If you are concerned about noise, consider one that has a remote blower motor. I have a 900 cfm Broan and my blower is mounted on the roof (you can also mount them on an outside wall or in-line). My floor plan is very open. If it's running on high (never necessary) and I am standing next to it, I can't hear the TV in the living room. But anyone in the living room watching the TV can hear it just fine. The noise I hear is not the motor, but the air moving through the mesh screens and the pipe. Most of that noise will be reduced or eliminated when I insulate and box in my pipe with a chase and replace the mesh panels with baffles. Since the motor is adjustable, I usually run it very low and can hear the TV just fine over it.

We sacrificed the cabinet space above and ran a line straight up into the ceiling joists. From there hops through a joist or two, but mostly running parallel, then up into the attic. The motor for the fan is located in the attic. It pulls the air from the stove and pushes it out the roof. I think there also exterior mountable fans that 100% pull the air from the home. The remote motor mount helps cut down the motor noise, but in our case we went with a 1200cfm unit and a 10" uninsulated metal pipe. Trying to move that much air through metal is a bit noisy. It's ok on lower speeds. You may need to replace or move the microwave as well. I'm my 1st house, they charged me $500 for a microwave that vented outside. I didn't get it, though (and missed it during the walk through), so I don't know if it's even possible. There are also down draft vents on some stoves. They pop up when you turn the stove on and pull the air downwards. I'm not sure how they connect to the rest of the house, but might be an option if it's easier for you to go down, like into a basement and out vs up. I've also seen some metal ducting that designed to fit between stud walls @ stud depth, but I'm not sure if code allows them to be used for exhaust from a stove. You could look into that if you don't have high cfm requirements. They're hit and miss depending what is in the stud bay, and kitchens tend to have a lot in wall infrastructure in them.

@opaone Thanks for the reply . My recent google search says that there is a direct formula to convert sones to decibels . Maybe this will help some people: dB = 33.2×log10(Sones) + 28 But for those a little less math oriented, there are conversion tables online. Of course, this formula does not help to tell you how loud a range hood will be based on published information since the published information is garbage . But at least it will help people prove that their range hood manufacturer is a liar . I did exactly that with a previous range hood and got a refund . So here are my data: Range hood make and model: GE UVW9631 professionally installed. Duct is about 1.5 meters long , right angle, externally exhausted , a reducer from 8” to 6” was required Ambient: 35 dB Low: 51 dB Medium: 60 dB High: 64 dB Boost: 67 dB Ambient: 35dB This is the published noise rating from GE (cfm/sones): 100/1.0 (Working Speed) 290/2.5 (Medium Speed) 380/4.0 (High Speed) 610/- (Boost Speed) For reference, the actual noise level of 51dB on low fan speed is about 5 sones , compared to the advertised 1 sone, 28dB. Way off.

When moisture and grease particulate laden air flows through a duct, there is turbulence and these two components will be brought in contact with the duct wall. Slow moving air will allow condensation; fast moving air will allow impingement collection. In general, both likely happen at different ratios depending on temperature and fan setting. I wouldn't change a duct diameter just for this reason. What you have for a hood is unconventional. Commercial hoods and "pro" residential hoods use baffles that cause two 180-degree turns in the air flow. The larger grease particulates are deliberately impinged on the baffle surfaces, and given some baffle angle will drain toward a collection point. Even without angle, the grease is trapped where it can't easily drip down onto the cooktop. I don't know the details of this fan scheme, so I won't comment on what it might be doing with grease. When you initially wrote that you were having the hood installed, I didn't bother with commenting on its design -- sorry. Whether grease dripping from the fan grid was intended or not I cannot say, but it likely is consistent with the physics of the hood design. Again let me pontificate for this forum that the farther a residential hood deviates from long-time optimized commercial hood design, some portion of performance and efficiency will be lost. Running the fans for a minute before generating any greasy effluent might help with condensation, but I think the fan blades here are causing grease collection around the grill as well as wherever it is supposed to collect.