How does the noise of a 2-to-4-seat helicopter compare with that of a similarly-sized fixed-wing aircraft?
I found one study comparing helicopter and airplane noise levels and signatures:
Aircraft source noise measurement studies summary of measurements...

Unfortunately this study does not provide conclusions or any  abbreviated comparison in plain language (or I just failed to notice them). It does, however, provide a near endless array of tables to shuffle though and compare.

According to the study there are similarities and differences in noise signatures between helicopters and airplanes, especially depending on the phase of flight and this hardly comes as a surprise.  In real life the biggest problem arises form the differences in how people react to sounds. Some of us might not care much about even the loudest jet noise (I, for example live a couple of miles form an AFB, and I don't mind), and some experience the slightest noise as a terrible nuisance.

What I would recommend, based on my very limited (but still academic) studies on acoustics, is to conduct a series of test flights and measurements to:


verify that legal (if any apply) decibel limits are not exceeded
give you measurement data for comparison and later reference
and, most importantly, to give you and the area residents an idea of the noise related to helicopter operation.


It should be noted that noise charts, such as those presented in the study above, are not easy to interpret. Decibel is a rather tricky unit, and does not necessarily correlate with the way one experiences noise. In comparison, for example, helicopters might be louder in decibels in some stages of flight than same size airplanes, but the noise might not be experienced any more disturbing (or pleasing, depending on how one is inclined)

Human ears are by no means precision instruments, and actually neither are high end decibel meters: their accuracy is typically somewhere in the range of 1 decibel, and considering it equates a difference of 26% in sound energy, and 3 decibels would be doubling or halving the sound energy, it's not very precise... To make things more complicated, an increase of 1 dB is just barely noticeable by human ear (or brain). 3 dB is perceived as about a quarter louder, and 10 dB feels twice as loud.

So: there's not much to distill from decibel charts or measurements unless the differences are huge across the whole frequency range (more than 10 dB), and even then it's still somewhat ambivalent... It's best to get real life experiences when it comes to sounds.