6. Monitoring engineering: long-term management

6.1 Factors influencing long-term trends

What determines the noise level of an aircraft heard on the ground? Obviously, it depends on the aircraft itself, its type, weight and engines. Figure 28 shows the average L _{A, max} levels for each type of aircraft as an annual average, i.e. over highly representative samples of several thousand movements for each type of aircraft. Remember that a reduction of 3 dB corresponds mathematically to a division by two of the square of the sound pressure, a reduction of 6 dB to a division by four, and so on.