# Stop the Air Conditioning Noise!



## SamSpade (Jun 3, 2010)

My new home theater construction is well underway (meaning it’s a huge mess). 

The heating and air conditioning are being installed and I am not happy with the noise of the moving air I hear when the air conditioning comes on. At my listening position, I measure 55 dB of noise when the air is on and it is so quiet with the air off, the meter does not show a reading. (I live on a 5 acre lot in the country.)

The noise is clearly coming from the airflow out of the supply ducts and the inflow into the return. I don’t hear any sound from the mechanical parts of the air conditioner (they are in another room). 

I have worked with the HVAC contractor and we have replaced the flexible duct he used with a flexible duct made for reducing noise. We have also increased the diameter of the ducts and registers. There are no straight runs of duct. Each run is in a serpentine configuration to maximize the absorption of the noise of the air rushing through the duct.

Any suggestions for reducing the sound of the air from the air conditioner would be greatly welcomed.


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## bpape (Sep 14, 2006)

What you need to do is to have a larger area that the duct feeds into before getting to the diffuser and increase the size of the diffuser. What you're looking to do is create a lower velocity system. When you dump the duct into a large box, you decrease the pressure and velocity without changing the amount of air passing through the system. Damping the box with duct board will further damp this.

Bryan


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## tonyvdb (Sep 5, 2007)

Hi, First thing have you removed the outlet vent cover in the room and does this reduce the noise? 
Round ducts with gradual turns can also reduce noise. also turning vanes inside the square ducts at corners really help.


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## SamSpade (Jun 3, 2010)

Hi Bryan,

I just spent $1,400 with your company on acoustical treatment for my home theater. I really enjoyed working with your engineer, Bryan.

I am a retired electrical engineer, so, other than one course in Thermodynamics, I have little knowledge of HVAC. Since I live in South Carolina, my complaint so far has been with the AC. I do know I have a heat pump and the air handler and evaporator are located in another room. Does that change any of your recommendations?

Also, the HVAC guy I am working with has no knowledge of the reduction of air-flow sound and not a great deal of knowledge of AC. If you could put your suggestions in terms a carpenter could understand, I believe I can get him to understand and implement your recommendations.

Thank you so much for your help.

Wayne Willenberg


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## bpape (Sep 14, 2006)

Hi Wayne

What you're looking for is to not change the amount of air flow but to simply lower the velocity of the air which will reduce the wind noise you're hearing.

As I said, what needs to be done is to build a large box for the duct to tie into. If the duct is say 6" diameter, I'd want the box to be maybe 12x12" x say 48" long. Then, at the far end of that box along one side, I'd want a long diffuser with a lot of surface area. The interior of the box would be lined with standard HVAC duct board. 

Bryan


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## SamSpade (Jun 3, 2010)

Bryan,

Here is a photo of the air handler and evaporator that is used for my home theater and another room:









The box on the left side is 25" long, 16" high and 22" deep. It is used for supplying air to the room. As you can see, I am somewhat hemmed-in by the water heater on the left.

The box on the right side is 19" long, 15" high, and 15" deep. This box is used for the return air. Since the return is the noisiest part of the system, and I have the room, I can have the box you were referring to replace the current box.

Your proposed description of the box was: _If the duct is say 6" diameter, I'd want the box to be maybe
12x12" x say 48" long. Then, at the far end of that box along one side, I'd want a long diffuser with a lot of surface area_.

Since I am using 10" duct, how about a box that is 24"x 24" and 60" long? In terms of a diffuser, I have several RPG Skylines that are 23" x 23" x 7". Would one of those be suitable as a diffuser?

It is not clear to me where the diffuser should be located. Are you saying it should be located on one of the 23" walls as far away from the duct attachment as possible?

Thanks again for your help.


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## bpape (Sep 14, 2006)

The box I'm referring to would be at the other end of the duct in your room - not at the air handler. This allows for a pressure drop and reduced velocity without changing flow.

Sorry for a terminology problem. The diffuser I'm referring to is the cover on your return and supply vents. Something with more surface area will again keep flow the same while reducing velocity by spreading the same flow over a larger area.

Bryan


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## ironglen (Mar 4, 2009)

bpape said:


> Hi Wayne
> 
> What you're looking for is to not change the amount of air flow but to simply lower the velocity of the air which will reduce the wind noise you're hearing.
> 
> ...


Sam, the correct sizing of ductwork to match a system is imperative, and often incorrect from what I've seen in the biz. Is your HVAC guy familiar with it? For every 12000 btu cooling capacity, the ductwork should handle 400 cfm of airflow, so a 24000 btu or 2 ton ought to be designed for 800 cfm airflow for both supply air (into room) AND return air (from room to the unit). I actually had noise from the return because it was undersized before I reworked it. If the supply duct is too constrictive in any way, it will cause noise and similarly, the return will do the same. I think it's important to note that while the return cannot be too large, the supply lines can be too large, but probably rarely happens as many HVAC techs appear to incorrectly oversize unit change-outs to insure the homeowner doesn't complain about insufficient cooling.

