Process plant noise: Evaluation and control

PROCESS PLANT NOISE: EVALUATION AND CONTROL

PETER SUTTON

Esso Petroleum Co. Ltd, Esso Refinery, Fawley, Southampton (Great Britain)

SUMMA R Y

The first step in the economic control of noise from a complete plant is to evaluate the contribution which each individual item of equipment makes to the total noise level. This involves determination of the sound power level of each source and estimation of the attenuation of noise between the source and the point where noise level is to be controlled. The second step is to identify the possible methods of noise reduction and to select the best by cost~benefit analysis. A computer program, based on the OCMA Specification, is helpful in this work.

INTRODUCTION

The usual basis for justifying expenditure in process plant is that of return on investment. This basis works well in the usual business situations but is not adequate for evaluating expenditure for environmental conservation. For environmental projects the need will be determined by the criteria set by the control authorities or by the good neighbour policy of the company.

In an extreme case the choice may be between carrying out the improvement work or shutting down the plant. If the cost of the required improvement would make operation of the plant totally uneconomic the decision is simple. This situation seldom arises, however. In most cases the criteria can be met at a cost which can be borne by the plant operation. The engineering need then is to find the lowest cost route for satisfying the criteria.

Cost/benefit analysis of neighbourhood noise control measures makes two requirements with regard to noise evaluation. First, there must be a consistent basis for the evaluation and comparison of alternative noise regimes. Second, there must be a reliable method of estimating the effect which treatment of

17 Applied Acoustics (9) (1976)--© Applied Science Publishers Ltd, England, 1976 Printed in Great Britain

18 PETER SUTTON

individual equipment items will have on the total neighbourhood noise level. These two steps will enable the benefit element to be expressed in numerical terms; the cost element can be determined by conventional cost engineering procedures. This paper is concerned primarily with the estimation of.the effect of in-plant silencing measures on neighbourhood noise level.

Frequent reference will be made to the UK Oil Companies Materials Association (OCMA) Noise Specification. ~-3 (See also paper entitled: Specification and Prediction of Noise Levels in Oil Refineries and Petrochemical Plants, by K. Marsh on page 1 of this issue.) This Specification was drawn up to assist the oil industry in the measurement, prediction and control of noise. It specifies measurement and calculation procedures and gives guidance in the setting of limits. The OCMA Specification is believed to be the most comprehensive procedural specification in noise control yet published and it may be used in conjunction with any local, national or industry specifications and regulations (see Marsh paper and reference 4).

NEIGHBOURHOOD NOISE MEASUREMENT AND EVALUATION

The measurement of noise from a continuous process plant in the neighbourhood area presents certain problems. Noise level measured at a distance from a works varies widely with weather conditions (Fig. 1). In addition, the measurements must distinguish between the noise from the refinery and background noise from other sources. Guidance on measuring procedures is given in the Standards ~'6 and in the literature. 7 - 1 o

The A-weighted noise level (dBA) is now generally used for measuring industrial noise in community areas. The Standards 5.6 give procedures for evaluating transient components and noticeable characteristics such as whine, hiss or throb. The correction factors to be applied to the basic noise level for transient peaks and other characteristics are quite high. With process plant noise it is economic to suppress the noticeable characteristics rather than to reduce the overall level by the compensating amount. This means that noise should first be rendered steady and characterless. The median noise level in dB(A) can then be used to evaluate neighbourhood noise from a refinery. 1 o This does not permit precise prediction of annoyance or complaint risk but it does give an indication. There is not, as yet, any more accurate method of prediction available.

