HVAC Acoustic Treatment Basics: Grille, AHU & Fan


HVAC equipment often generates a great amount of noise during operation. Hence, acoustic insulation or treatment is necessary to keep people from getting disturbed. However, acoustic is not a topic commonly discussed in HVAC. So, I did some research.

Essentially, acoustic is about soundproofing. In HVAC, people often refer to it simply as acoustic treatment. Usually, project contracts have a specific section for acoustic treatment requirements.

Acoustic treatment is sometimes known as sound attenuation.

Now, let's take a closer look at some of the noise sources in HVAC and what's the common acoustic treatment methods.

HVAC Acoustic Treatment

In an HVAC system, unpleasant noises can come from multiple sources including supply air diffusers, return air grilles, air handling units, chillers and exhaust fans.

Grilles and Diffusers

If certain supply air diffusers or return air grilles are noisy, it is often due to imbalanced airflow where some of the diffusers or grilles are having exceptionally high airflow.

When too much airflow is directed to a diffuser/grille, the air velocity and air resistance of the diffuser/grille increase thereby causing it to produce unwanted noise.

Hence, we can either balance out the airflow or install additional grilles to reduce the noise.

Besides, every grille has a free area value.

Free area is the amount of space available for the air to pass through.

Small grilles tend to have a much lesser free area ratio than large grilles. Furthermore, different types of grilles have different free area values.

For example, the free area of external weatherproof grilles is usually less than 50% while many louver grilles used for return air have a free area of 70%.

So, we need to make sure that we don't size grilles based on the wrong free area value thereby undersizing the grille.

See my post Return Air Grille Sizing Guide: CFM Chart & Calculation for a detailed grille sizing guide.

Nonetheless, despite having the correct grille size, some grilles are still noisy because of the high air velocity caused by the smaller duct size.

Ducts are usually sized based on an air velocity below 1500 fpm (7.62 m/s). However, the air velocity required for a quiet grille is often around 500 fpm (2.54 m/s).

So, if the duct is directly connected to the grille, the resulting noise level will be high.

Fire Rated Duct with Plenum and Reducer

To mitigate the problem, a plenum box with sufficient depth (about 300-600 mm depending on the application) must be installed before the grille and acts as a connector between the duct and the grille to reduce the air velocity and hence, the noise.

See my post What is a Plenum Box? (Purpose for FCUs and Diffusers) for more details.

AHU Room

Many times, air handling units (AHUs) are located near occupants. In addition, many people like to design the return air grille as one big piece on the wall of the AHU room to save cost.

Consequently, complaints regarding noise usually come from the AHU room, specifically the return air grille.

Normally, an adequately sized return air grille should not generate loud noise as the airflow is within the acceptable range and the air resistance is relatively low.

However, when a return air grille is placed on the wall of the AHU room, it opens up a path for the noise produced by the AHU to transmit out through the return air grille.

It is even worse when the return air system adopts the free return style where there is no return air duct.

Ideally, return air grilles should be spread out uniformly across the room. It is better for air circulation and noise transmission is greatly suppressed.

Nonetheless, if you already have a big grille on the wall of the AHU room, you can install a return air duct with internal insulation to suppress the noise transmission.

If the return air duct is present, simply install the internal insulation to reduce the noise level.

For air handling units, the internal insulation shall be 2 inches 2 lb Rockwool with the perforated metal sheet at 13 ft (4 m) length. Such internal insulation should be applied in both the supply and return duct.

See my post 4 Types of Duct Insulations (with Specifications) for more details about duct internal insulation.

If that's not enough, room insulation may be required.

Rockwool Insulation Perforated Metal Sheet

Many times, the brick walls of the AHU room or the chiller room are not enough to block the noise transmission. Hence, Rockwool insulation with the perforated metal sheet is installed on the wall inside the room for noise reduction.

Learn more about Rockwool room insulation in my post 5 Types of Insulation Used in Air Conditioning.

Modern office buildings often have strict requirements for workplace background noise levels. The below table gives you an idea of the noise level required based on different types of rooms:

Room TypeAcceptable Noise Level
Home30 – 40 dB
Office40 – 50 dB
Restaurant40 – 50 dB
Kitchen45 – 65 dB
Factory50 – 75 dB

In one of my previous projects, the office space noise level needs to be lower than 45 dB with the AHU room just beside. Hence, we did extensive internal insulation for both the supply and return ducts. Furthermore, the AHUs that we used are internally insulated at the fan discharge section for more noise suppression.

Chiller Plant Room

A typical screw chiller has a noise level of around 80-90 dB. Such a noise level is too loud for almost all types of rooms. Hence, room insulation is often necessary.

However, many chiller rooms are located either in the basement or on a dedicated mechanical floor where they are isolated from noise-sensitive spaces and thus, room insulation may not be required.