Bryan, is that much surface area for the supply needed to reduce the noise? I would think that a smaller increase would be sufficient without having a huge grill and airbox.


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## bpape (Sep 14, 2006)

As I said, the box is to create a pressure and velocity drop. I just feel that even an oversize vent cover, diffuser, whatever directly to the evidently undersized ducting is still going to cause a problem.

Bryan


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## ironglen (Mar 4, 2009)

Looking at your picture, it appears that the return box and possibly the flex duct used for return air (is there only one 10" flex connected?) may be a big culprit causing the noise. We tend to build quite large return air plenums or boxes and use large grills to reduce the static load on the blower motor, which is also quieter. We typically use a 16"x24" return grill for a 2 ton/24000 btu cooling unit and often see these undersized in homes.

Sam, the execution of Bryan's advice should be pretty straightforward and affordable using common ventilation materials, plus labor. Let us know how it goes (pics are always nice).


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## SamSpade (Jun 3, 2010)

Hi Glen,

Since my last post, my HVAC person has replaced the two 6” x 10” registers, connected to 8” diameter standard ducting, as the source of incoming air. I now have four 10” x 12” registers, connected to flexible sound absorbing ducting. The ducting is configured in a variety of non-linear paths. That has taken care of the sound of the incoming air!

The remaining problem is the sound made as the air flows through the 12” by 20” return. The return is connected by 14” flexible sound absorbing ducting. The speed of the air at the entrance of the return is 520 fpm. So, it is back to the drawing board to try to solve this last problem, and then I hope I will have a quiet room!

*I have sent you a private message*. When you get a chance, would you take a look at it?

Thanks


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## bpape (Sep 14, 2006)

You've done something similar to what I was suggesting - just without the pressure box. You've close to quadrupled the opening surface area for the same amount of air to flow through - same flow, lower velocity.

The same thing will work on the the return end. Doubling the area of the return will cut in half the amount of air moving through each return vent.

Bryan


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## ironglen (Mar 4, 2009)

Good! Rather than expanding our return, I elected to use a second matching grill/airbox along with a second flex duct into the main return plenum that's attached to the unit. To do so, I had to increase the return plenum's size. I believe you have the space at your unit, so you should be in stealth mode shortly! :T

I'm really new to HVAC, but seems that I've found many techs don't pay attention to airflow much, if any. After much personal studying on HVAC last year, I found that a good forced air system should be invisible: neither heard nor felt, which seems to be the exception rather than the rule today. That should be the case for all systems, not just HT applications.


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## sga2 (Nov 14, 2009)

Add a second return with same duct and grille size and connect back to the main return with a wye fitting (shaped like a "Y", not a "T"). This will cut the velocity down to approx 260 thru each grille. At that velocity any sound generated should be below the noise floor of the room.

I would also add that with flex duct, the installation is just as important as the size. The inside of flex duct is a plastic tube with a helical wire spring for structure. When the "spring" is stretched, you have a nice relatively smooth surface inside the duct but when it is compressed or bent sharply, the spring compresses and the plastic covering (which is no longer stretched smooth) has bends and folds which both decrease the effective inside diameter (i.e., increase velocity) and create turbulence and noise due to surface roughness. Make sure straight runs are reasonably stretched (not to the point of stressing the joints, but with very little slack) and turns are LONG radius. Want inside radius of the elbows to be no less that the duct diameter.

Regards,
sga2


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## mandtra (Aug 30, 2010)

Almost all modern air handlers are multi speed , simply by rewiring . I would go to a lower fan speed. Has your Hvac guy not suggested that?


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## ironglen (Mar 4, 2009)

mandtra said:


> Almost all modern air handlers are variable speed , simply by rewiring . I would go to a lower fan speed. Has your Hvac guy not suggested that?


Most blower fans are multi-speed rather than variable speed as only the high efficiency units use variable, and even some high efficiency units don't (our seer 16 doesn't). It is true that a med-high speed may be used rather than high speed in some instances , but ensure that sufficient airflow occurs or the indoor coil will freeze up.


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## sga2 (Nov 14, 2009)

ironglen said:


> I found that a good forced air system should be invisible: neither heard nor felt, which seems to be the exception rather than the rule today. That should be the case for all systems, not just HT applications.


I design HVAC systems for commercial/industrial facilities and you are right on. But I'll add that most owners also expect it to be free - or very cheap - and that expectation (read: lack of budget) usually trumps the first two requirements...

Regards,
sga2


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## ironglen (Mar 4, 2009)

Right on Sga2! 

I'm just learning on the job coupled with textbooks while I finish college (in another completely unrelated area). Anyways, I'm betting a whole lot of HTS members could benefit from your expertise in design, particularly general airflow requirements in their HT builds, if you can find the time. Surely the added cost for a proper performing system would be deemed valuable, after all, the HT builds I see on here are no small investment. Perhaps you/we could write up a brief llist of helpful tips/guidelines on said topic?


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