Comment by amluto

The constant fan noise issue always seems a bit absurd to me. There’s an excellent mitigation available that also happens to be to extremely cheap: insulated flexible duct. The cheap mylar-fiberglass-mylar sandwich stuff that’s reinforced with a steel spiral, is sold in big boxes, and is favored by cheap contractors because it’s easy to transport and install.

This stuff is amazing: not only is it safe (no airflow in contact with fiberglass) and fairly well insulated, but it has excellent acoustic insertion loss. If you take 100 feet of rigid galvanized steel duct and talk into one end, someone at the other end will hear you loud and clear. If you take that same duct and install duct liner (and incredibly annoying process) like a fancy commercial installer, you won’t hear much at the other end. If you use 100' of flex duct, you will hear basically nothing. This stuff mostly outperforms even the most expensive commercial acoustic solutions!

Here’s a spec sheet from a random brand:

https://www.flexmasterusa.com/Portals/2/Downloads/Flex/6B.pd...

Wow, 12 feet of 6" duct attenuates 250Hz sound by 43dB! That will make that frequency close to inaudible even if the equipment end of the duct is quite loud as HVAC gear goes. Use wider duct or a longer run (or both) to get it even quieter and to make a bigger dent in the lowest frequencies.

So you stick you fancy fan somewhere that you won’t directly hear it (in mechanical space with a fiber-insulated wall between you and it) and you connect it to the living space with ducting that contains at least a decent length of insulated flexible duct. And you keep the grilles and ducts large enough to keep face velocities low so that the ducts and grilles themselves don’t make much noise, and you have a fantastic system.

Or you use extremely expensive specialized semi-rigid ventilation duct or rigid galvanized steel or uninsulated flexible aluminum, and you’re sad because your duct is a speaking tube.