Engineering Method of Sound Power Determination for Marine Engines

Transcription

1 Engineering Method of Sound Power Determination for Marine Engines Minoru Ka1nata**, Hideo Ikeda***, Nobuhiro Baba*** The subject of this paper is the method to evaluate the sound power emitted from marine diesel engines, which is based on the sound pressure determination. In the highly reverberant sound field such as at engine room in a ship, accuracy of the sound pressure based methods tends to get worse. Through the data analysis of ten measurements, the authors propose a new method for the sound power evaluation in which the environment correction factor is estimated from PI index. This approach, named the hybrid method, is shown to have both precision and practicality. 1. Introduction The degree of noisiness of the sound is generally measured by the sound pressure level, but the sound power level is used as the index in order to evaluate the intensity of the sound radiated from a machine. This is because the sound pressure is diversely changed from the condition where only the direct sound is present (equivalent to the anechoic chamber or the extensive open-air environment) to the condition where the reflected sound from the boundary is large (the in-board engine room etc., or a reverberation chamber with the diffuse sound field in an extreme condition) according to the environment where the machine is placed, and the sound pressure can not be the index for the sound intensity. The sound intensity measuring technology has been disseminated in various fields as the method to directly measure the sound power, and the standards about measuring procedure are discussed in ISO 1). 2). However, special equipment is necessary for the intensity measurement, and the intensity measurement is time-consuming. Thus, the sound power determination method 3). 4). 5) based on the sound pressure which has been conventionally used is employed. Few applications of the intensity measuring technology to the super-large sound sources such as large main engines for marine use have been reported, or there is no guidance to the power determination in the sound field where a large sound source is present in a *Translated from Journal of the MESJ Vol. 31, No. 6 (Manuscript received Feb. 6, 1996) Lectured OCT. 11, 1995 ** University of Tokyo (Bunkyo-ku, Tokyo) ***Nippon Kaiji Kyokai (Midori-ku, Tiba City) narrow space such as the engine room. In these backgrounds, authors have examined the sound power determination method for the marine engines focusing on the analysis of the measurement results of actual engines. The intensity measurement which was achieved for the marine diesel generators was summarized in the reference 6). A new determination method referred to as the hybrid method is proposed in this report from the recognition that a convenient method for measurement with practicality is requested to be specified after various examinations and discussions. The history of various standards on the measuring methods and the outline of the conventional determination methods, and the fundamental ideal leading to a new determination method are shown in the reference 7). So in this report, the theoretical aspect of the new determination method and the detailed examination based on the data of actual measurement are described, and the guidance of the sound power determination of the sound source such as the engines and equipment for marine use is established. 2. Measuring Methods In this chapter, various sound power determination methods are briefly described, and the advantages/ disadvantages of each method are examined from various aspects. 2.1 Sound Pressure Method In the sound pressure method, the mean sound pressure level is calculated from the measurement of the sound pressure, and corrections are made thereto to obtain the power level. This method is classified into three types; the method which is based on the free sound fields, the method based on the diffuse sound field, and the comparative method with the known power level. February 1997 (1)

2 2 Minoru Kamata, Hideo Ikeda, Nobuhiro Baba Good accuracy can be got when the sound power measurement is carried out in special environment such as an anechoic chamber or a reverberation chamber (ISO3741, 3745). However, the data must be corrected according to the sound field condition of the measurement site, and the total accuracy is determined by the variation of sound field and the accuracy of the correction terms. Three kinds of formulae to calculate the power level are shown below. <Free sound field method> Lw=Lp+10log S S 0 -K (1) <Diffuse sound field method> Lw = Lp 10log T T log V V 0-14 (2) <Comparative method> where, Lv = Lwr + (Lp - Lpr) (3) Lw: sound power level of the sound source Lp: mean sound pressure level by the sound source Lwr: sound power level of the reference sound sourse Lpr: mean sound pressure level by the reference sound source S: measurement surface area of the hypothetical parallelepiped, So = 1 m 2 K: environmental correction factor V: volume of the measurement room, Vo = 1 m 3 T: reverberation time of the measurement room, To=1s The back ground noise level affects to the accuracy of every method. Though the effect of the sound from extraneous sound sources is the problem which can be solved only by the sound intensity method, it can be treated