SOUND POWER LEVEL DETERMINATION

It is necessary to estimate the contribution to the total noise level made by each individual item of equipment so that the most effective and economical silencing

PROCESS P L A N T NOISE; E V A L U A T I O N A N D C O N T R O L 19

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Fig. 1. Histogram of daily noise level readings taken at a fixed point in the ncighbourhood of a refinery over a period of one year. A reading was taken each night, taking care to avoid local background noises such as passing traffic and wind or rain noise. The noise generated by the refinery was virtually constant, so, variation in measured noise level was a consequence of varying attenuation effects, which, in turn, depended on the weather conditions. The median level was

49.5 dB(A), the 90 per cent level, 53 dB(A) and the highest standard recorded, 57 dB(A).

measures can be selected. To do this it is necessary first to determine the sound power level of each item of equipment.

Many noise measurement specifications deal only with procedures for measuring sound pressure level. As a consequence, much of the equipment noise data readily available are not adequate for the determination of sound power level. The ISO and various industry and national organisations are now drawing up more comprehensive procedural specifications for various classes of equipment. At present, however, the OCMA Specification appears to be the only comprehensive standard applicable to all types of equipment used in process plant.

There are considerable theoretical and practical difficulties in sound power level determination in plant or test-shop conditions. With small individual sources, such as electric motors widely spaced in the open air, the sound power level determination can be quite accurate. With large sources, such as furnaces, airfin coolers and large compressor sets, or with equipment which is closely spaced, it is necessary to use procedures which may be appreciably inaccurate.

The propagation of noise from large sources and the errors inherent in some 1 1 - - 1 5 standard measuring procedures have been discussed in several papers.

20 PETER SUTTON

CONCAWE is at present studying this problem and hopes to be able to derive correction factors to be used in connection with the OCMA Specification procedure.

For the present the OCMA procedure is the best available in most cases. Errors in sound power determination are most likely to be positive, that is to say the estimated sound power level will tend to be high. The error is unlikely to exceed 3 dB.

Pure tones are important in plant noise assessment. When measuring equipment noise it is necessary to note any pure tone components. Numerical evaluation of these requires the use of narrow band analysis. For most practical purposes it is adequate to note whether dominant pure tones are present as judged by ear. I f they are, the octave band or bands containing them can generally be identified from the octave band sound pressure level data. Evaluation of pure tones is discussed in the OCMA Guide 3 and elsewhere. 15

Directional characteristics may be important. This factor is allowed for in the OCMA Specification.

ATTENUATION BETWEEN PLANT AND NEIGHBOURHOOD

When the source sound power level has been determined, the noise level at a distant point can be predicted if the attenuation between the source and the distant point is known. The total attenuation can be considered as being made up of four components:

(i) hemispherical propagation effect (inverse square law); (ii) atmospheric absorption;

(iii) ground effects--absorption, scatter and partial screening; (iv) meteorological effects.

The first component is a matter of simple geometry. Numerically, the value is given by:

D = 20 logR + 8 (1) where: D = attenuation due to distance (dB) and R = distance from source to measuring point (m). The constant 8 is the conversion factor from PWL to SPL at a distance of l m for hemispherical propagation.

Atmospheric absorption has been studied extensively and there is a fair degree of agreement in the literature as to its value. Typical values applicable to temperate climates are given by:

A = 5 R f . 10-6 (2) where: A = value of atmospheric absorption (dB) and f = frequency of the sound (Hz). (See Fig. 1 of paper by Marsh, p. 7 of this issue.)

Ground effects and meteorological effects are far more difficult to evaluate in general terms. The effect of a simple finite screen can be determined theoretically. The effect of ground can be determined also if simplifying assumptions are made

PROCESS PLANT NOISE: EVALUATION AND CONTROL 21

as to the acoustic properties of the ground. However, with a refinery the ground between the plant and neighbouring housing is usually variable in nature and uneven and there is generally some complex partial screening by tankage, bund walls and other plant and buildings. It is not practicable to evaluate the effects of this theoretically so empirical data must be used. Unfortunately, there is very little empirical data available. 16- ~ 8

The usual approach in dealing with these two factors is to determine the total attenuation between the source and the measuring point and to subtract the values for hemispherical propagation and atmospheric absorption. The balance is then termed 'excess attenuation'. This is made up of ground effects and meteorological effects. The values taken by OCMA for representative refinery situations are shown in Figs. 2 and 3 of the paper by K. Marsh on pp. 8 and 9 of this issue. These are considered to be typical average values over a range of meteorological conditions and so the meteorological factors can be regarded as self-cancelling. It is thus reasonable to refer to this average excess attenuation as 'ground effects'. Ground effects will clearly depend on noise source height above ground level; the notes accompanying Figs. 2 and 3 of Marsh's paper show how this is to be taken into account.

