Who has solved the HVAC air noise problem?

I am in the redesign stage of a totally dedicated listening room which will inlcude tube amp, pre-amp and cd player. The reconstruction will include a flat ceiling and tons of sound isolation and insulation. The central air system for the house with constant air flow produces way too much background noise and hum through the vents. I want to add a separate dedicated a/c system for just this room (25Lx15W x 9.5H). I am looking for the advice of those who have managed to baffle internally in the ducts sufficiently to eliminate the air flow noise and machine hum, while still having an effective a/c system. I want an invisable a/c system with five overhead vents. Duct size should not be a problem as the walls will be at least 12" thick at one end. I presume that fiberglass or lined/insulated sheet metal with all surfaces covered are the way to go. BUT, I really don't know. What I know is that I want to be able to never hear the environment, only the music.

All thoughts welcome including my obvious need for more therapy.

Bill E.
The first thing you need to know is the cooling BTU requirement for the room. From that number total cfm
can be calculated.CFM will dictate the number and sizes of
ducts. To reduce vent noise you can go with larger ducts and registers and fewer of them. Use insulated flex duct. If you have room to snake the duct around instead of running it straight you can reduce noise further. Don't forget to install returns. You can use ductboard or flexduct for this to reduce noise further. Finally, have canvas vibration joints installed at the blower housing if any sheet metal ducts are used.
How will you heat this room?
A.A.S. in HVAC
B.A. in Management
M.S.Ed. in Counseling
If my suggestions aren't enough, we'll look at therapy
Roger: I am in stiches!!!

You say that you have suggestions about the counseling?

Bill E.
Installing staggered baffles inside of your vents will help. This means extending a padded piece of plywood, or whatever, halfway into the duct then doing the same on the opposite side a foot or so further along. four flanges should do it. Also, flare the air exit so the register is (3 or 4 times) bigger than the main shaft. This slows down the air-stream and associated noise. Consult a pro for the nitty gritty...

I've done very similar baffling in hvac systems in a prior home to reduce air noise. The staggered baffles were metal disks with the centers cut out which slowed down the air in line ahead of the vents by a few feet. It reduced the noise enough that I could tolerate being in the room, but I wasn't looking for zero noise and can't remember how quiet it was.

Your solution sounds more effective. As to the flaring, it makes sense to me. I can't imagine that the loss of back pressure to the vents would have any effect since I'm only trying to balance one rooms vents.

Is there an excellent solution for dust management?

Best wishes,

Bill E.
Why not just turn the HVAC system off when you are doing critical listening?
The biggest reason for a noisey HVAC system is generally a poor return system. Many will try to use 1 return air duct which is centrally located in the house. The better alternative is to put 2 in and try to keep them in hallways if possible. This is the way I did my house and I cannot tell when it is running. Another problem may be an air handler too big for the system.
If you use a separate system you will have to put a return air duct in the room which is not desireable. A better alternative is to increase the size of the existing system (heat pump I pressume) and install another return air duct. An experienced HVAC contractor should be able to remedy this noise problem for you. Also DO NOT use duct board unless you like breathing in fiberglass particles. You probably cannot find it anyway. I haven't seen it used for years and it is probably against code also. Metal trunk lines with flex duct is the way. Trunk lines should be wrapped with insulation. HVAC should not be noisey. Good luck
Several things:
1. not all blowers are created equal; shop around.
2. keep the blower as far from the room as possible.
3. make the ducts as big as possible; the drop in noise from doubling the size of the duct will astonish you and is probably more effective than anything else you can do. I cannot emphasize this enough.
4. Mechanically decouple the ducting from the pump/blower unit with a flexible coupling to eliminate vibration transmission.

There is tons of good info in F. Alton Everest's Master Handbook of Acoustics, around $40 in paperback. It is well worth getting a copy before you really start planning.
I would like to call attention to the central unit itself, the working part of the system. This is often located on the same floor level, and even worse if situated near the listening room.

Since this houses the electronics, blower motor and fan assembly, it is a major source of mechanical noise and vibration. Installing sound absorbing fiberglass within the walls of this utility closet will go a long way toward killing these noises.

I went one step farther. Had a custom (extra thick) solid core exterior door and frame with threshold built for my utility closet. It was then weather-stripped with the rubber gasket system for severe weather. Made an unbelievable difference by closing up the last cracks and vents that leaked noise into the space.