by using appropriate correction value in the conventional treatment if the sound pressure level is lower than that of the sound source to be measure by 3 db or more. Then, there is another problem how many points must be used to get the mean sound pressure level. This depends on the size of the sound source to be measured, and in the case of a super-large sound source such as a large engine, it is necessary to confirm if the number of the points of measurement and the condition of the sound field are sufficient to calculate the mean level. The standards8} for the internal combustion engine which were specified in 1987 by DIN, four kinds (9, 12,15, and 19 points) of number of measurement and measuring position are specified according to the size of the sound source, and the subsequent standards such as ISO generally followed them. In many standards, the measurement distance between the measuring positions and the surface of the sound source to be measured is specified to be at lm, but it is difficult to strictly satisfy this requirement in a difficult case where the shape is irregular and not like a rectangular parallelepiped. In the free sound field method, the environmental correction for the reflected sound must be carried out, but it is necessary to understand the acoustic environment to evaluate this correction term. More concretely, the equivalent sound absorption area of the measurement room is required, and this value is calculated from the information of the sound absorbing coefficient of the room wall and the area of the wal1, or the volume of the room and the reverberation time. Thus, it is necessary to correctly obtain these values. Though the maximum limit of the environmental correction factor is generally 7 db, the treatment for the larger value is not clearly defined. In the diffuse sound field method, it is necessary to obtain the reverberation time and the volume of the room, and in addition, to confirm that the variation of the sound field is small and the obtained mean sound pressure level can be the representative value for the room. In the comparative method, it is necessary to grasp the effect of deviation of the measuring position depending on the difference in size of the reference sound source and the sound source to be measured. 2.2 Intensity Method The sound power determination by the sound intensity measurement seems more direct compared with the sound pressure method because the transmitted power through the measurement surface can be obtained by multiplying the sound intensity by the area. As mentioned above, the standards for the intensity measurement have been established by ISO and the method for measurement of a large sound source such as the marine engine or in an adverse environment such as the operation shop of the manufacturer or the in-board engine room is now discussed separately. It is important with the intensity method to consider the trade-off between measurement accuracy to be required and scale of measurement points related to the practicality. In principle, it can be said about the method that the more the number of measuring points is, the less the difference between the measured value and the true value is. But the time required for the measurement (2) Bulletin of the M.E.S.J., Vol. 25, No.1

3 Engineering Method of Sound Power Determination for Marine Engines 3 reaches tens of hours if the number of measurement points reaches 1000, which is practically difficult at the actual measurement. From the consideration about measured data until now, it can be concluded that the lower limit of measurement points number is 200 if the required al1owance is l db, and 100 if the required allowance is 2 db. However, even in the case of the intensity measurement at 100 points, the required time of measurement is over 2 hours, and it is not only costly but also impractical to operate the sound source in a constant condition during that time. Though it is possible to slightly reduce the time by measuring continuously by the scanning method, the time of measurement is at most halved. On the contrary, in the scanning method, there may be some points where the continuous scan is difficult due to inferior scaffold, and in some cases, joint use of the scanning method and the discrete point method may be necessary. Also, the intensity method needs the intensity probe consisting of pair microphones and a 2-channel analyzer, and is costly compared with the simple sound level meter to be used in the sound pressure method. 2.3 Advantages and Disadvantages of Each Method Basic features of each method are as mentioned above, and in this chapter, their advantages/disadvantages are further examined from more various aspects. First, examination is made on the point for what purpose the emitted sound power level is required. The sound power level is treated as the index to make a judgement of the noisiness of a machine, i.e., the sound source without affecting the environmental condition of the place of the machine installation, and in that sense, the more excellent accuracy of the evaluation value is the better. In particular, radiation of the sound indicates one performance of the machine, the low level of the emitted noise can be one of the sales points, in addition to the point that the noise level of the control room in engine room is finally within the