Distance is an important factor in noise reduction. It is therefore desirable to locate the noisy process equipment as far as possible from the refinery boundary. It is logical to locate buildings, tank farms and bund walls in the intervening space. Landscaping which involves use of earth walls or natural hills between plant and housing is helpful.

The use of tree screens for noise reduction is sometimes advocated. Trees as a part of landscaping to conceal the plant visually are probably of psychological value. Dense tree screens of considerable depth are of some value for high frequency noise attenuation. 19, 2 o Data for excess attenuation by trees is generally based on tests where the noise source and the receiver are both close to the edge of the trees. The effect of a 30 m (100 ft) thick planting in the middle of, say, 300 m (1000 ft) total distance between a refinery and nearby housing would be less than that shown in Fig. 2. In any case, the frequencies above 2 kHz suffer so much atmospheric attenuation that they contribute little to total noise over distances of say 300 m or more. In field work it is very difficult to distinguish between the various factors which make up the total excess attenuation but on balance it seems that conventional tree planting gives little benefit in noise reduction. Theoretical con- siderations suggest that trees would have little effect at low frequency but would give greater attenuation at higher frequencies. This is supported by the rather scanty evidence in the literature. Figure 2 suggests that attenuation at low frequency is less through forest than over open flat land; this is unlikely to be true and is probably accounted for by differences in experimental method and in assumptions made as to atmospheric attenuation effects.

22 PETER SUTTON

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CALCULATION OF NEIGHBOURHOOD NOISE LEVEL

The noise level at any point in the neighbourhood of a refinery can be calculated if the individual source sound power levels and the attenuation between the plant and the point are known. The preceding sections have reviewed the OCMA procedure for determining source sound power level and attenuation, and have also mentioned the inaccuracies and uncertainties involved.

It is impossible to predict neighbourhood noise level to the nearest decibel but experience in Europe has shown that the OCMA procedure gives a useful approxi- mation (see Marsh's paper and reference 4). Figure 3 shows the relationship between measured and calculated neighbourhood noise levels for several sites. The measured levels shown are the median values f rom a large number of measurements taken at each location over a period of time. The spread of readings was typically + 10 dBA at each site.

There are several local factors which may affect the specific excess attenuation values for a particular site. In Fig. 3 it will be seen that the predicted noise levels

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Fig. 3. Comparison of noise levels predicted from on-site PWL determination using the OCMA Specification and measured noise levels (taken from the paper by Marsh on pp. 1.15 of this issue, with some additional data). Each measured noise level represents the median of a number of

readings at each point. The readings plotted in Fig. 1 relate to the location marked P here.

at Refinery D are generally higher than the median measured levels. This may be because, in this case, all the measuring points were to the south or west o f the refinery and at that location the wind direction is more frequently in the south-west quadrant than in any other quadrant . The predictions for refinery F are low. In this case there was flat land with very little screening between the plant and the

24 PETER SUTTON

houses but the OCMA 'significant screening' attenuation values were used. If the 'minimal screening' attenuation values had been used the predicted levels would have been about 3-5 dB higher, giving closer agreement.

