Hello. The whine from our neighbor's vent fan, directly opposite our dining room window, has been so severe and at such an unfortunate frequency that its noise could be heard even in the farthest corners of our big house. We tried a variety of acoustic solutions, some suggested on websites like this one - basically following the idea that a longer pipe, or a "tee," or stepping up the pipe diameter, or running it to the ground behind bushes, adequately kills he noise. No go in our case. We ended up piping the vent into an old plastic garbage can filled with discarded fiberfill pillows and comforters. They must be packed loosely enough to allow the very moist hot air to escape freely, but densely enough, obviously, to kill the noise. It's also important to drill air vent holes in the sides, top, and bottom of the garbage can. It's an experiment,but so far, so good. - SL

Not sure what part of country u r in or what type of construction or housing. In SoCal where I have lived, airing out a house is nearly daily even on chilly days up in the mtns. In the northeast, essential to have some outside air circulation in summer because of humidity and heat , essential in winter because of stuffiness and dry air. So in both cases we have managed with recirculating internal vents. THERE IS NO QU that externally vented are faster and more effective. ALL are noisy. In condos and apts we have lived in, there are restrictions on external venting... Can't vent within "x" feet of someone window, for ex., so do be aware of local code, Lastly, the effort and cost are a consideration depending upon where the vent will penetrate to the outdoors. There are split units btw, where the actual motor and blower are mounted exterior or remotely from the actual hood and fan... This is to reduce interior noise. Most don't come this way and the split is more costly to buy and to install... AND if it is a high volume (CFM?) I can tell you that the split is not that much more quiet. Good luck. You never know who and when you might be swept off into the rain bowed horizon

Magic is often over valued in real life, if not in anime. Historical views on the subjects raised by the OP can be found in this forum's archive by searching the key words. I will agree with anyone who notes that doing so can be tedious, albeit informational, due to the large body of commentary in these areas that has accumulated since at least 2007. Generally, the dominant noise in a simple blower-hood-duct system is the turbulence noise from the blower blade tips. This is true for VaH squirrel cage blowers as well as various other blower configurations. Baffle turbulence noise is secondary and is normally a hiss like sound, although this may vary with the quality of the baffles. (Air speed across the baffle edges is always going to be lower than at the blower blade tips due to the difference in areas.) Using an exterior blower with an intermediate silencer (muffler) reduces the major part of the blade tip noise, leaving only the baffle noise and some hood side duct noise. This allows conversation across the hood and is recognized as a noise solution. Where a silencer is not used, an exposed VaH blower vs. an internal-to-the-hood conventional blower may have modestly different sound levels. The purchaser will need to audit the candidate systems to truly know which is perceptibly louder. While VaH assert a factor of 1.5 in effective flow rate, this appears to be because there would be flow rate loss of that order if they had pressure loss from baffles due to the slope of their fan curve. Normal blower configurations can have a 1.5X like drop with baffles, ducting, hood transitions, and MUA pressure losses all included. VaH do not count most of these non-baffle sources of pressure loss, and hence will flow even less than their specified flow rate in a real physical installation, just as conventional configurations do. In a not too far back argument here, it was clear than once all the losses were considered, "conventional" blowers were better at dealing with the usual significant pressure losses that are likely to be present in a real system. VaH fan curve data can be compared with comparable Broan fan curve data (to pick on one source) to see which has the more desirable closer-to-vertical behavior when reacting to pressure losses. But to know the actual net impact on one vs. another, one has to estimate all the losses and then use the different curves to find the estimated flow rates. VaH cleaning convenience vs. baffle cleaning convenience has also been argued here, with varying opinions about VaH. As I don't have a VaH I can only direct the OP to the archives or to others who may join this thread. In fairness it is necessary to point out that VaH, like other manufacturers, makes changes to their products over time, and observations of earlier versions of products may not be fully representative of present models. (Viz., see measurements reported here a few years ago by "clinresga" comparing his VaH in one residence to a measurably quieter more conventional system in another residence.) kas

It depends a lot on how frequently you need the A/C and what kind of budget you have. One of the portable units pictured above can be purchased for $150-250 and it should be easy to adapt it to your 4" opening. On the other hand, purchase and installation of a mini-split would be several thousand dollars, and you still need somewhere outside to put the condenser. However, a good mini-split would air condition a very large sized apartment, or even an entire house if you use multiple head units. Bruce