Noise reduction

[JCole21961]

I have a lot of air noise from my AC outlet registers. The ductwork is all galvanized steel insulated on the outside. Outlet air velocity is about 300FPM. How can I reduce this noise?

[The Viking]

What type of noise are we talking about?

Vibrations?

Air velocity?

The solution will depend on a lot of factors...
None of which we are likely to find over the net without knowing a lot more about the situation. (and maybe not even then)

[JCole21961]

The noise is air-noise, not vibration. Removing the outlet registers does not improve the situation. All ductwork is rigid steel. I'll visit your site, thanks.

[Brian_UK]

Is it noise generated by the fan?

[JCole21961]

If you mean mechanical noise from the fan, no. Of course the fan moves the air and it is the air that is actually making the noise. It sounds like a gale but as mentioned above, actual velocity is low at 300 fpm except at the fan outlet where it is more like 1600 fpm.

[Brian_UK]

I wonder whether the joints in the ductwork are facing the wrong way and have not been sealed over.

The air moving in the duct can resonate over the ductwork edges and generate noise.

It might be possible to fit some sound attenuating foam to the inside of the ducts near the terminals to absorb some of the sound.

[nike123]

If you mean mechanical noise from the fan, no. Of course the fan moves the air and it is the air that is actually making the noise. It sounds like a gale but as mentioned above, actual velocity is low at 300 fpm except at the fan outlet where it is more like 1600 fpm.

With that speed of air at register I doubt that register is generating noise. What air velocity is in duct before register? Maybe ducts are of high speed construction (more than 800 fpm or 4m/s). Duct before register should have about 400 fpm or 2m/s air speed in order to be "quiet".

http://www.engineeringtoolbox.com/circular-ducts-sizing-d_206.html

[Chill'D]

Hi all,

Your best bets are to get ductwork attenuators installed before and after the fan, these are just like silencers.
If you have more than one outlet, one might be chucking out more than the rest, if so you would need to get them balanced.

cheers.

[JCole21961]

Thanks, that is probably a good explanation, the guys here are not experienced in duct design. I'll try the foam solution.

[JCole21961]

What form does a duct attenuator take? Can you give me some pointers so I can have them made locally?

[frank]

Have you had any work done on the system recently, or has the velocity noise always been there?

Attenuators will not reduce velocity noise in this case as it seems like it is undersized registers that is the cause.

Velocity is the result of area x volume. Reduce the volume or increase the area and velocity drops. What size is the register/outlet?

[nike123]

Attenuators will not reduce velocity noise in this case as it seems like it is undersized registers that is the cause.


If register is undersized than speed of air at register will be higher than measured 300 FPM.

[JCole21961]

The system has been noisy from the start. If anything, the registers are too large, 300 fpm is slow compared to my house in the USA which delivers 600 fpm from registers and is completely silent. Ductwork is sized correctly, keeping velocity constant at about 750 fpm. My fan is on a VFD and as I slow it down the noise decreases, but even at half speed the system is noisy compared to my US system.

[nike123]

The system has been noisy from the start. If anything, the registers are too large, 300 fpm is slow compared to my house in the USA which delivers 600 fpm from registers and is completely silent. Ductwork is sized correctly, keeping velocity constant at about 750 fpm. My fan is on a VFD and as I slow it down the noise decreases, but even at half speed the system is noisy compared to my US system.


For the low noise, final run of duct before register should have air speed =< than 400FPM. You have 750FPM. That speed is for main run or branch run of duct.

Check this article! (http://www.google.com/url?sa=t&source=web&ct=res&cd=9&url=http%3A%2F%2Fwww.acoustics.salford.ac.uk%2Fstudent_area%2Fbsc2%2Fenvironmental_industrial_noise_ control%2FNotes%2FFan%2520Noise.doc&ei=YRnJSJiIApKg0gXj5uCSAw&usg=AFQjCNFjSCY9mm79xr4ZUJ6nK-il-UUzDg&sig2=eihmoj6B3TAfS6hW_hS_Ig)

Also, noise is relative to acoustic of space, and same noise level in one space could subjectively sound much noisier in other space.
Could you post picture or make and model of registers?

