Airports use noise monitoring terminals to assess the sound. On the hardware side, I know what is used in Calgary, Montreal and Toronto is a Brüel & Kjær type 2250. Brüel & Kjær is a Danish company. They are the world's oldest manufacturer of sound measuring equipment, going back to 1942.
The equipment these airports use is installed at 80% of the major airports in the entire world. These are type 1 sound level meters. Sound level meters are type 0, 1, 2 or 3. Type 0 is used as a reference in a laboratory to calibrate other instrumentation, and type 1 would be the next level. From a practical perspective, type 1 is the most accurate of all of the equipment used. This data is then sent in real time to servers in Australia via 3G communications.
It's just measuring the data. They're measuring it in terms of the best quality of the signal itself. Next though, the key is what you use that data for. Is it just put there on a server where it's archived or do the airports actively take that data, use it to respond to complaints and monitor infractions, etc.?
I believe that a lot of airports, while they're measuring very good-quality data, are doing very little effectively with the data.