Office A/C vent issues

MichaelMurphy

MichaelMurphy

New member
Hey everyone :)

I'm working on converting my work office to a production room. Everything is going well except I'm having some serious issues with the ceiling air conditioning vent. Here's what's going on:
  • The a/c produces the usual low rumble, but it's loud enough that both the mic's lowcut switch, as well as some Izotope RX3 EQ'ing, don't remove it without cutting out necessary frequencies.
  • Due to space restrictions, the only good mic placement spots are within 6' of the vent. So not only do I have rumble, but the air itself hits the mic. Windscreen doesn't seem to help much.
  • It's a corporate building, so I can't shut off the air conditioning.
  • The mic is a RODE NTG-1, which I have to have pointed downward. It uses a super-cardioid pickup pattern, so it picks up quite a lot of the vent noise as well.
I thought about clamping a small sound blanket to the ceiling frame around the vent, but I don't have one with me right now, so I can't experiment. Has anyone something like this before?

Any suggestions would be greatly appreciated! I'm usually great at rigging some unique solutions, but this one has me stumped.

Thanks,
Michael
 
If modification to the room's construction is an option, installing a vent that is at least twice the diameter/width of the air duct can cut down drastically on the "rushing air" sound. As for the air hitting the mic, the only thing you can do there is to flag the airflow. Literally, put a flag on a C-stand overhead to redirect the air away from the mic by placing it close to the ceiling and angling it to "bounce" the air toward a wall.
 
Thanks for your reply, Alex! Unfortunately there isn't anything I can do to the ducts or vents since the building is technically rented. I do like the flag idea, though. Definitely will give that a shot! :)
 
I believe Alex is suggesting an external add-on duct that you can hang from the ceiling to "extend" the length, reduce the veolicity (by making it larger diameter), and divert the air outlet somewhere else in the room (away from the microphone). If the LF noise is ONLY coming out of the HVAC grille, then it is possible that an external duct extention could also be designed to reduce the LF noise. But if the LF noise is "ambient" (from the ducts in the ceiling, etc.) then there isn't much you can do.

Now that LF rumble seems quite likely to be a show-stopper. If you really can't filter it out without affecting the signal of interest (are you a Bass?) then you could very well have an untenable position. As Scotty (Star Trek) was oft to say: "Ye cannae violate the laws of physics!"

There are things like masking background noise with music, SFX, etc. But since we know nothing about what you are trying to produce, we have no idea whether this is even a possibility.
 
Richard - that's interesting! I'll have to give that a look. I have a feeling that it might get in the camera frame, though, since I'm shooting in a fairly small space (10' W x 16' L x 8' H). Worth a shot though! The room is a green screen room where I record training scenarios and interviews, so I want it to be as quiet as possible. I was nominated to also be the spokesperson because of my voice, which, unfortunately in this circumstance, is quite bass-y. Maybe I should present my videos in falsetto. :P

Thanks for the tips, guys!
 
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MichaelMurphy said:
... since I'm shooting in a fairly small space (10' W x 16' L x 8' H). Worth a shot though!
Click to expand...

Just out of curiosity... do you have any kind of acoustic treatment in this room? In a box like that, if everything is hard office walls and ceiling (is flooring carpet or tile?), low frequencies are probably going to be enhanced. Reducing the velocity of air coming out of the vent will help. If, as Richard mentioned, the low rumble is transmitting up into the air ducts and then through the ceiling in general, there's not much to be done.

However, knocking down reflections and adding bass traps to the corners of the room can mitigate a host of other issues. Corners are where bass frequencies are strengthened and can contribute to rumble problems, also enhancing any bass-heavy voices. Reflections between hard surfaces also cause issues with muddying the sound, and this may be wreaking havoc with your NTG-2. Shotguns are not usually the first choice for highly-reflective interiors.

Take a look at Auralex's "Roominator" kits for solutions that offer both absorption for the walls and bass traps for the corners.
 
