Sound Pressure Level Calculation

1. 1.1 SOLAS RULES

The new IMO/SOLAS rules, compulsory for new vessels built after 1st July 1998,specify strictly rules to be follow for General Alarm / Public Address System.

M.S.C. ( Maritime Safety Committee )66/24/ Add. 1

General Emergency Alarm system

- The General Alarm system shall be capable of sounding the generalemergency alarm signal consisting of seven or more short blasts followed by onelong blast.

- The system shall be capable of operation from the navigation bridgeand also from other strategic points.

1.2 The system shall be audible throughout all the accommodation and normal crew working spaces.

- The alarm shall continue to function after its has been triggereduntil it is manually turned off or it is temporarily interrupted by a messageon the public address system.

- The minimum Sound Pressure Levels for the Emergency Alarm tone ininterior and exterior spaces shall be 80 dB (A) and at least 10 dB (A) above the ambient noise levels duringnormal equipment operation with the ship underway in moderate weather.

1.3 The Sound Pressure levels at the sleeping position in cabins and incabin bathroom shall be at least 75 dB (A) and at least 10 dB (A) above ambientnoise levels.

M.S.C. ( Maritime Safety Committee ) 66/24/ Add. 1

Public Address system

- The Public Address system shall be a loudspeaker installationenabling the broadcast of messages into all spaces where crewmembers orpassengers, or both, are normallypresent and to muster stations.

- It shall allow for the broadcast of messages from the navigationbridge and such places on board the ship the Administration deems necessary.

- It shall be installed with regards to acoustically marginalcondition and not require any action from the addressee.

- With the ship underway in normal conditions, the minimum SoundPressure Level for broadcasting Emergency Announcements shall be:

* In interiorspaces 75 dB (A) and at least 20 dB (A) above the speech interference level.

* In exteriorspaces 80 dB (A) and at least 15 dB (A) above speech interference level.

1.4 Sound Pressure Level

All loudspeaker data sheet should quotethe sound pressure level of the loudspeaker measured in decibels. Increase ofSPL

The basic rule is that each time thepoints of diffusion aredoubled the S.P.L. increase by 3 dB (A).

Sample:

N. 10 Loudspeakers with SPL of 70 dB (A)

N. 20 Loudspeakers SPL will become 73 dB(A)

N. 40 Loudspeakers SPL will become 76 dB(A)

Increaseof SoundPressure Level

1.5 Decrease of SPL

The basic rule is that each time thedistance is doubled the S.P.L. decrease by 6 dB (A). ( in open space )

Loudspeakers with SPL of 90 dB (A) at 1 meter

will become 84 dB (A) at 2 meters

will become 78 dB (A) at 4 meters

will become 72 dB (A) at 8 meters

will become 66 dB (A) at 16 meters

will become 60 dB (A) at 32 meters

will become 54 dB (A) at 64 meters

will become ONLY 48 dB (A) at 128 meters