Understanding L_den

During Lecture 6A, Prof. Snellen introduced the L_den metric as a method to combine the noise levels from all aircraft over an entire year. Let’s look into this parameter a little deeper and try to calculate a value for L_den using several different calculation steps.

Important parameters for L_den

Recall from the lecture that the parameter L_den takes into account flight movements during three different segments during the day: daytime, evening and night-time. These are defined as follows:

Daytime: 07:00 – 19:00h
Evening: 19:00 – 23:00h
Night: 23:00 – 07:00h

For the evening and night-time flights a penalty of +5 and +10 dBs is applied respectively. The value of L_den is calculated for an entire year, consisting of 365 days of 24h each.

Definition of L_den

To work with L_den, it is convenient to represent the matric in the form of an equation. One way to do this is by using a simplified form. This takes into account the different flight movements over an entire year and the penalties applied during the different time periods, while assuming that the noise from all other sources is negligible. Such an expression is given by:

where:

Δt = time duration of each noise producing flight movement [s]
L = the noise level per aircraft [dB]
Nd = total amount of flight movements per year during daytime [-]
Ne = total amount of flight movements per year during the evening [-]
Nn = total amount of flight movements per year during night-time [-]

Example calculation for L_den

With the equation for L_den now known, let’s try to solve an example problem.

At a certain airport the amount of airplanes that pass over each hour is recorded. For this location it turnes out that a total of 25 planes pass over per hour during daytime, 4 planes per hour during the evening and 3 planes per hour at night. Each planes produces a 70 dB noise for 25 seconds. Can you calculate the value of L_den using these values?