5.2.3 LOUDSPEAKER SELECTION AND SOUND PRESSURE LEVEL
The calculaon of sound pressure level (SPL) is also dependant of distance from the loudspeaker:
The calculaon is according to the formula:
SPL =A+(10·LOG
10
(B))-(20·LOG
10
(C))
WHERE:
SPL: Sound Pressure Level gure at a specic point [dBA]
A: Loudspeaker specic data - SPL gure at 1W / 1m (according to loudspeaker data sheet) dBA]
B: Loudspeaker specic data – Output power ((tapping according to loudspeaker data sheet) [W]
C: Distance from the loudspeaker to the measurement point [m]
FOR EXAMPLE:
The mounted loudspeaker is a DNH HP15T, (108 dBA at 1W/1m. The distance to the loudspeaker is 10m and the tapping of
the loudspeaker is 4W,
The SPL in that point is calculated with A=108 B=4 and C = 10,
THUS:
SPL =108
dBA
+ 10·LOG
10
(B) - 20·LOG
10
(C) = 94
dBA
Please note that under no circumstances are sound pressure levels in accessable spaces
dBA
.
5.2.4 NUMBER OF LOUDSPEAKERS
To determine the required number, type and tapping of loudspeakers in a Compact MPA system it is necessary to make an
installaon SPL requirement chart over the site/vessel. Within this chart noise gures must be known or esmated. This,
along with the environmental requirements will form the basis for the loudspeaker selecon.
When this is prepared, an SPL calculaon can be performed to nd the requirement for loudspeaker density. The SPL formula
in secon 5.2.3 can be used to establish the number and type of loudspeakers. This will also include informaon about
tapping and output power.
It is also vital to take duplicaon into consideraon. Whenever the Compact MPA system handles General Alarm or the vessel
is intended for passenger transportaon the system shall be fully duplicated (A & B system) using separately located central
equipment with physically separated cables and loudspeaker segregaon. All spaces shall be covered from both duplicaon
loops.
5.2.5 CABLE ARRANGEMENT
In a Compact MPA system that handles General Alarm (PA/GA) or if the vessel is intended for passenger transportaon each
loudspeaker distribuon cable shall be organised as a closed loop starng and ending at the central equipment point. This
increases redundancy regarding in the event of cable problems.
5.2.6 ZONES
In most installaons there is a requirement for dividing the system into dierent paging zones. The Compact MPA has 6 zones
available for paging separaon. Each zone is individually controlled from the MPA 1601 control unit.
The zone selecon is important for loudspeaker distribuon and the cabling plan preparaon.
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