Reducing the sounds from HVAC systems

Often, people try to carefully curate the sounds in their lives to create an enjoyable day-to-day environment.

The importance of soothing sounds has led to sleep apps to be valued in the millions and YouTube videos of people folding towels or reading softly to attract an audience of millions of viewers. At times, I can get sucked into a wormhole of soap cutting ASMR, or autonomous sensory meridian response, videos that are so oddly satisfying and relaxing that time effortlessly flutters away without any realization.

A well-functioning HVAC system will produce a humming that easily blends into the background and becomes a pleasant white noise like ocean waves. However, many HVAC mechanical noises can create a disturbance when they become too loud or stand out from the other room sounds.

Location

HVAC location is important for noise control. Noisy mechanical equipment needs to be as far away as possible from noise-sensitive locations in your buildings such as classrooms, performance areas, offices, etc.

It is recommended for the mechanical equipment to be in a basement room, below grade, far from the sensitive areas of the building. Buffer spaces that are not sensitive to noise should be located closest to mechanical rooms.

If the unit must be adjacent to noise-sensitive spaces, then the equipment will need to have a noise-blocking enclosure. Consideration must additionally be given to the equipment itself in this situation, and the absolute quietest system needs to be selected, which typically adds to the cost of the project. Walls may need to be thicker and may also require double stud wall partitions or double-width concrete masonry unit (CMU) walls.

Sound can be transmitted from one level’s floor to the deck of the level below. Sound can also be transmitted from sidewall to sidewall. This is an important aspect to consider because the noise from the mechanical room can still propagate if noise transmission relief is not properly considered.

Rumble

The continuous low sounds like distant thunder, a passing train, or a convoy of tanks passing through streets is rumble. It is also a common noise complaint in HVAC systems because it is so hard to focus on anything other than the sound.

The first step to finding the solution is cleaning the fan blades on the air handler. Although this would be the quicker and easier solution, the fans can be large and heavy making it more difficult. When cleaning the fan blades, check every part to ensure it is running efficiently. Balance the spindle, lubricate the bearings and pay close attention to detail to ensure it rotates smoothly and silently.

The second step in stopping the rumble is to isolate both the air handler and compressor mechanically from the building structure. Typically, spring isolators are used here, either mounted to the floor or hung from the ceiling.

Ducts

Larger ductwork, particularly near terminal outlets, reduces air velocity. Ducting is a common source of noise because they tend to funnel the air handler’s noise directly to the space that is being heated or cooled. If you remember connecting two cans with a piece of string in your childhood, this is a similar concept, the sound is meant to travel down the specified path. Ducts can be lined with insulating material to keep the noise inside the duct.

Silencers

There are three types of silencers: dissipative, reactive, and active. When selecting a silencer, there are three main considerations to address:

  • Insertion loss – the difference between sound-pressure levels measured at the same point before and after a silencer is installed
  • Static pressure drop – the pressure drop across the silencer
  • Regenerated sound – sound generated by airflow through the silencer

A silencer that is placed closer than three duct diameters from a duct element, such as an elbow, will produce a significant increase in static pressure drop across the silencer. This will typically increase the fan and silencer’s airflow-generated noise.

The side and center pods of dissipative and reactive silencer are made of perforated sheet metal, and the cavities of dissipative silencers are filled with either fiber glass or mineral wool. The cavities of reactive silencer pods are tuned chambers with no filling materials. Because of the tuning in a reactive silencer, sound absorption is more difficult to achieve.

Active duct silencers produce inverse sound waves, cancelling unwanted sounds. They reduce low-frequency, pure-tone, and broadband sound effectively. An active duct silencer consists of a microprocessor, two microphones placed a distance apart in a duct, and a speaker placed between the microphones and mounted externally but radiating sound into the duct.

Active duct silencers should not be used in situations where airflow velocity is greater than 1,500 fpm, or where there are duct elements or transitions that can generate significant turbulence.

Acoustics

The designs of modern spaces, particularly offices, provides much more open space for collaboration and a lot of industrial designs that include hard materials such as metal and glass. These designs promote productivity, boost workplace culture and attitudes, and look beautiful. However, sound tends to bounce around in areas with these design features.

For improved noise control, incorporate sound-absorbing materials into your workspaces. For example, acoustic panels can be incorporated in a larger room to absorb sound, soft materials such as drapes and pillows is a decorative way to achieve noise reduction, and introducing partitions into a room will block sound from outside areas.

 What works well for one building may not work for another, ultimately the best strategies will come from looking at your building occupant’s specific needs. Start the conversation in your building to address how the occupants in your building would like to improve sound control in the building to provide more day-to-day comfort.

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