This specification defines the procedures for controlling noise in plant and equipment for use in the petroleum, petrochemical and allied industries; it applies both to the design and construction of new plants and to the modification of existing.plants where noise reduction is required. Noise control is a continuing process which starts a t the earliest stage of design and carries on throughout the operation of the plant; it is a process which requires close cooperation between the Principal, Contractor and Equip- ment Vendor. Essentially, the specification deals with the stages up to the commissioning of the plant or equipment, but it i s sufficiently comprehensive to provide guidance on noise control during opera- tions, particularly in the diagnosis of sources of neighbourhood nuisance.

The acquiring and reporting of noise data in a standard manner is a key feature of design for noise control and is a field in which there is an urgent need for uniformity. This specification is there- fore intended to provide standard methods for doing this.

Although the measurement and calculation procedures in this specification are practicable and adequate for use in most cases, special situations may be met where they cannot be employed precisely as specified. In such cases it may be necessary to modify the procedures. An explanation of the theory and philosophy behind the procedures is therefore included so that a consistency of approach and accu- racy can be maintained. Some special cases of noise-level prediction - particularly indoor situations where enclosures or screens are to be employed - are beyond the scope of this specification, and the methods of calculation described in standard texts must be employed.

This document is Revision 2 of the OCMA Specification originally published in 1971 with the title "Procedural Specification for Limitation of Noise in Plant and Equipment for use in the Petroleum Industry". It contains many changes in text and replaces Revision 1 issued in October 1972. These changes are the result of experience in the use of previous documents in the petroleum and petro- chemical industries and the publication of new technical material in the intervening years.

The aim o f this revision has been to present a more complete version of the measurement and calculation procedures, and this has extended the document so that it may appear complicated to new- comers t o the subject. OCMA has therefore prepared a further document, NWG-3. which gives a guide to the use of the specification. It describes the principles to be applied in selecting noise limits and gives worked examples of the calculation methods.

The member companies of OCMA declare that they are at liberty to buy goods which do not comply with this specification whenever they desire to do so.

Part 1 defines the scope of the specification and the division of responsibilities between the three main parties in a construction project; the Principal, Contractor and Equipment Vendor. In any given project there may be contractual requirements which define in detail the relationship between these parties, particularly with reference to performance guarantees, and it is not within the scope o f this specification to provide a precise legal contract for the project. Nevertheless. the general framework given here defines the essential features of any such contract.

The notation and terms used in this specification have been chosen as far as possible to be in accordance with current international practice.

Patt 2 defines the mezsurement and reporting o f equipment noise; i t s aim is to specify measuring pro- cedures which can be used in the practical situations encountered in plants and factories. These pro- cedures are based on the use of a precision sound-level meter equipped with octave-band filters a n d an accelerometer; equipment which could be readily available in plants and factories.

They include a correction for semi-reverberant environments: for small wurces this is derived from readings on two measuring surfaces (so that no special equipment is required). Where this is not practicable, alternative methods are proposed which are based on the use of a partial second surface or the Room Constant, in descending order o f merit.

The preferred measuring surface is a hemisphere in which t h e source is small in relaiion t o the measuring distance; this is called the "small-source method". The use of ihis method is not determined bv the physical size of the source but by the nature o f the environment - the absence of high background noise and o f physical obstructions, so that measurements can be made sufficiently far from the source.

Where it is cot possible to get far enough away from the source - because of background noise or physical Obstructions ~ it is necessary IO measure in the geometric near field of the source, This means that the source dimensions are large compared with the measuring distance, so it is called the "large- source method". The preferred measuring surface is then the conformal surface (a cuboid with rounded corners and edges1 Eut if the source radiates unevenly it is necessary i o divide ihis inio separate s u r . faces (called "measuringmrface zones"), to calculate their wund-power levels individually. and then t o add them up. There are still some sources which da not suit this method usually they have identifiably separate radiating components - and these have t o be treated as a composite o f sources, using the small- source or large-source methods as appropriate. A procedure is also given for determining the sound- power levels of linear sources.

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