic noise from the fan itself. In addition, due to the varying speed, the air flow patterns are disturbed leading to noise from the exit points such as diffusers and grills
• Faulty drive system may not directly generate noise, but loose or broken belts may lead to power air flows which will result in noise at the egress points. In addition, vibrations levels of the unit itself will increase leading to higher transmitted noise.
• Monthly check of the vibration levels of the AHU will help identity the condition of the assembly and proactively carry out repairs to the motor/fan etc.
Ventilation System noise: The flow of air across orifices and openings generates noise inherently. Thus, the aim of the design is to minimize the noise that is generated. Poor design such as abrupt change in direction, sudden change in duct diameters to accommodate aesthetics, etc. lead to increased noise. In addition, leakages through duct joints also contribute to the noise as this is uncontrolled and often not detected as the ducting is over the false ceiling and not seen. Imbalance in the system during operations is another major contributor to the noise in the ducting system. The air balancing issues can come up due to change in configuration of the office, blockages of the exits by the employees or non-functional dampers. The approach to reduce ventilation system noise is:
• Carry out six monthly checks for leakages and disruptions in the ventilation system. This will highlight the disruptions in the air flow and enable proactive repairs to get the system back to design point.
• Annual measurement of CFM at designated points where the arrangements exist. This will allow the O&M team to assess the health of the AHU and system and undertake repairs where needed.
• Quarterly or at least six monthly IEQ checks which cover noise measurements.
• Whenever any office layout change is planned, a detailed study of the ventilation system and the flow patterns should be carried out to ensure that the air flow patterns are not changed drastically to keep the noise within acceptable limits.
Background noise usually remains that – background noise and is not given that much importance in modern work spaces, unless it is a specific requirement as in a library or auditorium. However, the detrimental impact of high ambient noise in other work space are well documented and have an impact on productivity and employee wellbeing. While the design usually caters for the least amount of noise from the AHU and ducting systems, changes in the operating profile as well as improper maintenance cause the noise levels to increase beyond design levels. There is, thus, a need for the O&M teams to firstly understand that noise in the workspace from the AHU/Ventilation system is a problem and then plan mitigation strategies to minimize the noise so as to have a healthy and productive workspace.
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