Page 71 - samyn_ebook2

69
runs overhead acts as a reflector for the projected sound. Vertical speaker
arrays are placed between the columns of the footbridge at the points of
support in the vicinity of the elevators.
- Horizontal loudspeaker arrays are placed at large distances on the platforms
shielded by a glass canopy outside of the cover. An added advantage of
speakers placed outside is the reduction of the noise pollution level for the
neighbouring inhabitants because of the concentrated spread of the sound
signal.
- A number of small, classic loudspeakers are placed in areas which can not
be reached by the loudspeaker arrays:
- The heated refuges;
- The elevator areas, both on the platforms and on the footbridge.
Figure 17 shows the lay-out of the loudspeakers of the Public Address (PA)
system.
3.4 Wind control study
Draft and suction lift: study of the microclimate underneath the cover
The comfort of passengers and personnel is largely dependent on the microclimate
underneath the cover. Avoiding great air currents (draft) under the cover in the
direction of the railroad tracks is of particular importance. The design provides a
number of active measures in this respect.
The various elements of the platform layout (pedestrian bridge, elevators, shelters,
signs…) act as air stream obstacles. The air current resistance under the cover is
increased which lowers the flow rate and the air velocity.
It has proven to be disadvantageous to increase the height of the cover in the
centre while keeping the area at the narrow sides equal. It would seem that a
higher roof level would decrease air velocity, because the total area of air stream is
increased. This potentially beneficial effect is largely countered by an increase of
the flow rate, a result of the lower air current resistance caused by the higher roof.
The curve of the arch of the cover was not severely increased for this very reason.
A very sharp curve of the arch would also cause turbulent air streams under the
cover, resulting in higher air velocities.
Due to the absence of large obstacles in the vicinity of the station, the air pressure
difference between the entrance and exit openings of the cover is very slight.
Combined with the existing air flow resistance under the cover and the specific
shape of the roof, this results in inside air velocities lower than the outside wind
velocity.
Expanded metal wire mesh is utilized in the construction of the pointed tips at the
narrow sides of the cover. This material filters the strong winds common in the
station district. It also breaks the pressure wave caused by passing trains.
The fact that, in this way, the openings at the narrow fronts are reduced to a minimal
size, has a particularly beneficial influence on the microclimate underneath the
cover.