Previously, I've encountered a project where the chiller plant room is located directly above the building's grand ballroom where noise and vibration controls are critical. Although room insulation and vibration isolators are in-place, the building's management ends up putting fiberglass insulation on the ceiling in the grand ballroom to reduce the noise to their acceptance.

Such a soundproofing method is not very common in HVAC but it is well-known as the ceiling acoustic treatment.

On the other hand, vibration is another contributor to noise. Uncontrolled vibration may produce screeching or whistling noise that is both disturbing and irritating.

Thus, HVAC equipment such as chilled water pumps, smoke spill fans, chillers and even small air conditioners have some sort of vibration isolators.

In chilled water systems, large chilled water pumps and condenser water pumps often sit on a vibration isolator called the inertia block. Basically, an inertia block is a platform that uses springs as the “leg” to reduce the vibration transmission from the pump to the floor.

Condenser Water Pumps on an Inertia Block

Normally, one pump will require one inertia block which may not be necessary for smaller pumps. Sometimes, multiple pumps can sit on one big inertia block.

Ventilation Fan

Smoke spills fans (smoke extraction fans), pressurization fans and other large ventilation fans are hung on spring isolators that are anchored onto the soffit slab. These spring isolators reduce the vibration transmission from the fan to the soffit slab.

Large ventilation fans often need four supports and thus, 4 spring isolators are needed for each fan.

However, vibration is often not the main source of the noise but the fan itself.

A typical smoke spill fan can deliver an airflow of around 10,000 cfm or more. Hence, it produces a significant amount of noise during operation.

Normally, smoke spill fans and pressurization fans that involved fire protection don't reduce noise transmission by simply installing internal insulation in their ducts as the AHU does.

Instead, they use a silencer that complies with standards like the BS EN12101-3 for fire resistance up to 2 hours at 250°C (482°F) for noise suppression.

Systemair Fan Silencer

Silencers for ventilation fans are factory-made. They are designed to withstand high temperatures.

One of the major contributors to noise is high air resistance. Hence, silencers with a static loss of less than 0.2 in.wg. are preferred.

In the event that the noise level requirement is strict, the cabinet type of fan can be used instead of the conventional axial fan.

Cabinet centrifugal fans have the centrifugal fan, which is the source of the noise housed in a cabinet. Therefore, cabinet fans are much quieter than axial fans.

See the different types of HVAC fans in my post 5 Types of Fans in HVAC (Characteristics & Applications).

Fan Coil Units in Open Space

Not all fan coil units (FCUs) are located above the ceiling or in a dedicated room. Sometimes, FCUs are installed in open spaces where there is no ceiling or wall to help reduce the noise.

Previously, I had a project where two 10-ton ceiling ducted fan coil units were installed in a hall without any ceiling. The two units are free hung up high in the double-volume hall.

During the commissioning stage, the noise level measured inside the hall is about 66 dB which is too loud despite having an internally-insulated plenum box on the return side of each FCU.

FCUs Wrapped with Rockwool Insulation for Noise Reduction

Hence, we try out wrapping the FCUs with 2″ Rockwool insulation and a layer of PE insulation for better aesthetics in an effort to reduce the noise level.

As a result, the noise level measured in the hall after the insulation wrap was about 59 dB which is now acceptable.

By wrapping the FCUs with Rockwool insulation, the noise level drops about 7 dB, which is significant when we experience it on the site.

How to Reduce Noise in HVAC?

Apart from the acoustic treatment I mentioned earlier, there are also other ways to reduce noise in HVAC, particularly from the design standpoint as follows:

  • Design a low fan static pressure ductwork system.
  • Select a fan that can run at peak efficiency.
  • Use smooth reducers at the fan inlet and outlet.
  • Locate the equipment away from noise-sensitive areas.

In addition, let's recall the common acoustic treatment strategies that I mentioned earlier as a summary in the following:

  • Use a properly sized grille.
  • Check the airflow to diffusers and grilles.
  • Use a plenum box to reduce the air velocity.
  • Avoid putting a grille on the wall of the AHU room.
  • Internally insulate the supply and return ducts.
  • Put insulation on the inside wall of the AHU and chiller room.
  • Use vibration isolators for pumps and fans.
  • Use silencers for smoke spill and pressurization fans.
  • Insulate the ceiling if necessary.
  • Wrap the fan coil unit with Rockwool insulation.

Conclusion

Acoustic is an important topic in HVAC. Many times, noisy HVAC systems are realized after completion which is too late and costly for rectifications. Often, people just live with it.

Hence, understanding the basics of HVAC acoustic and knowing the common acoustic treatment methods can help avoid such unfortunate circumstances.

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Yu Chang Zhen

I enjoy working on air conditioners and ventilation fans because Malaysia is a very hot country.

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