reference value or not. Not only manufacturers but a1so customers wish to obtain the correct values. On the other hand, where the sound power measurement of the marine engines can be carried out is restricted to the test bench of the manufacturer or on board the ship, because the engine load is necessary for operation, and the environment of measurement is very bad in either case. Especially, the in-board engine room is a narrow space, and its reverberation is high, and the evaluation value is apt to be larger even with the same engine. Moreover, because the engine room is surrounded by the steel plates, the effect of the structure-borne sound can not be neglected. Under such circumstances, the mean sound pressure is calculated through the measurement of the sound pressure at a plurality of points (about points) around the engine by a handy type sound level meter in a conventional method, and the sound power level is obtained after correction theory by the free sound field method. Though the power level can be easily evaluated with the time of measurement of about 30 minutes in this method, several data of the acoustic environment of the measurement room are required for the correction of the environment. Some of these values can be obtained correctly, but if some values me obtained only from the estimation, they may be the factors for error. Generally, the maximum limit of the environmental correction factor K is specified as 7 db. But in the case of the engine room, the value K calculated from the intensity measurement result often exceed 7 db. Therefore, the free sound field method can not be fundamentally applicable in the engine room. The diffuse sound field method may rather be suitable for the measurement in the engine room, though the distribution of the sound pressure in the engine room is not fairly uniform. In addition, in the case that the accuracy (standard deviation) of the evaluation value is 5 db, it is impossible to compare between measured data of different object under different condition when the difference is around 3-4 db. Thus, it still remains suspicious whether the accuracy required for the practical use is secured or not. In the diffuse sound field method and the comparative method, there are little experiences of application to a super-large sound source such as a marine main engine, and further examination is necessary. On the other hand, in the intensity method which is extensively used in various fields recently, the scale of measurement becomes large as mentioned above, and costly, which is a large disadvantage. Though the examination by the authors revealed that reduction in number of measurement to an appropriate scale can be made without the loss of the accuracy, it takes more than 2 times of the measurement time than that by the sound pressure method. In the discrete point method, the accuracy is greatly affected by the fact the measuring point can be regarded as representative of the mesh to be concerned, and it is risky to reduce the measurement points too much. Even in the scanning method, it is necessary to keep the appropriate scan interval, and in some cases, the scan at a constant speed is difficult on unstable scaffold, and little experiences are achieved for the marine engines by this method, and further examination is required. However, this intensity method has great advantage of the points that the evaluation of the room environment is unnecessary and the sound February 1997 ( 3 )

4 4 Minoru Kamata, Hideo Ikeda, Nobuhiro Baba power can be directly obtained. Therefore the intensity method should be employed in the case where the excellent accuracy is required and the measurement on sufficient scale can be carried out. 3. Proposal of Hybrid Method As summarized in the previous chapter, there are two methods for the sound power determination of the marine engine. One is the sound pressure (free sound field) method which is practicable but low in accuracy. The other is the intensity method which is used when the high accuracy is required. But each method has its own disadvantages, and the method where the accuracy is improved while the practicality is kept is desired. In the sound pressure method, the sound pressure is the scalar value, and the sound pressure level is not changed greatly if the measuring position is slightly changed due to the diffusive characteristic of the sound. Because the difference between the maximum value and the minimum value of the measured value of the sound pressure is less than l0 db at the positions, and on the scale of number of measuring points specified by ISO, etc., the accuracy of the calculated value of the mean sound pressure is fairly good. However, the calculation of the environmental correction factor needs the information on the reverberation time or on the sound absorbing coefficient of the wall surface at measurement site. The accurate values of these are not generally obtained, so it may be a factor of great errors in some cases. At this time, we happened to think of new foi1ow-ing index, PI index (δ pl), which is obtained at the intensity measurement with no further labor and indicates the acoustic environment, may be used for the environment correction factor in the sound pressure method. The PI index is the difference between the sound pressure level and the intensity level at the measuring