If detailed analysis is to be made with a view to optimisation of noise control expenditure in a refinery, some site survey work must be done. It is necessary at the very least to make a full plant sound power level survey and a neighbourhood sound pressure level survey. The neighbourhood sound pressure level survey should preferably extend over a period of not less than six months including summer and winter periods. It should include not less than 25 sets of readings at each point covering a representative sample of weather conditions. Calculated and measured neighbourhood noise levels can then be correlated in octave band and A-weighted terms. It may be worthwhile also to carry out an attenuation study using synthetic noise sources. This also must be done over a period of time so as to cover a representative range of meteorological conditions.

EVALUATION OF PLANT NOISE SOURCES--COMPUTER PROGRAM

To find the best noise reduction plan in cost/benefit terms it is necessary to determine the significance of each plant noise source and to predict the effect of each of the possible alternative noise reduction steps. In a whole refinery the number of noise sources is so large that a computer must be used.

A computer program can be written using appropriate values for attenuation. These would consist of eqns. (1) and (2) together with values for additional attenuation. The graphs given as Figs. 2 and 3 of the paper by Marsh (pp. 8 and 9 of this issue) may be used and best fit equations derived; these may be modified if adequate site data are available. The computer input should include, for each so urce:

(i) a code reference; (ii) octave band sound power levels;

(iii) co-ordinate of the source location; (iv) elevation of the source.

It should also include the co-ordinates of each neighbourhood point to be investigated.

The print-out should first tabulate the input data. It should then tabulate under each neighbourhood point in turn:

(i) each source code reference; (ii) each source octave band SPL and total SPL (linear);

(iii) each source octave band SPL A-weighted and total SPL A-weighted. (iv) total SPLs and A-weighted SPLs for all sources together.

Table I is an example of a computer print-out of this type. A more sophisticated program may be written to include such operations as


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allowance for noise source directivity, evaluation of pure tone perceptibility, calculation of A-weighted sound power levels and of source power level from reference radius pressure level or other source pressure level data. I t may be found convenient to include the prescribed noise limits in the input. This would be helpful if the limit is not the same for all neighbourhood points, so that the appropriate limit can be included in the print-out tabulation under each point.

As an example of the use of a simple computer program, the plant expansion project given in the OCMA Guide as a calculation model can be used. This project consisted of:

1 furnace; 2 sets of airfin coolers: 1 elevated, 1 at grade; 2 sets of pumps; 1 compressor and motor.

One neighbourhood point 500 m away was used and the noise limit was 45 dB(A). The ground effects values in Fig. 2 of Marsh 's paper (see p. 8 of this issue) were used. The print-out, Table 1, uses this plant model.

The A-weighted SPL print-out makes it easy to identify which sources make the greatest contribution to the A-weighted level and in which octave bands. I f an octave band limit is set, this can also be investigated. I f there is no separate octave band limit, it is convenient to make up one based on the overall A-weighted limit. This facilitates the identification of dominant source/octave-band figures. This can be done by setting out the A-weighted octave band spectrum which has a value at 1 kHz equal to 5 dB below the overall A-weighted limit. In this case the 1 kHz octave band limit would be 45 - 5 = 40 dB. With eight separate sources a guide A-weighted octave band limit for each source would be 5 to 10 dB below the total, say 40 - 5 = 35 dB. In the print-out (Table 1) the octave band levels which exceed this guide limit have been italicised in the A-weighted SPL columns.

A useful addition to the simple program is the facility to delete any individual source and substitute the octave band sound power levels corresponding to that source when silenced. The print-out will then show the effect of silencing each source in turn. It can also show the effect of silencing several selected sources together. The effect of noise reduction on each of the air fins, E1 and E2, is shown in Table 2. It is assumed that alternative fans with a sound power level 5 dB lower than that of the original fans can be used. Modifying E1 reduces the total noise level from 51 to 50 dB(A). Modifying E2 shows no effect, The reason for this is that E2 is at grade and is subject to greater ground effects attenuation than El . E2 thus has less effect on total neighbourhood noise level than El .

