Quieting Noisy Pipes, Ducts and HVAC Equipment

Mechanical noise is its own category. A furnace, a rooftop unit, plumbing or a neighbor's heat pump produces steady, low-frequency sound that travels two ways at once: through the air inside ducts and openings, and as vibration through the structure the equipment touches. Fixes that work on voices often disappoint here, because they treat only one of those two paths. The questions below cover both, from hush boxes to duct-borne rumble.

First, find the path

Put your hand on the wall or floor when the equipment runs. If you feel it, the noise is structure-borne: the equipment is shaking whatever it sits on or hangs from, and the building carries it to you. If you only hear it, and loudest at grilles and openings, it is traveling inside the duct or through an air path. Most real cases are some of each, which is why a duct fix alone or a wall fix alone often gets half a result. Our article on flanking noise explains why sound seems to come from everywhere at once.

Structure-borne: isolate the source

Vibration wants a rubber break. Equipment on isolation pads, pipes in cushioned hangers rather than strapped hard to framing, and flexible connectors between units and ductwork stop the shaking before the building amplifies it. For pipes already buried in walls, wrapping accessible runs with our sound control pipe and duct wrap and decoupling the drywall in the affected room are the practical moves.

Airborne: mass and lining, not foam

Thin sheet metal is nearly transparent to sound, so bare ducts both leak noise and carry it between rooms. Wrapping ducts with mass loaded vinyl duct wrap adds the weight the metal lacks; damped drywall around duct chases quiets the rumble a chase turns into a speaker. Where ducts serve two rooms that need privacy from each other, the duct run itself is the highway, and it needs lining or rerouting, not more drywall.

Enclosures and hush boxes

Pumps, fans and compressors can be boxed, and several customers below have built successful enclosures. The recipe is mass on the outside (damped double layers), absorption on the inside, no rigid contact between the equipment and the box, and honest ventilation, because equipment that cannot breathe fails. A hush box built as a sealed bare-metal shell disappoints on all counts.

Living next to the noise?

If the source is not yours, like a neighbor's heat pump at the property line, treatment moves to your wall: the same decoupled, damped assembly from our noisy neighbor wall guide, with special attention to windows and vents facing the equipment, since low frequency noise walks through weak points.

Related: Soundproofing a Condo Ceiling and How to Soundproof.

Real customer questions about pipes, ducts and HVAC noise

My neighbor's heat pump runs all day - can my wall be made to stop it?

Customer question: My neighbors new heat pump, two fans exhausting to my dining room wall 9 feet away is driving me crazy. The two worst frequencies are 63 and 125 hz, dificult to deal with and it passed thru concrete and vinyl walls outside and my r13 wall inside. Initially I thought about doing only a portion of my facing wall (10'x14') with 5/8' drywall and green glue but I now understand flanking noise better and really nothing will help except for a complete tear down and rebuild of all the wall in the dining/living room..way too costly. Would just the main wall facing this heat pump with the added 10'X14' help even a little bit or should I just give up and crank up my local stereo and stay out of that room? Thanks.

Trademark Soundproofing answer: It would help somewhat. Best would be if you can build something right next to that pump if your neighbor agrees. You can just put up a frame with MLV on both sides

The boiler's vibration comes up through the floor - how do I stop it?

Customer question: I have a sleeping problem where the heater/boiler in my basement is vibrating through the floor and into my bed (probably even the walls too). I can feel it going through my body when I'm trying to sleep. I am quite noise sensitive, so even though it is quiet, it is extremely hard for me to sleep well. Unfortunately I don't have much control besides making adjustments in my own room and possibly some small ones to the heater/boiler. What could I do to improve my situation, ideally without making huge adjustments like tearing out my hardwood floor? I am considering 1. adding a rug/carpet to help with this and 2. putting some suspension clamps on the pipes along with removing some unnecessary poles in there used for other purposes. If that helps, does the rug/carpet need to cover the entire floor or just under my bed?

Trademark Soundproofing answer: Sorry to hear that. Without major adjustment i would say your best of with premium carpet underlayment and plush carpet on top. While we always recommend doing the entire room in this case just under the area that your bed is enough.

Rooftop HVAC units rumble through our office ceiling - what helps?

Customer question: Our office is on the second floor of a two story building. The HVAC units are on the roof and create a droning sounds in our office when they turn on. Anyway to fix this?

Trademark Soundproofing answer: Vibration pads under the HVAC unit may help. In the meantime you can try the 3M damping tape on the machine and on the ducts.

Is the noise coming through the ducts or through the drywall?

Customer question: Yes ac ducts… But shouldn’t the sound just come through the ducts then and not from the drywall? Is there a machine that can tell you where the sound is directly coming from?

Trademark Soundproofing answer: It is very difficult to pinpoint. Keep in mind that the Ducts are running above the drywall too and can be transmitting sound in those areas as well. Especially if it is rigid metal duct work.

Can I drywall over an air duct with Green Glue to quiet it?

Customer question: I am planning to attach 5/8' drywall directly to an air duct. Would a layer of Green Glue between the metal duct and the drywall be effective in reducing sound from the duct?

Trademark Soundproofing answer: Not really. You can put some metal damping tape on the duct for best results.

Pipes in the wall carry noise and vibration - what actually stops it?

Customer question: Hi, I have problems with sound transmission through walls/pipes in wall, and maybe some resonating in my room. So, I am wondering how would I figure out what kind of walls are between me and my side neighbor? I think the walls are hollow inside (they sound so) but how do I figure out if there are 3 drywall pieces between me and neighbor (bad triple leaf), or just 2 drywall pieces, or cement plus drywall on each side? I cannot tear down existing walls, don't have any approvals. If I hire a contractor to just green glue +mlv another layer of drywall on top of my room wall can that make the situation worse (if I don't know what's between me and neighbor)? Thank you, Karina

Trademark Soundproofing answer: Hi Karina, You can try obtaining the original construction plans from the builder otherwise you would need to make a hole in your wall. Adding more drywall with Green Glue to your existing wall or adding MLV will only make it better.

