Many different metrics are used by airports, and it changes from country to country. For example, the European region uses something different from what we use in Canada, and what we use in Canada is different from what is used in the U.S.
A lot of the metrics are average-based noise metrics, where they'll measure the sound over an extended period of time and give you an average.
For example, in the U.S., they use something called Ldn, or in some states, like California, Lden. They take the daytime noise over the entire day, and the nighttime noise over that 8-hour period. They add a 10 decibel penalty, then come up with this one single-value number to represent that entire 24 hours.
In my opinion, it's not an appropriate metric to use for impacts that are cyclic, where we have an aircraft flying in anywhere from every 90 seconds to several minutes. It is that frequency of the aircraft, the coming and the going, as well as, if you think about nighttime noise, the Lmax levels. It's not that eight-hour average over the nighttime that's waking you up; it's the maximum levels, the high-impact sounds.
Europe does do a better job, for the most part, than what we do here in Canada.
To answer your other question, yes, certainly there are better metrics out there. With respect to human perception and how we hear sounds, there's another factor that really isn't being taken into account in evaluating aircraft noise, but it is being used in other industries, and that's the human impact of the sound.
A typical metric would be a loudness metric, where it takes not only the sound pressure level, but also includes other factors that affect the quality of the sound, like the frequency, whether it has modulation or is sporadic. All of these have significant impacts on the impression of the sound we hear.
In other words, with psychoacoustics, it's not necessarily how loud or how quiet the sound is, but also how good or bad the sound appears to the human.