You might start with searching for "clinresga" using Google and targeting this site: ths.gardenweb.com. He has a ventilation system that is probably as quiet as residential hood parts will achieve. I don't recall that it was for 48-inches though.

My largest Wolf Pro-Island hood is large enough and is used with a Wolf/Broan 1500 cfm roof fan and in-line silencer [muffler] by Fantech. It is quiet enough to talk under at full speed while wok cooking, but I wouldn't call it quiet. I'm not sure the usual meaning of quiet is possible using a normal hood (perforated ceiling system perhaps). I suspect that a similar flow rate commercial up-blast fan might be a bit quieter, albeit more obvious on the roof. Most noise from my setup is air turbulence at the baffles, but some motor rumble and lower duct and hood transition turbulence is perceptible.

In any case, it is apparent that you are just starting the glorious journey into the complexities of kitchen ventilation, and should perhaps review the references at my Clippings, reached via My Page on this site.

Also, search on MUA on this forum. What goes out has to be replaced. You cannot put in a hood suitable for serious cooking on a 48-inch range without providing make-up air from somewhere; this somewhere cannot be from back-drafting combustion appliances; and unless you live in a climate like San Diego but without any dust, this air will have to filtered and probably heated and have its own blower.

Think of this as your next hobby for a while. Or find a HVAC firm that can spell MUA.

kas

Thanks for your comments. WOW, I had no idea this was so complex. Now DH and I are discussing the stovetop being on the island (so he can see the TV in the FR while cooking). I guess that adds another level of complexity into the whole vent issue. SIGH.