VO-BB - 20 YEARS OLD!

[Moosevoice]

Any ideas on how to quiet an A/C vent in my studio?
I'm in the process of installing audio blankets around my recording space so I'm sure that'll help but wanted to see if anyone had any tips for addressing the actual vent.
Here is a picture of the vent in relation to my mic.
Thanks,
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www.moosevoice.com

[vkuehn]

A/C noise can be one or both of two components: Actual mechanical noise traveling through the duct from the system blower or mechanical noises entering other outlets and coming through the duct work. Older systems with hard metal ductwork and no fiberglass lining are prone to do that. If the distance from the blower mechanism to your outlet is a very short distance, that can contribute to mechanical noise.

Most of us have to deal with the "velocity noise" of too much air coming through too small of a pipe and opening and the air moving too fast. Air moving 800 to 1,200 feet per minute is typical because installing small ducts keeps the cost low. What you really want if you are installing something new is air velocity down about 200... maybe 300 feet per minute. And you want the newer duct systems that have a lot of soft insulation on the inside to keep the noise level low.



The solution in your case may be what I call a "dog house muffler". You could start by experimenting with something fabricated from cardboard. Maybe 18 inches wide, 12 inches high, 24 to 36 inches long. Cut a hole in the top of the muffler near one end just the right size to fit over you current vent outlet. Put a similar size, maybe slightly larger opening somewhere near the other end of the box. It could be on the end, on one side, or facing down. If you have some fiberglass insulation, glue it to the insice surfaces of the muffler. One third of the way down the length of the box, put in a partial baffle that forces to are over to one side. 2/3rd of the way down, put in a similar baffle the forces the air over to the other side. Mount this thin up against the ceiling temporarily with some painters masking tape or something that will not destroy your ceiling paint. See if it gets the job done. You may listen and decide you want to make it a bit larger, or shaped differently. Experiment. Figure out a final shape and size that is least disruptive to the decor of the room.

If the contraption gives you good results, consider constructing a more durable, more decorator friendly version from thicker, good looking materials. Attach to the ceiling carefully and neatly and when it needs to come down some day, all you have to repair is to spackle a few screw holes.

If you still have noise left, consider re-arranging your seating position so you can re-orient the mic so the sensitive side of the pattern is more away from the vent. (Careful. This could result in the sensitive side of the mic picking up added reverberation or slap back. There is no free lunch in this venture!)

If your software allows you to see the frequency spectrum, record some room tone and analyze it. If the a/c noise tends to be down in the 110 hz and below area, depending on the tonality of your voice you might be able to reduce some A/C noise by using a high pass filter and take out sound below 100 hz. Sch filtering can damage the tone of some voices. For voices like mine.... it will never be missed!!!!

Trying to totally solve the a/c noise problem with software based noise reduction usually leaves you with a lot of ground hamburger sound. But if adding a muffler, orienting the mic and some high pass filtering work well, just a hunt of 2 or 3 db noise reduction can add the finishing touch. Trying to reduce A/C noise 6 to 12 dB tends to be disaster in the final product.

[Moosevoice]

Interesting. Do you have any pics of this 'dog house muffler?'
_________________
www.moosevoice.com

[vkuehn]

[Lee Gordon]