[mchild]

Are all registers noisy or just a few?

What is the CFM air flow from the noisy ones?

How do you know what the air speed is?

You reference both 300 and 750 FPM, which is it?

What size is the duct on the noisy branches?

[JCole21961]

Two registers are very noisy. They are square taps along a main run, simply square holes in the bottom of the trunk with short extensions. The 750 fpm is trunk-line speed. 300fpm is average register outlet speed. There are no curves in this ductwork, every bend is square. If you can receive Autocad I can send you the drawing.

[mchild]

Two registers are very noisy. They are square taps along a main run, simply square holes in the bottom of the trunk with short extensions. The 750 fpm is trunk-line speed. 300fpm is average register outlet speed. There are no curves in this ductwork, every bend is square. If you can receive Autocad I can send you the drawing.


Post here as a .pdf or take some pics.

I am assuming you have measured these speeds and not simply taken it from plans. The stated speed of 300 FPM, in and of itself, should not pose a problem. Something else is the issue here (I suspect the true speed on these two short runs is much higher than the average). If you have not acutally measured the speed and the CFM you must do so otherwise anything you do to remediate is just a guess. Measure, make chages and then remeasure so that you know what impact the change actually has. Guessing does not work.

If the 300 FPM is the average I would speculate that the problem outlets are, in fact, much higher. The short distance from the main truck is an issue as you may be hearing the 750 FPM that it has. With residential ducting the main truck speed would be better at around 600 FPM. If the noisy ones are higher than 300 FPM, and thus probably too much CFM also, then balancing dampers will need to be installed where the branch comes off the trunk to control the volume of air flowing to these branches. The register itself is a poor way of controling the air flow and will probably whistle and such.

Having two 90* turns in the braches that are noisy will reduce the noise, but will also increase the static pressure. The impact on static pressure will be minimized by using 90*s that have both an inside and outside radius and turning vanes. No hard turns as they create a lot of turbulence and increase static pressure.

Also, using short sections of flex duct, about five feet or so, will help attenuate the noise. Flex duct that is size right, stretched tight, and with no turns is fine for air flow. Done any other way and it is a problem.

The location of where these branches come off the trunk will also impact the air flow and noise.

With the air speed you report in the main trunk, you may already have high static pressure. Have you measured that?

[nike123]

If you can receive Autocad I can send you the drawing.
If you dont know how, send me ACAD drawing in personal message (http://www.refrigeration-engineer.com/forums/private.php?do=newpm&u=13885) and I will convert it to PDF and post here.

[icecube51]

many times i have being could out for noise on kitchen exhaust fans. so i put silencers on the start and end of the tube and is till now the best in result.
they are available from 200mm to 400 &more.

just a hint,

Ice
for info, www.airflux.be (http://www.airflux.be)

[JCole21961]

I have measured the speeds at the inlet and outlets. The duct speed is a calculated average based on duct dimensions. I'll be out for a week but will resume on my return. Thanks for all the input.

[nike123]

This (http://www.mediafire.com/?mv5zgdyoqi6) is your drawing in PDF!

[mchild]

This (http://download280.mediafire.com/omdgrwnvz3tg/2r7ssbuzv3q/Brazilian+houseMaster2004.pdf) is your drawing in PDF!


Link isn't working.

[nike123]

Link isn't working.
It is sorted now!

[Prince Vaillant]

I wonder whether the joints in the ductwork are facing the wrong way and have not been sealed over.

The air moving in the duct can resonate over the ductwork edges and generate noise.

It might be possible to fit some sound attenuating foam to the inside of the ducts near the terminals to absorb some of the sound.


http://www.pipelagging.com/as15240a-sound-insulation-sound-insulation-sheet-p-502.html