How do I build a hush box for a noisy fan?

Customer question: Hello, how are you? I would like to make an acoustic/hush box for a centrifugal fan in a VOC sensitive environment and was wondering if Green Glue products would work on the following ... Glue two 1/4 inch Masonite Hardboards together with Green Glue Noiseproofing Compound and cut the sheet into the correct dimensions for the box - length approx 2 foot and Height and Width approx 1 foot. I would then screw the pieces together to make the box and use Green Glue sealant on the joins/edges. 6 inch holes in the front and back of the box would be cut to allow inlet and outlet airflow through acoustic ducting - Green Glue Sealant would be used to seal the acoustic ducting to the masonite edging the hole. Just wondering if the Green Glue products and masonite would be okay for this box. Thanks

Trademark Soundproofing answer: They would be OK. 1/2 inch boards would be better.

Building a sound-deadening box for a pump - what should the walls be?

Customer question: I am building a sound deadening box (18’ x 10” x 12”) to muffle the noise produced by an electrically powered high pressure, three piston water pump. Unmuffled, a (free, iPhone app) acoustic meter reads 70dB, peak frequency of 80 Hz, at distance of 5 feet from operating pump. Usual seating position is 15 feet from pump’s location, so it’s appreciably quieter at that distance, though still rather annoying. The pump is part of my veranda’s mist cooling system; it’s placed on the concrete floor. It requires fan-driven ventilation at the back (inlet) and front (exhaust) of the pump. Openings for this ventilation are 4”x4” square at inlet and 3.5” diameter circular hole at outlet. The box includes openings which replicate these louvered openings in the pump’s casing. Your suggestions/feedback on following overall plan: Six-sided box with following layers, from inside out, on top and 4 sides: 2lb MLV (attached with screws), 3/4” MDF, Green Glue, 3/4” MDF, 2” thick pyramidal sound foam. Bottom of box is single layer of MDF with 2lb MLV screwed to upper face; it does does NOT have the sound foam, Green Glue, or second MDF layer, but does include four 4” x 4”, 1” thick vibration isolation pads on the underside to decouple from the concrete floor. The box is held down onto the bottom piece using multiple loops of lightly stretched shock cord to ensure minimal sound leakage at side/bottom interface. The MLV on the bottom extends ~ 3/4” beyond the outer edge of the box on all sides. All inside joints are caulked and taped; box assembled very tightly with glued and nailed butt joints (accurately cut; I’m a woodworker). Over each ventilation opening, stand-off spacers of ~ 1.25”, then a 5”x5” piece of 3/4”MDF with 2lb MLV screwed to face closer to box. These are intended to allow sufficient airflow while forcing sound to “make a 90 degree turn” before reaching my delicate ears. Several much smaller additional openings for water tubing and electrical cable; no baffles on these. Over the box described above is placed a second, 5-sided, much larger box (24” square) of 1/2” MDF with 2” pyramidal foam glued to all inside surfaces. Joints caulked, glued, nailed as with smaller box. Crushable weather stripping along bottom edge to minimize leakage at box/concrete floor interface. Larger box includes ventilation openings as on smaller box, and aligned with those in smaller box. No sound baffles on larger box’s ventilation openings. This box also conceals the filter and excess water tubing; it is painted to match the veranda walls’ color. (This beautification serves to reduce spouse-generated noise). I anticipate receipt of my (single tube) of Green Glue this afternoon; very soon I’ll be adding that and outside 3/4” MDF layer to inner box. Suggestions? Anything missing, or “unsound” in this plan? Any guesses as to degree of noise reduction I may realize with this approach? Any help much appreciated.

Trademark Soundproofing answer: Sorry for the late reply. This is a wonderful build and should eliminate nearly all the sound. One thing I would switch around would be on the first box. I would put the sound foam on the inside of the box rather than the outside. Wondering how we can incorporate the spouse generated noise into our business :) Let us know your results. Best, TMS

How do I quiet vacuum pumps under a sheet metal panel?

Customer question: Trying to reduce sound emission from vacuum pumps under a table which will have sheet metal panels boxing in the space under the table. We are thinking of using a 2' layer of Owens Corning 705 against the panels and then hang 1/8' MLV in front of the 705 - sound hits the MLV first .....or....should we attach the MLV-PSA directly to the metal [to create a CLD, Dynamat type product??] and have the 705 as the first acoustic layer

Trademark Soundproofing answer: Hi Ed, I would attach the MLV directly to the metal and have the 705 absorb the sound first.

Where does pipe and duct wrap work best?

Customer question: Hi, I notice your product of noise damping pipe and duct wrap. [it is not possible to install isolation in my room but the idea is to put such a layer above the strings and underneath of my grand piano. Can you recommend this product or an equivalent. I need only small quantities, can you propose the smallest quantitiy [about 4square meter max]? Best regards, Erwin

Trademark Soundproofing answer: It is available in squares of 2' x 2' or 2' x 4' here. https://www.tmsoundproofing.com/Ceiling-Tiles-Sound-Barrier.html

Sound leaks through vents and gaps at the eaves - do they matter?

Customer question: Hello. I have a question about the ceiling. All along the eves there are many, many holes I guess for venting. Would it be possible to cover the eves with joint compound or MLV to eliminate the traffic noise with out doing any damage to the house? Thank you

Trademark Soundproofing answer: You would have to check with your insulation contractor.