HVAC Noise Reduction Definitive Guide 2022

HVAC systems play an important role in our homes. They help control impurities, regulate temperatures, stop condensation, etc. unfortunately, vents are noisy and can be truly annoying.

Several early studies pinpoint Noise as a contributing factor to poor indoor environments and sick building syndrome. (Source) Additionally, Noise has been shown to cause serious productivity decline and mental health and physiological problems.  

Imagine a situation where you have a noisy HVAC  in your home? Balancing between livable room conditions and fighting through the Noise can be quite a challenge. In this guide, I shall be taking you through some of the best ways to soundproof a noisy air vent in the bedroom as well as other rooms in your house.

Reasons to Soundproof a Noisy HVAC?

Soundproofing an HVAC system is important for homeowners, landlords, and workplaces for several reasons highlighted below.

• Soundproofing HVAC helps you stay within the Government Regulations enforced by the Occupation Safety and Health Administration (OSHA), Environment Protection Agency (EPA), Federal Housing Administration (FHA), US Department of Housing and Urban Development (HUD), and other state and local ordinances.
• Prolonged exposure to loud noise from the ventilation system reduces productivity, causes stress, and could result in accidents.
• Unwanted Noise is annoyance resulting to too much echo, reverbs which gable speech intelligibility and clarity.
• Pressures from insurance carriers often get receive claims filed under the workman’s compensation laws.

Types of HVAC Sounds

Sound and vibrations are generated by the source and transmitted through different paths until they reach the receiver. It’s important to understand the part which is the source for effective soundproofing.

Each type of Noise dictates something wrong with the different parts of the HVAC system. Below are the different kinds of Noise you’re likely to encounter that will help you pinpoint the exact culprit.

• Break-in Noise
• Break-out Noise
• Duct borne Noise
• Terminal End Noise
• Radiated Equipment Noise

Duct Break-In

Duct break-in noise originates from equipment noise transmitted to the ductwork and travels down the duct system. Flexible ducts are highly prone to this kind of Noise compared to non-flexible Noise because they’re lightweight.

You can minimize break-in noise by:

Avoid flexible ducts. You can install or replace the existing lightweight flexible ducting with heavier ducts made of sheet steel.

Alternatively, you can enclose all the flexible ducts in a solid enclosure made from timber, steel sheet plasterboard material, etc.

You can also soundproof the flexible ducts with sound-insulating material such as Rockwool or glasswool. The material should be at least 1 inch thick.

However, before enclosing the duct, it’s important to note that Noise from the ceiling cavity is likely to penetrate/leak through the ceiling. This is likely to happen when lightweight lay-in tiles are used. Plasterboard ceilings give better sound performance compared to lightweight ceiling tiles.

Duct- Break Out Noise

Duct breakout noise comes from the duct wall affecting the adjacent space. Breakout noise can occur in the following scenarios:

• Fan noise transmitting through the duct
• Regenerated noise, commonly known as aerodynamic Noise, comes from obstructions in the duct.
• Turbulent airflow results in duct walls rumbling and vibrating, causing low-frequency airborne Noise.

There are several ways to deal with breakout noise, they include:

• Ensure that the air velocity in the main duct doesn’t exceed 1500fpm, while the velocity in the branch ducts shouldn’t exceed 600fpm.
• The duct fittings should be smooth to allow for smooth velocity changes.
• The ducts should be made stiffer. You can make the ducts heavier by wrapping damping materials around them. External bracing of the HVAC ducts will also increase stiffness.
• Add damping compounds such as spray-on or self-adhesive compounds around the ducts.
• Use acoustic silences to eliminate fan noise if you have an acoustical tile ceiling.

Radiated Equipment Noise

This type of Noise occurs due to vibrations of the mechanical equipment such as the air compressor, pump, fan, air handler, and motor. It is transmitted through the floor or walls into the adjacent space.

And because the natural frequencies of the walls, floors, beams, and columns range from 10-60Hz, the components can be significantly excited into greater vibrations when they’re located near equipment in the same frequency range.

Therefore, the Noise can be transferred directly or through the duct to the building structure. Alternatively, it can be radiated from the HVAC duct.

Excessive vibrations can be attributed to:

• Fan speed too high
• Faulty or unbalanced fan wheel
• Improper or lack of lubrication
• Incorrection direction of fan rotation
• Worn-out belts, belts too loose or oily

To deal with this kind of noise, it’s highly recommended you isolate the fan and the duct from your building structure. Simply use a flexible coupling between the duct and the fan to isolate.

