Noise Reduction for HVAC ducts 1

[MedicineEng]

Hi All:
Pretty soon I will be handed a big bag of lemons and asked to turn it in a big bag of sweet oranges.
I explain:

In our property we have several HVAC systems that are noisy since these were installed and most are even slowed down well below airflow design in order to not to disturb users.
I can categorize the noise in 2 big types:

-Low frequency drumming
-Air noise at the air diffusers

A couple of potential solutions were already put forward in paper, like:
-Installation of turning vanes in large ducts whenever there is a 90 degrees change of direction;
-Increase of air diffusers coming out of the same VAV in order to reduce airflow in each one

Up to now We were discussing with the main contractor the rectification of these issues, but I think the contractor is ready to jump ship and let us deal with this by ourselves and I'm seeing this landing on my lap.

The issue that worries me most is the locations where I have the low frequency drumming and vibration noise as I'm not sure how am I going to address this. This drumming comes from the high velocity in the duct and I believe that the contractor might have used ducts with strange aspect ratios and eventually some lower thickness sheet metal. Replacing ducts is out of the question so I'm looking into mitigating measures that can allow me to drive these systems closer to its original airflow design. The ducts are already wrapped with rockwool insulation but I wonder if rewrapping it with a some acoustic insulation liners would improve the situation or the difference would be so negligible that wouldn't compensate the investment.

Any ideas or magic products out there that can help me with this issue?

Thanks a lot and Happy New Year!

 

[Drazen]

You need to verify sources of noise before considering mitigation measures, otherwise you are blindly pouring money.

Air velocity in reducers can be calculated and checked against recommendations, as well as ducts air velocity. Specialist should be tasked with noise measurements at locations you deem critical.

Vibration is related to fan units supporting structures, the same can be the case with low frequency noise, if it's structural noise, it has to be dealt with in modification of support assembly.

 

[willard3]

You should hire an Engineer with ventilation experience.

Making recommendations here may be misapplied or misinterpreted and then you will hire an Engineer to solve the problem if you want to fix it.

It's more expensive, but it will solve the problem.

 

[Strong]

Unless you know a lot about sound and vibrations and noise control, then hire an expert noise control engineer. I assume you have a commercial project that could involve sound criteria, noise codes, complaints and law suits that would justify the expense of evaluating the real problem and identifying one or more corrective actions. You could contact a material supplier, but they may be limited on engineering or only specify materials they sell. Here are two links for your general information:

Search: reduce air duct drumming noise

https://www.bing.com/search?q=reduc...TU0xkjVi7MSqoqvL9PUeH8CkZDL8jMYUAfb0YyIFdIIVF

Search: reduce air duct diffuser noise

https://www.bing.com/search?q=reduc...-30&sk=&cvid=BE9A854446F342009BE4C68A46242345

I am a Board Cerified Noise Control Engineer
Walt

 

[carlosgw]

Would installing stiffeners on the duct help with the rumbling/oil caning? (see SMACNA reinforcement)

 

[berkshire]

I used to work for a company that did air quality testing and design , we got saddled with a noise mitigation project in the late 1970s.
We ended up with two P.E.s working on this for a month, with a full spectrum sound analyzer, to sort out a deep rumble in a ducting system that turned out to caused by vortices in the duct impinging on the duct wall. It was one of those " It couldn't possibly be that. " situations. But it was.
B.E.

You are judged not by what you know, but by what you can do.

 

[MedicineEng]

Thanks a lot for all your feedback.

It seems that I will have to at least propose to take this to specialists, otherwise I will end up wasting a lot of time and money and might not achieve the results that the management wants.

Thank you for the extra resources that you guys pointed out to me.

If anyway anybody has other suggestions, please let me know, as in the end with or without specialist, I will be responsible to deliver the results or potential solution.

Thank you and Happy New Year.

 

[Emrelug84]

If you have cad drawing of the existing ductwork, it had better make engineering calculation such as duct acoustic calculation with respect to octave band frequencies according to ASHRAE HVAC Applications Sound and Vibration Control. It will give you sound levels at any point in your system theoretically and this may address you the possible error locations.
Regards,
Emre

 

[dbill74]

Air movement noise (whooshing sound) at the diffusers can be easily fixed by replacing the diffusers and runouts with larger duct and diffusers with larger necks. Also make sure flexible duct is not kinked, that will create a lot of noise.
Another noise maker at diffusers is manual dampers at the diffuser neck. If this is the case, move the dampers back to the take-off junction.

Low frequency drumming is going to be your real problem child. More than likely it is coming from the AHU fan though the structure or ductwork.

Couple things you can check before getting a noise expert involved are:
1. Verify there flexible connections between ductwork and AHU are installed properly and in good condition.
2. Verify unit vibration isolation is properly installed and haven't bottomed out or otherwise failed.
3. If units use chilled water, verify the piping connections have a bit of flexible tubing.