PA system on Oil Platform

This is an example of calculations with a PA voice alarm system for an off-shore oil platform. The background noise is assumed to be 85 dB(A) with a pink noise spectrum, i.e. the same SPL in all octave bands. Even though speech is played back from the loudspeakers of the PA system, pink noise spectrum has to be assumed instead of the male speech spectrum.
 
The reason is that this kind of loudspeaker has a very limited frequency range, and sound is mainly emitted in the three octave bands 500 – 2000 Hz. As a result the the frequency response of the speaker dominates over the frequency response of speech. For security reasons, there are two independent loudspeaker installations, and calculations has to be made for each of them and for both together. Only one of them is active in this example.
oil1

For the simulations, both point and surface sources have been used. The PA system is represented by a series of point sources and noisy structures are presented by surface sources. The main oil platform geometry has been placed inside a box with walls of 100% absorption to represent the open air. ODEON can be used for open air situations as long as the geometry is enclosed in a dummy box. Less than 20% of the rays used in the simulation are allowed to escape from a model via unintended openings, otherwise ODEON cannot perform the calculations and displays a warning message. However, there is no problem with rays being absorbed at the dummy box’s surfaces.

oil1

Grids are defined on all floors of the platforms and a color map of SPL and STI is calculated in the job list.

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The regulation implies that STI shall be at least 0.35 in the area where people can have access, but at the same time a 1 kHz alarm tone shall produce a SPL that does not exceed 115 dB. The number of loudspeakers, their position and power level must be selected in order to fulfill these requirements as closely as possible. Both SPL and STI results are shown in the figure.