You can use vibration isolation pads, similar to the isolation pads for the air compressor machine below the fan equipment.

You can also use sound-absorbing blankets or soundproof blankets to treat the mechanical room roof and walls.

Regular maintenance of the motor and fan drive is highly recommended to prevent vibrations in the future.

Duct Borne Noise

Duct-borne Noise is mainly attributed to airflow and directly depends on air velocity. Also known as regenerated Noise, duct-borne Noise travels through the ductwork and exits through the grille or diffuser.

The noise increases as the flow velocity is increased and is affected by:

• Objects such as grilles, dampers, registers, etc.
• Jet noise, discharge, or inlet noise via the orifices
• Constrictions in duct area, silence splitters, etc.
• Fluctuations of airflow around duct branches or bends.

To prevent duct borne Noise, you should do the following:

Use quiet fans and provide a good fan outlet condition as ASHRAE requires. You should also route the duct in compliance with the SMACNA guidelines. You should not turn the air in the wrong direction.

Terminal Noise

Terminal Noise is the noise that comes through the terminal air devices such as the grilles, registers, diffusers, and vent cover that go in the room.

Choosing a terminal device with a noise criteria rating NC-30 or lower is important.

HVAC Noise Reduction Strategies

In all the HVAC systems, the source of Noise is a combination of the different processes, including mechanical Noise, pumps, fans, compressors, control dampers, air outlets such as dampers, grillers, etc.

The kind of HVAC noise that finally reaches your indoor space is comprised of:

Low-frequency fan noise. The culprit here is the centrifugal fans that generate the highest amount of Noise in the low frequency.

Medium-frequency airflow or turbulence- noise. Terminal units such as grilles variable air-volume boxes generate the highest noise levels in the medium-frequency range- 250, 500, and 1000Hz.

High-frequency diffuser and damper noises. Grillers, dampers, and diffusers generate the highest noise levels- 1000Hz and above.

Sound travels via air and structures, and so does Noise produced by an HVAC system, and therefore, it’s important we deal with each.

Location of the HVAC Equipment

Equipment such as air-cooled condensers, chillers, exhaust fans, and cooling towers should be placed in a room with thick walls and far from people.

You can create a fence between the equipment room and your house to significantly reduce the amount of noise leaking into your home.

The room should also be soundproofed- start by soundproofing the walls, doors, and windows using materials such as mass-loaded vinyl or soundproof blankets.

Eliminate Vibrations

Motor-driven HVAC systems produce many vibrations that can travel through a building structure through the walls, floors, and ceiling.

You must deal with the vibrations.

You can use anti-vibration mountings between the HVAC machine and the supporting structure, in this case, the floor. A good example of materials used as vibration isolators is cork, rubber, glass fiber pads, and steel springs.

You should also consider installing a floating floor, especially if the room is on a different floor from your house.

It’s also advisable to use flexible connectors on fans at every duct connection. However, the connectors should be long enough to provide flexibility and fold when the fan is turned off.

Use Quieter Fans

The type of fan you have will determine the kind of Noise you experience. For example, Axial-flow fans generate high-frequency noise while centrifugal generate high low-frequency noise.

If you have a noisy HVAC fan, replace it with one that operates at its peak efficiency without producing too much noise.

Please note that a smaller fan on a larger shaft will produce more Noise, while a bigger fan installed on an undersized shaft will generate low-frequency Noise.

However, when choosing a fan, pick one that generates lower sound at lower frequencies as sounds in the higher frequencies are easier to manage than low-frequency noise.

The fan speed also determines the amount of Noise your HVAC system generates. Reducing the fan speed will go a long way in reducing noise transmission.

Fan Speed Reduction	Noise Reduction
10%	2 dB
20%	5 dB
30%	8 dB
40%	11 dB
50%	15 dB

 

Reducing HVAC Ductwork Noise

As earlier mentioned, air passing through the ducts causes the duct walls to vibrate, resulting in rumbling noises.

Additionally, the noise passing through the duct can be converted to airborne noise and then transmitted to the atmosphere.

To prevent airborne noise, it’s important to understand that:

Small ducts are often noisier and more vulnerable to noise transmission than large ducts. This is because of the higher pressures and airspeeds. Oversizing the HVAC duct is often recommended.

