I bought the Broan 637004EX with the external 331H external exhaust fan with quietness of operation a close second to effective exhaust in mind. I chose this model since it was listed as amongst the quietest at 0.4--1.5 sones/100cfm. Also, for reasons of quietness of operation, I opted for the external exhaust, even though it complicated the installation. The 331H exhaust fan is rated at 600CFM, which translates to 2.4--9 sones. After installation per instructions with 10 inch ducting, the unit measures 46 sones at the high setting to 42 sones at the lowest setting (83--81dB). The technicians at Broan were only able to reassure me that the controls and motor are operating as designed but were not able to help me attain the noise levels advertised on their website. It should be added that since the sliding fan control is a voltage regulator (120V--80V), further voltage reduction could slow the fan further and result in quieter operation, but that is not an option offered by Broan. Our neighbors have a GE exhaust hood with an excellent range of operation, so I would recommend looking elsewhere if quiet operation is a consideration.I
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