The previous suggestions coupled with mine can reduce the various sources of noise to almost inaudible levels.
For return ducting, internally lined sheet metal ductwork would be my first choice for noise reduction, followed by ductboard. By the way, ductboard IS code approved for residential and commercial HVAC systems. I'm not crazy about the fiber from ductboard either. Internal insulation lining is MUCH better. Wrapping the outside of metal ductwork with insulation is done primarily to prevent condensation and avoid energy losses through the duct walls. In my experience wrapping the outside of metal ductwork with insulation does little to reduce or dampen air noise inside the ducts-which is what Bill wants to accomplish.
Make sure any metal ductwork is heavy gauge. This will help prevent ductwork noise often experienced when the blower first comes on and pressurizes the duct system.
As to filtration, an Aprilaire Space Gaurd media type filter installed by the blower does a great job and adds to noise reduction due to its thickness and composition. Some electronic air filters will marginally outperform the Space Gaurd but require maintainance and make some noise.
Hope this helps.
The ducts are not the problem I found when building my room ( see http://www.angelfire.com/mn2/mzn50/ and click on the CAD) you will see that I have installed insullated ends, like mufflers, they do a great job and my room is dead silent.

Good luck
I have returned to re-read this thread after so much time has elapsed. The comments have been right on point. Thank you. It is the return system which has caused the problem. I have a dedicated 2.5 ton hvac system with hydroair heat directly below the green marble shelf. The returns which draw air across the amps produce 50db at 25hz.

The initial attempt to baffle them in the return boxes, each of which has it's own 12" round duct to the blower unit reduced the volume but did not eliminate it.

I have been advised to have the ducts interrupted by making a horizontal turn, then a downward turn, then a horizontal turn, then an upward turn and tie on back to the system. Each leg to be treated similarly and each duct lined with hospital grade accoustical liner and wrapped with fiberglass insulation.

Your thoughts please?

Bill E
Problem Solved!...
build an earthship. We have NO need for heating or cooling year round - not even a crackling fire ;-)

then again solar electric power and tube amps can be a problem.

Gardengirl must have taken lessons from my wife. I don't understand a thing she says either.

Bill E.
The proposed fix by the acoustical consultant is re-routing the duct to make several turns and line the duct with acoustical liner. The contractor prefers to install baffle boxes since shop time would be higher and field time would be almost none.

Results to follow.
I would advise full use of the on/off switch for the AC. You are spending mucho dinero and still may not fix the problem. Spend the money on concert tickets or a great new pair of headphones for the times when it is just too hot or cold, or turn it off just prior to listenting sessions. You paid good money on your gear to hear "gnat belches at 50 ft". I understand. Unfortunately a 35-40db HVAC system is not tolerable.

As a recording engineer just when you think the venue is quiet you put your headphones on and hear some motor or HVAC system noise close by. It never fails. I remember JA from Stereophile recalling just an instance recording Cantus.
I will never compromise or surrender. It's much more fun to have intricate interiors to metal boxes. We feel that the baffle boxes will solve the problem and eliminate the hum. It's really quiet, but annoying.

If I could deal with the heat and no air it would be different. The room is tight, and shortly will be sealed.
Therefore, it's necessary to have the a/c on to bring in intake air.
Bill- weren't you planning on running ac in the winter in this room? As a form of heat to offset the BTU's of heat from your amps, or had the plan changed? In any case I am quite curious how this will turn out.
Yes, I am.

The room hvac is designed to run a/c year round. and it needs to. Last week when the heat was on in other parts of the house and the system was off, the room was 80 degees. The heating system is below the room. Although there is insulation and three layers of buildup in the boiler room, it still heats the listening room.
I studed a wall across the rear of my room, insulated and dry-wall with a door. Installed my heating system at one end of this long area. I did not take this wall all the way to the ceiling...I left it 19" short.

I then built a giant muffler 19" high X 41" deep X 127" long. I installed 4 return openings into the muffler to reduce any intake noise...results: dead quiet return and you never hear the furnace kick on or off. AC unit is of course out-side. Five ceiling registers of good size are also quiet.

Of course this would not work as well in small rooms because floor space is limited...my room is still 23 ft. deep after giving up 4 ft. so this is just an idea that works for a large dedicated room with it's own heating/cooling system.

Another suggestion is to put your system into a cabinet with an independant cooling/air flow system. Your room will always need air conditioning so I am not suggesting you do without, but if you were to build a cabinet for your system that has a vent coming if from below and then a vent at the top you can use a remote(attic) fan to move the air. This will avoid adding excess heat to the room during the summer and you can not use it during the winter when you want the extra heat.

All of the above suggestions using baffles,etc. can work but today's 'high efficiency' systems might not be able to force enough cooling air around all of your modifications to keep the air flowing at comfortable levels. One solution to this is to get a central blower that is well over-rated for the square footage of your house. Be very careful when restricting air flow: you might wind up outsmarting yourself and creating the a very quiet room....from hell.