The print-outs here are shown with values given only to the nearest 1 dB for clarity. In practice it is necessary to print out values to the nearest 0.1 dB. Calculated noise levels cannot be accurate to the nearest 1 dB in fact but it is realistic to express the effects of small changes to the nearest 0.1 dB if these are regarded as comparative, not absolute, values. I t is necessary to work to this level to compare

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alternative noise reduction options. In the case of the air fins in Table 2, the effect of silencing E1 is about 0.8 dB, and of E2 about 0.3 dB.

The worked example in the OCMA Guide shows the effect of the following silencing measures:

furnace: conversion from natural draft to forced draft; air fin coolers: use of lower noise fans; pump motors: use of silencers; compressor: enclosure of compressor.

Each of these changes can be evaluated by the computer in the same way.

WORK AREA NOISE

At this point it is appropriate to briefly consider work area noise. In most refineries there is an incentive to control work area noise as well as neighbourhood noise. In the OCMA example the silencing measures employed reduced the work area noise to below 90 dB(A) as well as reducing neighbourhood noise to below 45 dB(A).

The work area noise levels for the original plant and the silenced plant are shown in Fig. 4. An excess above 90 dB(A) is accepted inside the compressor enclosure. This is reasonable because it is easy to ensure that personal protection is worn in such clearly defined and restricted areas. I f high noise areas are accepted, it is necessary to consider the demands of operation and maintenance by, for example, taking control panels and auxiliaries outside the enclosure.

Figure 4 shows an example of a typical work area noise survey. The 90 dB(A) isopleth is plotted, together with representative spot readings in the high noise and low noise areas. Octave band readings would be taken at the peak noise locations--marked A and B in Fig. 4(A)--so that the adequacy of alternative forms of personal hearing protectors could be checked.S

In Fig. 4(A), it will be seen that El does not give a noise level above 90 dB(A) at grade, while E2 does. This is because E1 is elevated (7 m above grade). The noise level on the access catwalks under the E1 fans would be about 91 dB(A). For this reason a full noise survey would give noise levels on platforms as well as at grade. This generally involves using two or more sheets for each complete plant survey.

COST/BENEFIT ANALYSIS OF ALTERNATIVES

With the minor plant addition project discussed in the previous sections, the number of options available is limited. When dealing with a major project such as a complete pipestill or reformer, or with a whole refinery, there will be far more alternatives to consider.


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30 PETER SUTTON

As an example of the approach which can be used, consider an established refinery. The average noise level at the nearest housing is 53 dB(A) and it is required to reduce this to 50 dB(A). Energy consumption must be considered and operating cost credits and debits are to be taken into account, as well as capital cos t .

A full plant noise survey is first made. Noise level measurements are made as necessary to determine the sound power level of each item of equipment. Work area noise levels would be measured at the same time so that the effect on work area noise of any silencing measures considered could be estimated. Neighbour- hood noise level data, in both dB(A) and octave band terms, will have been obtained already to establish the existing average noise level.

The sound power data and equipment locations are put into the computer and the print-out of neighbourhood point noise levels is obtained. This is then compared with the measured levels. Exact agreement will not be obtained; differences of up to _ 5 dB must be accepted. If the calculated level at the controlling point is, say, 55.2 dB(A) against an average measured level of 53 dB(A), this is satisfactory. It can be assumed that silencing measures which give a calculated reduction of 3 dB(A) will give a reduction of close to this amount in fact. Thus a calculated level of 52.2 dB(A) at this point will correspond to a measured average level of 50 dB(A).

The various items of equipment which make the greatest contribution to the noise level at the controlling point are identified from the print-out. The practicable methods of noise control are then considered and the effect of each one is evaluated by the computer. The approximate cost of each of these measures is estimated.

When selecting items for neighbourhood noise reduction work area noise should also be considered. Thus, if there are alternatives available for neighbourhood noise reduction with similar cost/benefit values, the items which give greatest benefit in the work area should be selected. The design of measures selected for neighbourhood noise reduction should be consistent with the desired work area noise targets in any case.

