dealing with air noise from A/C Vent

[conwaylemmon]

ventpic.jpg

ventpic.jpg

Hello everyone,
This is my first post in many years. I've been a long time fan of this forum and I have learned a lot over the years. Thank you to all of the contributors and creator(s).

I just moved in to a new sound room and I'm starting an audio post business in Times Square. The room is 12' x 14' x 8' and already has relatively good "soundproofing", including three layer drywall double walls, and a layer of whisper wall on all walls and ceiling. I'll probably be building some bass traps as well.

The only problem that I don't quite know how to fix is the air noise coming from an A/C vent in the ceiling. There is building AC coming in on a constant 24 hour flow and I need to build or buy a baffle of some sort to quiet the air noise. Luckily the AC unit is very far away and the air comes in via flexible duct, so there is no compressor or fan motor noise. It is only air flow noise. If I cover the vent with a board, the airflow stops and the room is silent.

I've looked around the web for off-the-shelf products that I might be able to install over the vent, but I haven't been successful, and I am wondering if anyone here as a suggestion or idea. Or am I better of building a plenum type baffle. I won't be able to get into the ductwork above the ceiling, so anything I do will have to be hung from the ceiling around the duct frame.

I've added a pic of the vent. the metal frame surrounding the 2 duct outlets is 24"x14" and the far edge (bottom) of the frame is about 30" from the wall, which is just about at the bottom edge of the pic.
anyway, thanks for looking. I'm happy to provide more info if needed.

[Soundman2020]

If you can't get into the ceiling to fix it, then your chances are not good. The basic issue is that the air is most likely flowing way too fast: ideally it should enter the room at no more than about 300 fpm, max, and preferably 100 fpm. So you have to slow it down. The only way to do that is to increase the cross sectional area of the duct greatly, but you need lots of space to do that, because you still need a clean, uninterrupted run of about 5 times the duct diameter after the expansion, so that the airflow has enough time and distance to smooth out again, so there is no turbulent flow.

What I would suggest (if you could get into the ceiling) is to just build a large silencer box up there, take the duct into it at one end (with a variable damper, so you can control the flow a bit better), and at the other end have your register into the room. If there is absolutely no way of putting your silencer in the ceiling, then it's going to take up an awful lot of space hanging below: I suspect it will need to be pretty large. And you'd still hear some air nose anyway, since flex-duct doesn't isolate very well.

In order to figure out how big the silencer needs to be, you'd have to measure the speed and volume of the airflow coming out of that duct, then work from there. The math is fairly simple.

But I'm curios about the photos: it looks like there are TWO flex-ducts up there! Why two?

- Stuart -

[conwaylemmon]

Thanks a lot for the suggestions soundman. the second vent is from another AC unit that is much too powerful for the room so it is unused. I think a previous tenant had a big analog board or something. But my setup doesn't produce much heat.

I have spoken with the AC guy and he is going to help me install an attenuator at the output of the AC unit to slow down the airflow a bit. I think that will help a lot. And I did find a way up the the cavity above the ceiling so I'm either going to install a silencer box up there or hanging from the ceiling.

[Soundman2020]

That's good news, on both fronts. It would be great if you could update this thread with the results, so others can learn from your experience.

- Stuart -