An internal blower of the same flow rate vs. pressure loss will be noisier than an external blower due to proximity to the cook's ears. One can always get a larger external blower operated at lower blade speed and improve noise performance for a given CFM.

Ducts may filter but cannot amplify. They can resonate at some frequency, but this can be ameliorated by covering the duct with vibration reduction material, such as the stick-on pads (Q-pads) sold for automotive use.

If Fantech were to shorten their silencer, one would find that the lowest frequency muffled would rise. Whether this was still worth it is unclear to me; the main value of the silencer is suppressing blade tip turbulence noise, which, while conceptually broad spectrum in noise output, may have stronger and weaker noise bands such that the shorter silencer could still be useful.

A search on "duct silencers" may provide alternative sources. For example, there is this presentation: https://www.youtube.com/watch?v=l2ZxWRckLes. And this link: https://www.dbnoisereduction.com/.

However, first I would ask if there is a reason the duct cannot rise into an attic where more room for the silencer may be present?

Hi kaseki,

Thanks for the response.

Right above the kitchen is a bedroom and there is no room in the wall for ducts to go up to the attic or roof. That's why the duct has a 90 degree elbow around the ceiling to go outside. It is currently a 6-inch duct.

Thanks for the info and some options I can try.

One idea I had was to use a Fantech 8" silencer which is around 24" long. This length will barely fit but I will need a 10x8 reducer and increaser because the Wolf hood and blower both have 10" vents. The 8" silencer may reduce the airflow a bit. With a Wolf 1500 CFM blower, it should provide enough airflow for a 36" range top with around 65K BTU total.

Yes, a constriction in the duct path will add pressure loss and move one to the left on the fan curve (more pressure loss, less CFM). However, I use a Wolf 1500 CFM exterior blower with 10-inch duct and silencer and presently passive MUA and still get 90 - 100 CFM per sq. ft. under my Wolf hood, which is the largest Pro Island hood. So I think you will be OK. Blower noise will still be attenuated, and I would expect that any noise increase from use of the reducers would be less than the blade tip turbulence noise being attenuated by the silencer. Consultation with Fantech might be useful in this case.

What are the size parameters of your candidate Wolf hood?

Thanks. I will check with Fantech. Fantech customer support is very responsive.