Yep! Just last week I covered the entire room (except the far short wall) with 2" Auralex wedge panels. The reflections are completely non-existent now, so at least now the bass frequencies are coming from a single spot. Amazing difference.
 
So paint the external duct green! Remember it can be wide and shallow so it doesn't hang very far below the ceiling.
Seriously, the expectations of what you can do in that space significantly exceed the possibilities of such an unsuitable room.

I question using a very sensitive mic like that in such a hostile environment. It seems like a poor choice.
You would be much better off using a headset arrangement with the mic an inch or two from your mouth.
Remember that proximity is your friend when you are trying to overcome ambient problems.
 
Yeah, the room is definitely not the ideal space. Unfortunately it's the only one available in the building to set up shop and I'm an in-house employee, so it's a bit difficult.

Fortunately, we're looking into getting some lav mics, which would be fantastic to have in this situation. We'll see!

I'll post any updates as they happen. Thanks again everyone :)
 
Don't assume you can't make modifications, ask. Unlike residential, commercial landlords are more open to minor modifications as long as it doesn't cost them anything and you are willing to restore the changes if requested at the end of the tenancy. Do you have a false (drop) ceiling? If so then the AC ducting in the ceiling will terminate in a box. That box is the only thing you would need to modify to achieve your goal. There's a few things you could maybe do.

You could seal off the box at the cost of no a/c. I've done this in our office once by simply dropping a piece of plywood into the box. The pressure holds it in place and the air goes elsewhere in the building. I would just push it up with a pole or remove it if it got hot. Don't know if this is a violation of building code but it did reduce the noise by a guestimate of 80%. I work in telecommunications and the equipment rooms are unbelieveably loud between the a/c and several hundred thousand little fans in all the rack equipment. The office is annexed from the equipment floor and connected to the same a/c compressor so it is overly powerful and much noisier than an office a/c should be.

You could modify or replace the box with a baffled box. Forcing the air through a maze robs the volume and can be made even more effective lined with some absorbent materials like acoustic foam or insulation products. There may be commercially available solutions but I know for sure there are plenty of designs online, particularly on music studio/recording forums. I'm planning to do this in my own home once it's built.

Or you could redirect the aircon in the false ceiling, similar to richard's suggestion but in the ceiling. Simply add more ducting to the end and take it somewhere else or just have it travel further, do a lap of the room before coming back to the box.

If it's not a false ceiling then modifications will be more involved and may even be structural. A landlord may still permit it though, especially if it is a commercial solution to be performed by qualified tradespeople. No harm in asking. Might be worth finding out who installed/maintains the a/c as well. They may have a solution for you.

Good luck.
 
If you have a drop ceiling redirect the vent/duct to blow air inside the ceiling (if that makes sense) cover the opening with another ceiling tile and in the furthest corner create a 1ft opening so air can escape into the room. They sell baffles and you can also wrap your duct too.
 
Since no one else has suggested it, I will. Take the grille off. Just take it off. This will effectively increase the outlet area (and therefore decrease air velocity some), and it will take obstacles out of the airflow path and therefore decrease "whistling".

Alternatively, if it's really bugging you, you can always take the grille off, completely seal off the back (lots of pink fluffy and duct tape), reinstall it, and seal around it. Done well, that should end your noise problem. It will likely cause other problems, like temperature and humidity problems, but one thing at a time eh?

If the outlet is on the floor, you can always drop a sound blanket over it when recording.
 
Thanks everyone for your suggestions! I'll talk to the higher-ups about the duct/vent ideas. We have to go into the ceiling to run some power cables anyway, so maybe we can kill two birds with one stone.

Speaking of birds, I'm delighted (read: about to lose my mind) to announce that there is now a nest of newly hatched birds in the ceiling rafter directly above my office. If anyone needs a recording of birds chirping for several hours, let me know! :P Ughhhh
 
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