A strategic approach to mitigating noise issues in HVAC systems

By: Matt Downey

 

MITIGATION OPTIONS

Some of the most common noise abatement solutions include:

  • an operational change, such as running equipment at a lower capacity (often by installing a variable frequency drive to operate equipment on demand rather than at a constant output) to reduce noise emissions
  • relocating equipment to avoid impacting sensitive receptors, such as neighbouring structures
  • replacing or decommissioning a piece of equipment and replacing it with a quieter unit
  • installing OEM supplied solutions such as insulated casings, compressor blankets or low-noise fan blades to reduce operating sound levels on noisy componentry
  • installing customized noise attenuation components around the noise source, such as silencers, noise barriers or acoustic louvres

When noise from mechanical equipment becomes a problem for facility owners or managers, they often turn to mechanical service partners for assistance. Whether it’s a noisy chiller on the roof or a recent equipment change that exceeds regulatory noise ordinance levels, mechanical service firms are often the first line of defence.

As a trusted partner, you’ll go above and beyond to support your clients when they face a mechanical challenge. But when it comes to noise abatement, you may lack the specialized acoustic expertise to handle the task. Cue the inevitable project uncertainty, stress and frustration.

Mechanical noise issues are typically complex, challenging to mitigate and nearly always involve significant bottom-line risk. Rudimentary solutions often focus on the physical components that can be used to address the issue such as sound barriers or silencers, but they frequently overlook the potential consequences. Installing and integrating those solutions as part of a complete building system can give rise to a series of unintended, long-term issues.

At times, simply informing the client an acoustic problem will need to be immediately addressed is difficult enough, let alone making it understood that inaction could result in far more serious consequences: fines, a legal challenge or even a facility shutdown imposed by a regulatory agency in extreme cases. That is why it’s essential to tackle a noise problem strategically by developing a comprehensive plan and executing it.

So, your client has a noise problem. Now what?

Gather as much information about the problem as possible by asking basic questions: Is the noisy equipment operating at full capacity? Has it been serviced lately? Are there any external factors that could be impacting its performance, such as impeded airflow or malfunctioning parts? In some cases, a simple mechanical fix will address the issue.

Once those potential problem sources have been ruled out, be prepared to engage a reputable acoustical consultant. The consultant will conduct an analysis using in-field sound monitoring and/or computer-aided acoustical modelling, and will use that data to assess mechanical noise levels against regulatory ordinances to determine the extent of the problem.

The consultant will produce an acoustic assessment report identifying the source of the noise and its potential impact on surrounding receptors, including reduction requirements for the noise source based on eight-octave band frequency strata. This information is critical in assuring the mitigation strategy selected is designed to quantitatively fix the problem.

At this point your client may ask a very reasonable question: How much will this problem cost to fix? The answer depends on several factors, two of the most important being the structure of the existing facility and equipment specifications.

If noise control solutions such as rooftop sound barriers or silencers are required, the building may need to be reinforced to support the added weight. Gaining access to the facility’s original structural drawings will be crucial to understanding its structural constraints and to better understand the full range of mitigation options. In the absence of drawings, it may be necessary to hire an engineering firm to evaluate the building’s structural integrity and capacity.

It’s important to determine the operating capacity and design conditions for the equipment. Reference technical data sheets to understand the component’s key performance parameters. Everything from system air flow and pressure drop allowances, to clearance requirements and maintenance accessibility considerations is pertinent. With this information in hand, you can better advise the facility owner/manager on next steps.

 

Why do site constraints matter?

Understanding the source of a noise challenge and developing a mitigation strategy are necessary first steps, but it’s the process of navigating often complex logistical site constraints that can pose the greatest threat to a project’s success. It’s important to note almost all noise mitigation solutions require some form of on-site construction during the installation process. Whatever the specifics, site constraints will always impact a project’s budget and completion of timelines and should be a prime consideration in the design of an effective abatement strategy.

This was the case when Parklane was engaged by Toronto Western Hospital to address unwanted noise from cooling towers located on the roof of hospital’s central utility plant (CUP). In this case, the mitigation solution involved the installation of acoustic plenum silencers across each of cooling tower inlets.

Through the project posed challenging acoustic requirements and structural limitations, the location of the cooling towers posed a specific concern with respect to the installation of the final solution. The CUP building was adjacent to the main ambulatory care entrance, drastically limiting timelines for onsite construction operations.

The design methodology for the abatement system was therefore deployed based on two fundamental principles: design accuracy to avoid interference and time-consuming field fitment, and prefabrication (modulization) to minimize field time.

Advanced 3D scan to point cloud modelling was used to generate an accurate base model of existing site conditions. The silencers were built using this model to ensure an accurate fit. Silencers and plenums were constructed in large, pre-fabricated modular section and mechanically connected onsite. The approach reduced the installation to a five-day construction period, down from a projected six weeks using conventional design and construction practices.

What is an acoustic assessment?

An acoustic assessment measures sound emission from facilities or mechanical components, and determines whether the acoustic output falls within acceptable limits under local regulatory noise guidelines. Acoustical consultants will measure noise emanating from each piece of equipment, as well as background levels surrounding the facility. From there, the consultant will build a model that predicts the acoustic impact on sensitive receptors neighbouring the facility and create a final assessment summarizing three key details: an inventory of noise sources on-site, identification of sources deemed out of compliance, and the amount of sound reduction required at each problem source to bring the facility into compliance.

The takeaway: Facility owners/managers need to understand retrofit noise abatement can be a challenging and methodical process. Solution design should be heavily influenced by installation constraints.

Duct and chiller silencers are among the noise attenuation components that can be installed around the noise source.

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