Noise calculations

[mechWatt]

Is there any equation or recommendation that I could use in order to see if my plenum box (with grill attached) will produce noise?

Grills supply air to a public area and therefore, I would like to eliminate as much noise as possible. Air speed thru the grill is around 1,2 m/s.

 

[Strong]

Search: flow induced air noise from grille

https://www.bing.com/search?q=flow+...YKa*TVyIL2!s!nw2F5Fzio*RINJ4UxNUHNvWVlQbhIJWA

I would not count on a simple calculation if the application is critical. Grille design, local flow velocity (not average velocity) and approach angle could make an unwanted noise (structural rattle or air whistle) even with low velocity. Test it before you ship it!

Walt

 

[GT-EGR]

You’ll need to give more information since what you just mentioned has a variety of acoustical analyses that would apply

Sounds like you are worried about the noise from the grille. That’s the easiest source to determine since the grille manufacturer can provide that based on the grile type, size, and airflow. Just be sure your space is similar to theirs as they take attenuation credits for things like an acoustical ceiling.

If your plenum velocity is low, in the 500 fpm air speed range - then it’s doubtful you are getting noise from that unless you’ve got really bad fittings.

Is your plenum box attached to a piece of equipment - if so then you need to get the inlet, discharge, and radiated sound levels from the manufacturer - and then the most conservative case would be to assume those noise levels go right through the plenum into the space. But if this is too high - you really need acoustical modeling software like Trane taps program to determine how much sound makes it out of your unit and into the space.

Mcquay has a pretty good acoustical analysis PDF/white paper online that’s free - but it’s really complicated to get through, and beyond any tip you can get in this forum.

 

[lilliput1]

make sure you put an accessible volume damper at the branch connection to the main. Make sure it is far from an outlet so as not to create noise problem.

 

[Emrelug84]

There is recommendation in ASHRAE 2015 HVAC Applications. Please refer to "Path Noise Estimation and Control" in Chapter 48. You will find exact equations and explanations for noise phenomena for plenums.

 

[Strong]

It is always nice to get a response or feedback from an OP!

Walt

 

[mechWatt]

Sorry to everyone for not providing feedback, work has been hectic lately.

I've attached an image of a sketch regarding the noise from plenum boxes.

Grille dimensions (and plenums width*lenght) is 1025x75 mm.

Taking into consideration an effective area of the grille, air speed thru the grille will be 1.58 m/s.

Given the limited time I had to check on advices and informations all of you provided, I think that there should be no problem with noise.

My biggest concern is the dimensions of the plenum box (mostly the depth). At the moment, plan is to make it between 300 and 400 mm in depth. Wondering if this could maybe generate some additional noise?

I am grateful to everyone for sharing your knowledge.

 

[Emrelug84]

According to "Sound and Vibration Control" Chapter Section 46, Table 4; maximum free opening airflow velocity is given as 1.8m/s for Design RC 25 (Noise level). when looking at your velocities, it seems appropriate for the standard. In table-3, maximum recommended duct velocity for duct located within occupied space is 4.8m/s for Design RC 25. It is right since you have around 3.5m/s.
I think air volume damper will be more critical than plenum box. Can you please tell the pressure drop of the damper and the distance from the grille?
(I assume you need Design RC 25 to be safe side. Values are RC25-35 for office areas; RC40-45 for office corridors and lobbies. It had better to specify this also.)

Regards,

Emre