The following are examples of noise reduction measures which may be available. 21 Similar alternatives would be available at the design stage of a new project. Costs involved in conversion of existing equipment are likely to be higher than the incremental costs of including quiet equipment at the design stage.

Furnaces Natural-draft furnaces can be fitted with muffled burners. It is generally not

satisfactory to fit silencers to existing burners but replacement burners with integral silencers may be acceptable. An alternative arrangement is to fit new burners in a silenced plenum chamber.

The more expensive but technically preferable method is to use forced draft burners. A forced draft system is inherently quieter than a natural draft system.


Additional silencing can be incorporated by treatment of fan intake and trunking. There is some additional operating cost since power is required for the fan but there are far greater savings available as a result of the increased efficiency from better combustion air control. Conversion of existing furnaces may in some cases permit increased heat duty and hence increased plant throughput. With a new furnace, forced draft may allow design for higher heat density and thus give a smaller furnace with lower capital cost.

Airfin coolers There is now a wide range of fans available. The quieter the fan, the more

expensive it is in capital cost. The efficiency is not necessarily related to noise level so there may be no significant difference in running cost between quiet and noisy fans.

One method of reducing noise in a new unit is to use more tube rows. This reduces the amount of air to be moved and hence tends to reduce fan power. The effect of sound power is not great but it may be achieved at zero effective cost since, although the capital cost is higher, the operating cost is lower.

Compressors Generally speaking, the only available means of noise control are enclosures

and in-line silencers or pipe lagging. These may be expensive but they have a negligible effect on operating cost.

Electric motors Alternative quieter designs may be available at little extra cost. In particular

quieter cooling fans may be available. The cost of integral silencers can be small compared with the cost of the motor. Remedial silencing after installation is often more expensive and troublesome.

Steam vents, control valves, etc. A variety of silencing measures is available. These are unlikely to incur significant

operating costs. In the case of the refinery envisaged here Table 3 gives the alternatives identified,

together with the cost and effect in each case. From the list in Table 3, the items marked * are selected. These are the best

cost/benefit items, rating less than £50k per dB(A) (except for the compressor where there is an added incentive of noticeable pure tone suppression).

The total reduction required is 3 dB(A). The arithmetic sum of the benefits shown in Table 3 for the selected items * is 2-3 dB(A). However, the calculated total benefit for these five items taken together is 3.1 dB(A).

This anomaly is a consequence o f the use of a logarithmic scale. The subtraction of a decibel quantity in fact represents a proportional or percentage reduction.

32 PETER SUTTON

TABLE 3 POSSIBLE NOISE REDUCTION MEASURES IN AN EXISTING REFINERY

Benefit Cost Cost/Benefit Item (dB(A)) if.k) (£k/dB(A))

Furnace No. 1 burner muffs forced draft*

Furnace No. 2 burner muffs* forced draft

Airfin No. 1 fan change 1" fan change 2

Airfin No. 2 fan change I fan change 2

Compressor set No. l enclosure*

Pump motors group 1 (20 items) Pump motors group 2 (30 items) Steam vent silencer*

1.3 25 20 1.4" 90 -- 60 = 30 (1) 20* 0.2* 10 50 0.2 50 - 20 = 30 (1) 150 0.3* I0 35* 0-4 30 70 0-1 10 100 0.1 30 300

0-1" (2) 10 100 (2)* 0-2 10 50 0.1 15 150 0.3* (3) 5 15 (3)*

Notes: (1) Nett operating credit from increased efficiency is represented by an equivalent capital sum

subtracted from gross cost. (2) Source of audible pure tone, therefore enhanced benefit. (3) Noticeable individual source, therefore enhanced benefit. * Selected items.

For comparison purposes, each individual reduction is calculated as a single change on the base level of 53 dB(A). I f several changes are made in sequence, the base level is reduced successively for each change after the first.

