We’ve already broken down the primary reasons for wanting soundproof air vents, but in order to solve the problem, we need to understand its cause, just like with other HVAC noise problems we've solved. The best way to approach noisy air vents is to understand the two types of sound that are being generated.

Airborne Noise through a Duct

Airborne noise: Airborne noise is sound that travels through the air, like music from a speaker, a dog’s bark, or in this case, the noise passing through your the air vents of your central HVAC system. 

The two most common cases of unwanted airborne noise in this scenario are: the rushing of air through the vents and room-to-room noise.

It’s common for offices with an air conditioner making noise that the unit is too big for the space, which leads to having noisy vents. When the air conditioner’s output is too much for the size of the ducts, the air pressure inside the duct increases and the sound of blowing air is louder than it ought to be.

Unwanted room-to-room noise is common in both offices and homes. Anywhere there is shared ductwork, you have the possibility for annoying room-to-room noise. Even if you’ve taken steps to seal gaps seal gaps around the doors and soundproof the wall and/or ceiling, airborne noise can travel through the ducts to “leak” over from the next room over.

Structural Noise Through the Ductwork

Structural noise: Structural noise (or impact noise) is created when two objects contact each other and energy is transmitted through the physical structure as vibrations. In this case, sound is traveling through the metal ductwork.

When your air conditioner kicks on, it may cause vibrations that run through your ductwork and escape out the vents. The vibrational energy travels through the metal air ducts and creates an unpleasant structural noise that tends to sound like a buzzing or rattling noise in the background. If you have structural noise, the only way to stop it is by damping the vibrations with our Damplifier Pro sound deadening mat. You can also decouple the ceiling from noisy air vents using sound isolation clips, our RSIC-1 EXT04 is great for that. You would install the Damplifier Pro on the ducts if you have access, or directly on the vent that is rattling. If you stop the vibrations, you’ll stop the structural noise.

There are a few solutions that will work together to give you the most optimal results. While, these solutions will all work together, each also will independently take a bite out of those noisy air vents. The steps we outline here are in the order we’d recommend, should you choose to do them all.

steps to soundproof ductwork and air vents

These are the steps you need to take to soundproof the ductwork in your office, home, and/or building:

1.  Change the structure of the ductwork
2.  Line the interior with sound absorption
3.  Create a soundproof barrier for exterior
4.  Add baffles for soundproofing inside the air vents

step one: Soundproofing ductwork with new ducting

The best way to eliminate airborne noise in ducts is to add 90-degree turns within the ductwork. Noise traveling along a straight line inside your ducts won’t hit any barriers to block its path. However, if that noise is forced to make a true 90-degree turn, the sound coming out the end of the duct will be reduced by about 6 dB. We recommend adding two or three 90 degree turns for the best soundproofing. After that third turn, diminishing returns have kicked in and you won’t see much reduction.

The space between the 90-degree turns doesn’t matter. All that matters is that you have them. Also, do not use flex ductwork! Flex ductwork is not able to make a true 90 degree turn, and so you will not get the sound reduction. Only rectangular, hard ducts will be able to execute a full 90-degree turn.

step two: Air Vent noise reduction using Sound Absorbing Material

The second step is to line the inside of your ducts with a sound deadening foam. Simply open the air vent and line the interior surface of the duct. The material sticks with a spray adhesive – easy install! Our EcoVerb™ Roll is rated for use in ducts, and does an excellent job of absorbing airborne noise. It’s cost effective, simple to install, and comes in three sizes depending on your size of duct.

The key here is to remember that any amount of EcoVerb Roll you install in your ducts will reduce noise, but the total square footage you install will determine the size of dent in your noise problem. If all you can do is reach inside the vent, then you should do that as many places as you can. For the best results, you should line the entire interior and also oversize your ducts by 30% to allow for the use of thicker version of the EcoVerb roll material.

If your ducting is 10” x 10” or smaller, use the ½” thick EcoVerb Roll. For larger ducts (12” x 12” or larger), use the 1” thick EcoVerb. The thickest EcoVerb (2” thick) is best used in commercial settings like soundproofing a warehouse with commercial ducts measuring 18” x 18” or more.

step three: Create an HVAC sound barrier by lining the exterior

When you line the interior of your vents, you attack airborne noise. When you line the exterior of your vents, you can attack structural noise, too.

We recommend installing both Damplifier Pro™ and mass loaded vinyl — either Luxury Liner Pro™ for a decoupler option or Luxury Liner™ Foil Faced MLV for radiant heat protection — on the exterior of your ducts. Damplifier Pro™ is an excellent deadener, meaning it will silence those structural noises transmitting through the metal of the HVAC duct. Mass loaded vinyl is a sound blocker, meaning it will prevent the noise inside of the duct from escaping through its sheet metal walls.

You don’t need that much coverage with Damplifier Pro (30%) to effectively deaden the structural noise. If you choose to cover 100%, the Damplifier Pro will help block airborne noise as well. If your vibrating HVAC unit is the main culprit of the structural noise, you will need need to decouple it from the surrounding ductwork by leaving a small gap and sealing with acoustical sealant. If the problem is airborne room-to-room noise, completely wrap the duct with your MLV. Using both materials with 100% exterior coverage of the air ducts will give the best possible performance.

step four: Create a duct sound baffle to reduce air vent noise

If steps 1 to 3 are not options or if you have extremely high performance criteria (recording studio or similar), then your last step is to build a baffle. Baffling the interior of your ductwork is a great option if you have access. In many cases with existing ducting… you won’t. In these cases, you will need to build the baffle into the room. We consider the baffle an effective, but last-resort solution for those who have other constraints but have decided, “Alright, I gotta fix this noise issue.”

We recommend installing our BlocknZorbe™ sound absorbing panels in a staggered pattern within your air duct. BlocknZorbe panels are an awesome option for baffles, because they are non-fibrous and one of the few products rated to both block and absorb sound (no need to build something custom). There are several rules to follow when building the baffles:

• Use a baffling material that blocks and absorbs sound. We recommend our 2" thick BlocknZorbe™ for best performance.
• The baffle should cover 75% of the width of the duct. For 12” wide duct, the BlocknZorbe panel should cover top to bottom and be 9” across (75% of 12”).
• The distance between the baffles should be 2-2.5x the thickness of the baffle. For a 2” thick baffle, you should have 4-5” between each BlocknZorbe sheet.
• The more baffles you install, the better the performance. We recommend a minimum of 5 baffles.

There you have it! You now know more about soundproofing HVAC ducts than 99.9% of the people you know. Depending on your situation, you may not be able to do all of the steps we listed above, but any amount of soundproofing you do will help the problem. And then the more closely you can follow the guidelines with appropriate soundproofing materials, the better. If you need to soundproof a drop ceiling and HVAC vent noise is only part of the issue, then we can still help. We also can help build a soundproof HVAC closet as well! If you have questions along the way, please reach out to us at 1-800-679-8511 for assistance at any point in your project. We’ve got you covered!