You bring up a very important point, and I thank you for that. Sound quality is critical when we're looking at aircraft noise. The dependence up until this point in terms of regulation has always been sound level—how loud the noise is—but that does not explain the variance between one individual being highly annoyed from automobile traffic at a certain level and being the same amount of annoyed for aircraft noise levels that are lower than those from automobile traffic. It's not all about loudness. It is about loudness to some extent and sound quality to another. The frequency composition of the sound is very relevant.
Typically, when you have the presence of pure tones, which is one dominant frequency, that tends to elicit a very strong reaction from a receiver, from a person. High frequencies also tend to do the same. Low frequencies penetrate the home, for example, more easily, and may be a cause of vibration.
It is something that has been suggested as an alternative route for research going forward. A European study actually took the sound profile of different jet types and asked community members to adjust certain sound quality aspects in order to get a more pleasing overall sound. It was not reduced in loudness, mind you. The composition of the sound was just different, and it had a reduction of annoyance associated with it.