Air traffic noise can be determined in two ways: either it is calculated or it is measured. A measurement can only register the actual status at a point. With a calculation, however, it is possible to represent noise burdens over an area in the future. 

How is air traffic noise measured?
Noise is measured with the aid of sound level measurement devices. The results acquired depend on measurement conditions such as the weather situation and possible ambient noise, and apply exclusively for the local situation and the respective measurement period. A measurement can be carried out at the impact location or at the source location. In common practice the impact location is measured. Measuring aircraft noise is a complex technical process which places high demands on technology and the persons involved for the measurement itself and for the subsequent evaluation. Pursuant to Section 19a of the Air Traffic Act [Luftverkehrsgesetz] every airport connected to the scheduled air traffic system is obliged to measure air traffic noise. These measurements are generally taken at fixed measurement stations. The measurement of air traffic noise is based on a technically and logistically complex and expensive procedure. Until the noise registered at the measurement station becomes a noise level which can be represented, it has to pass through various technical procedures and checks and controls by personnel. The following questions have to be answered,

-    Is the validity of the measurement affected by environmental influences?
-    Is the noise measured actually air traffic noise?
-    Can the sound event be attributed automatically or manually to a certain cause, i.e. an aircraft?

The airport operator has to register and evaluate the data and then make them available to the approval authorities and the air traffic noise commission. Detailed information on the measurement methods can be found here.

How reliable are the results of the measurement?
Despite the use of state of the art technology, there are limits. A measurement accurate down to the decimal place is not currently possible. In the assessment of the Swiss Federal Laboratories for Materials Science and Technology (EMPA) commissioned by the RDF, it is assumed that under standardised measurement conditions for air traffic noise an uncertainty of around 2 dB is unavoidable. This uncertainty can, under certain circumstances, be considerably larger if specifications made for good reason by DIN cannot be observed. Detailed information on uncertainties in air traffic noise measurement is available here.