Design the Ultimate Home Theater– On a Budget Page 5
Your heating, ventilation, and air-conditioning (HVAC) system can be a significant but overlooked source of ambient noise in your home theater. In many cases, this might be the first place to make improvements. Norm offers the following suggestions for reducing HVAC noise:
1. Position the machinery as far away from the room as possible and isolate the equipment against the structure-borne vibration.
2. Use a low-air-velocity system that will allow 10 to 15 air exchanges per hour, depending on the size and heat load within the room.
3. Use dedicated sends and returns.
4. Place intake and outtake vents away from the listeners and speakers.
5. Use duct liner board, or round metal ducts lined with glass fiber.
6. Include a plenum silencer near the fan discharge.
7. Introduce right-angle bends downstream from the fan and upstream from the outlet.
8. Use isolation hangers to suspend ductwork when possible.
9. Use large, non-tympanic air-terminal diffusers. Avoid multilouvered vents.
10. Apply liberal amounts of acoustic caulking to seal penetrations made to the structure.
11. Eliminate as many obstacles from the air's path as possible.
Where possible, we employed a number of his suggestions, which included building a plenum silencer. The silencer consists of a duct-lined box that's larger than the duct that feeds it. It's like a car muffler, only bigger. It has a vent supplying air at one end and an output, which then feeds the room, at the other.
We also learned some tips from Steve and Bob at Air Products International, a Van Nuys, California–based company that sold us our HVAC supplies (few places sell to the public). The main concept is that velocity creates noise. Less velocity but more quantity (i.e., more ducts move more air more slowly) will create less noise in the system. We used ducts that consistently got larger or broke off into multiple outputs so that more air could travel through them with less velocity. For a total cost of about $400 in parts, we ended up with an extremely quiet air system.