Thus the total effect of several noise reduction items taken together is greater than the arithmetic sum of the effects of the items taken individually against the initial case. This is another reason for using a computer program for noise work since there is a considerable amount of calculation involved in studying all the possible alternatives individually and collectively.

CONCLUSION

It is possible to calculate the noise level in the neighbourhood of a refinery or other similar plant with a useful degree of accuracy using on-plant measurements or predicted equipment sound power levels at the design stage.

The OCMA procedure, which has been in use for over four years, was developed largely out of a procedure introduced by the author at Fawley (UK) refinery about ten years ago. The OCMA document is believed to be still the only published procedure adequate to deal with all refinery plant. It contains a number of innova- tions, possibly the most outstanding of which is the standardised allowance for excess attenuation classified as ground effects. Values for ground effects similar to those adopted by OCMA were first published in 1968.15

The OCMA Specification is still open to some criticism on both theoretical and


practical grounds. CONCAWE is working with OCMA on the main problem areas of large source sound power level determination and improvement of attenuation values. Both OCMA and CONCAWE would welcome information on the use of this or any similar specification and comments on, or criticism of, its usefulness and accuracy.

REFERENCES

1. Specification No. NWG 1 (Rev. 1): Procedural specification for limitation of noise in plant and equipment for use in the petroleum industry, Oil Companies Materials Association, Cecil Chambers, 86 Strand, London WC2, 1972.

2. Specification No. NWG2 (Rev. 1): Equipment Vendor's Extract from NWGI. 3. Publication No. NWG3 (Rev. 1): Purchaser's and Contractor's Guides to use o f NWGI. 4. D. S. TOWNEND, Ann. Occup. Hyg., 14 (1971) pp. 101-7. 5. 1SO Recommendation R1996. Assessment o f noise with respect to community response, Inter-

national Standards Organisation, 1971. 6. BS 4142, Method o f rating industrial noise affecting mixed residential and industrial areas,

British Standards Institution, London, 1967. 7. P. SUTTON, Ann. Occup. Hyg., 14 (1971) pp. 109-17. 8. P. SUTTON, Protection handbook o f industrial noise control, Alan Osborne & Associates

(Books) Ltd, Unit 5, Seager Buildings, Brockmili Road, London SE8, 1974, 2nd ed., 1975. 9. Guide to the evaluation of noise around refineries, Stichting CONCAWE, The Hague, 1968.

10. Community response to refinery noise, Stichting CONCAWE, The Hague, 1972. 11. Z. MAEKAWA, Applied Acoustics 3(3) (1970) pp. 225-38. 12. G. HUBNER, J. Acoust. Soc. Amer., 54(4) (1973) pp. 967-77. 13. L. SCHREInER, Erdol und Kohle-Erdgas-Petrochemie rereinigt mit BrennstoffoChemie, (10)

(1972) 594-8. 14. L. SCHREIBER, No~e around large open industrial plants, Inter-Noise 73. Copenhagen, 1973. 15. P. Su'r'roN, J. Sound Fib, 8(1) (1968) pp. 33-43. 16. P. H. PARKIN and W. E. SCHOLES, J. Sound Vib., 1(1) (1964) pp. 1-13. 17. P. H. PARKIN and W. E. SCHOLES, J. Sound Vib., 6(3) (1967) pp. 424--42. 18. A. H. MIDDLE'ION and J. G. SEESOLD, Propagation of machine generated sound within and

around a p r o c e s s plant, Inter-Noise 72, Washington, 1972. 19. D. AYLOR, J. Acoust. Soc. Amer., 51(1) (1972) pp. 197-205. 20. G. REETHOF, J. Amer. Pol. Cont. Assoc., 23(3) (1973) pp. 185-9. 21. P. SUTI"ON, Chem. Eng., (237) (1969) pp. 119-29.

Process plant noise: Evaluation and control

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