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.

Sorry if I was unclear, but the silencer removes noise that is beyond the silencer relative to the listener. So, not that it would matter, but hood baffle noise would be partially silenced when listening on the roof. Relative to the cook in the kitchen, the silencer removes noise beyond it. So, hood baffle noise and local duct noise and local hood interior transition noise won't be affected by the silencer when listening at the cooktop, but duct noise on the roof side of the silencer and most importantly blade tip turbulence noise from the roof blower or the in-line blower that is past the silencer (relative to the hood) will be attenuated when listening at the cooktop. The blower noise is typically the loudest, so the silencer is important to minimizing noise. (It is worth pointing out as I think I did sometime in the past that blade tip turbulence noise dominates baffle noise given well designed baffles. I only have personal experience with my Wolf/Independent hood, so it is possible that a particular baffle design could be noisier than a particular blower design.) So the silencer should be as near as feasible to the hood (past any dampers), and the motor closer to the roof or on the roof. A duct that passes heated rooms should be insulated from the rooms passed, and possibly partially coated with damping material (I use automotive sheet metal dampener sheets). A damper is needed at the hood (probably part of the hood) and another should be at the roof. The Wolf/Broan low profile blower I have has its own damper which opens upon flow. Abbaka and Broan blowers look similar and appear to use the same approach. An upblast or downblast quasi-commercial blower and housing design may need a conventional damper just below it. Dampers should be checked for function without hanging up on errant screws before workmen can escape the project. A straight run will have minimal duct bend turbulence noise (caused by flow separation, perhaps). It may or may not make adding a silencer inconvenient, architecturally. kas

@opaone: your numbers, while still higher than my estimates, are at least within an order of magnitude, which in this business probably is reasonable correlation. I agree that this could rise to a significant concern with 10 people really cranking away in a poorly ventilated room. So we're on the same wavelength. @spurpura: you are correct in that I've not invested in air quality monitoring devices or hired PhD's to test my house. But my calculations are grounded in simple physics. The only assumption I have made is in the estimate of CO2 production during exercise, and that was from an academic sports physiology laboratory, authored by a PhD and published in a peer-reviewed journal. If you have figures showing that CO2 production during exercise is roughly 10 times higher than my estimate (which is what would be needed to support your observations), I'll await the citation. There are really only three possible scenarios: You are measuring local concentrations of CO2, presumably close to where you are exhaling. Your PhD consultants and their equipment are wrong. There's an exogenous unidentified source of CO2 infiltrating your exercise room. Enough of that dead end discussion. But...as to your original question, I think you're on the right track. As noted upthread, our 1200 cfm hood runs through a straight run of 10" duct, about 25' upwards, with an attic-mounted Fantech LD10 silencer and FKD10XL blower. It measures 60db (LCS weighted, ear level, roughly 4' from baffles) on high. So we hit the mark you are shooting for and I expect your setup will too. Good luck!

If you're wokking, you need CFM. And you need something to "catch" all the stuff the wok kicks out. That ceiling-mounted vent is not going to capture anything. First because it will be far from the countertop (especially with a shorter countertop height). Second because there's nothing to guide all the greasy, steamy stuff into the actual vent. Fumes don't just rise, they spread. Imagine a cone, starting at 30 inches wide (at the level of your counter) and spreading out as the steam rises through the room up to the ceiling... 8-foot ceilings shouldn't be an impediment to a vent hood, especially if the counter is shorter. They usually need to be 30-36" from the countertop. You can look up the height of the various hoods and see what will be right for your island. The right-angle turn to vent horizontally is much less of a concern. Depending on how long the run is you can look at adding in a silencer, or just sticking the blower on the outside of the house.