<< Click to display Table of content >> Navigation:  The Model > The sources > Road sources > Tunnels

The exit of a tunnel is defined by the dimensions of its cross section:

•height h,

•width l,

i.e., LW the level of acoustic power per metre for the group of lanes inside the tunnel, Q the flow, and V the average speed V50.

The acoustic power per metre of the source line placed at the level of the tunnel mouth and halfway up is given by the following formula:

with α = 0.03 for non treated smooth concrete, α = 0.7x in the case of a mouth of a tunnel treated on a proportion x of its perimeter.

where α is the equivalent absorption factor for a road noise spectra in dB(A).

If the two lateral walls are treated:

In the case of a covering of a limited length L, it is necessary to place a source line at each exit.

The acoustic power per meter of each source is:

• LWb        if LWb < LW + 10log(L/21)

• LW + 10log(L/21) if LWb > LW + 10log(L/21)

The second relationship is used in the case of a short tunnel. The total power under the cover is half of the two extremities.

Example :

With a height of 10m, a width of 10m, a road power of 90 dB(A), a Flow of 1100 vehicle, an average speed of 110 Km/h and an absorption α of 0.03,

LWb = 97.67 dBA.

if we choose a lenght of 100m,  LW + 10log(L/21) = 96.99 dBA

the tunnel power will be 96.99 dBA.