A Fantech silencer is about 4 inches larger in diameter than the duct it fits, and 3 to 4 feet long. See chart. It will greatly reduce blade tip turbulence noise and duct turbulence noise from the duct between the silencer and the blower. It will not reduce any blower motor rumble or hiss from the hood baffles.

A larger blower operated at lower speeds can reduce noise, but this involves cost and adaptability issues.

It may not be wise to lower a given blower's low speed operation if the blower is intended for a specific hood, at least if the hood has baffles. Too low an air flow velocity in the baffles will defeat their function of centrifugal extraction. Also, very low speeds for air in cold ducts will increase grease particulate condensation on the inside duct surfaces.

With respect to motor control, that would depend on the existing control scheme. A rotary motor control that had a lower limit might work, or you may have a motor that is wired for different speeds and you would need more of an engineering analysis to determine feasibility. In any case, I would argue against operating a too low of an updraft as you may not get sufficient capture and containment, even when the plume velocity is quite low. In such cases, capture depends on the hood flow overcoming turbulence from human movement and drift from cross drafts.

Is the fan still too noisy when you have it on “low” speed?