If possible, avoid sudden changes of flow direction and cross-sectional changes in the ducting system. In addition, stiffen the duct surface and apply soundproofing materials to the vibrating duct to prevent sound transmission to the building structure.

If you have centrifugal fans, you will be facing low-frequency noise, which is difficult to contain. The best way to soundproof against low-frequency sounds would be by adding mass. Simply increase the duct gauge and wrap it with sound blocking materials such as mass-loaded vinyl to increase mass and enhance sound attenuation.

The best way to reduce fan breakout noise is to use a circular duct. This is because circular ducts have high hoop strength.

However, because I know that most ceiling plenums cant accommodate circular ducts, a good approach would be to size the duct to an almost circular shape.

You should also avoid sharp bends on ductwork and if bends are unavoidable, ensure that they are gradual. Gradual bends with turning vanes reduce noise compared to sharp bends.

For ducts more than 54” wide, you should spilt them. This will divide the air stream and the sound carried by each split.

Below is a table that shows how duct splits attenuate sounds.

 

% Of total air flow	60	50	40	30	20	15	10	5
Attenuation	1	3	4	5	7	8	10	31

 

Below is also another diagram illustrating recommended engineering practices for HVAC ductwork noise reduction

HVAC Duct Soundproofing Against Airborne & Impact Noises

To dampen air-borne noise traveling through the ductwork, you should line the duct with sound-absorbing materials such as fiberglass, elastomeric foam, and polyamide. Sound absorption highly depends on the liner thickness and the exposed surface area.

Sound absorption is measured in reference to Noise Reduction Coefficient (NRC).

1-inch fiberglass has an NRC of 0.70 and absorbs about 70% of the noise problem. On the other hand, 2-inch-thick fiberglass gives an NRC of 0.90 and absorbs 90% of the noise.

The location of the duct liner plays a vital role in the amount of noise absorbed. A duct liner should always be installed at the start of a duct system to dampen fan noise and near the outlets to dampen noise from fittings and dampers.

The amount of soundproofing highly depends on the use of space. For example, an apartment, classroom, or office requires moderate HVAC-noise soundproofing. Such projects only require 10-20 ft duct lining immediately from the fan. Alternatively, you can use a duct liner 10ft from the HVAC fan discharge.

Thin liners are perfect for high-frequency noises, while thick liners are effective in dampening low-frequency sounds.

Soundproofing Against External HVAC Duct Noise

Besides airborne noise transmitted inside the duct, you should understand that impact noise in vibrations can be transmitted through the walls into the surrounding rooms.

You can soundproof against impact noise by covering the duct with glasswool or Rockwool. You can get better sound attenuation by adding a layer of mass-loaded vinyl or sound-blocking blankets over the insulation.

Devices for Noisy HVAC Systems (Devices to Soundproof HVAC Noise

To reduce noise from HVAC systems, manufacturers have come up with different devices that help significantly cut down the amount of noise generated by an HVAC unit. They include:

Acoustic Louvres

Acoustic louvers are commercially available devices that only allow air to pass through them while at the same time restricting the passage of noise.

They act as standard louvers, but they are unique in that they have hollow acoustic vanes rather than flat sheet vanes. Some use fiberglass.

Vanes made of fiberglass rather than metal sheet significantly contribute to sound absorption and vibrations at the HVAC piping angles.

The acoustic performance of these sound absorbing louvers is measured by transmission loss at each octave band. This allows for a direct comparison between the louver and the wall it replaces.

Additionally, acoustic louvers are commonly installed in facades of houses where they’re structurally acceptable; they offer essential sound dampening.

Plenums for Sound Reduction

Plenums are great for sound reduction in outlet noise. Acoustical plenums refer to either a large volume expansion in the HVAC ductwork or simply an air handling unit enclosure. They cause a drop in static pressure and effectively dampen low-frequency noise.

Frequently Asked Questions (FAQs)

Why Is HVAC So Loud?

Most HVAC systems use an Air Conditioner blower motor and a fan which can become noisy as it ages. Modern HVAC units have quieter brushless motors that run almost silently. However, it’s important to note that any all-HVAC units produce some noise due to the fan and motor unit.

Which Device Is Used for Minimizing the Noise in Ducts Of HVAC?

There are two popular devices used for minimizing the noise in ducts of HVAC. They are acoustic louvers and plenums for sound reduction.

Some sound attenuating materials used for noisy HVAC units include fiberglass, soundproof blankets, glasswool, mass-loaded vinyl, etc.