point and given as follows. Comparing the equation (4) with (6) shows that there is strong similarity each other though the former is related to each point while the latter is related to the whole mean level. That means, it can be said that both K and δ pi have the same form except that the operation to get mean value is carried out within logarithm or outside, if the mean value is calculated from the PI index of each point. Thus, it seems nature to substitute the mean value ofδ pl for K. The Pl index is originally the index to express the degree of reverberation (the degree of the effect of the reflecting sound) at each measuring point, and the mean values among all measuring points should be taken to grasp the condition of the sound field of the whole room. Here, to keep the convenience of the sound pressure method, the mean value on the scale of the number of the measuring points by the sound pressure method (about points) is considered. It is often happened that the intensity level may be extremely small or negative in some conditions of the sound field. Then, it is feared that the mean value is greatly affected by these bias data, therefore the δ pl which exceeds 20 db is omitted from the calculation of the mean value. The new evaluation method of the sound power level proposed herein uses the intensity measurement though it is based on the sound pressure method, and referred to as the "Hybrid method". This hybrid method is practical at the view point of the measuring time because it is fundamentally based on the sound pressure (free sound field) method. It has also the advantage of no need to separately obtain the information on the sound environment of the room because the environmental correction factor can be (4) Bulletin of the M.E.S.J., Vol. 25, No.1

5 Engineering Method of Sound Power Determination for Marine Engines 5 evaluated from the intensity data. However, though the scale of measurement is same as that of the sound pressure method, the intensity probe and an analyzer are necessary for measurement. Also, the effect of other sound sources can not be canceled, because this method is essentially the sound pressure method. So it is necessary to make a correction in a similar manner to that of the conventional method. The accuracy of the hybrid method is examined in the next chapter. 4. Results of Measurement and Consideration The authors have carried out unit now ten measurement; 7 main engines (hereinafter, abbreviated as ME), 3 generator sets (abbreviated as DG.) In this chapter, the effectiveness of each method is examined through the analysis of these data. 4.1 Results of Measurement Main particulars of the engines to be measured, and the outline of the scale of the sound intensity measurement are shown in Table l. (For details, refer to the references (6) and (7).) The mesh size and the number of measurement points are determined under the restriction of the measuring time, and can not be thought to be sufficient except three cases of DG and ME-A. However, as mentioned in the examination of the previous report, the difference at the sound power evaluation is within 1 db even when the scale of the measuring point is reduced to one half, and the subsequent discussions will follow by regarding the power evaluation result at these intensity measurements as a tentative true value. In the fol1owing discussion, the overal1 value is calculated as the total sum between 125Hz and 4kHz octave bands on the linear scale because the spacer of the intensity probe (i.e., interval between the two microphones) is set to 12mm. In the method based on the sound pressure, the mean value of the sound pressure level is necessary. The requirements specified by DIN (9 or 19 points) are applied to the number and the position of these measurement points according to the size of the object to be measured. If the measuring point is not available at the prescribed position, the value is given by interpolation from the surrounding points. The results of the sound power determination by each method are given in Table 2. The results are examined hereafter. 4.2 Sound Pressure Free Sound Field Method In the sound pressure free sound field method, the correction for the measuring environment is necessary. The correction term was evaluated using the mean value of the reverberation time measured at four corners of the room for ME-C, F, G and DG-B, C. For other engines, the mean sound absorbing coefficient was used in evaluating the correction term. As for the volume of the measuring room, the shape is so complicate of both the test shop of the manufacturer and the in-board engine room that it seems meaningless to calculate the actual volume in detail. The results show that the cases with the in-board engine room are evaluated slightly worse though the cases of measurement at the test shop of the manufacturer are acceptably evaluated. In particular, the errors are large in the case of MEs. Detailed examination of the data shows that the evaluation by the sound pressure method is found to be too high in each case, i.e., the environmental correction factor K is smaller. Though the maximum value of K is generally prescribed as 7 db, the K value may, which is counted back from the mean sound pressure level from the true value of the power and the area of the compartment to be measured greatly exceed 7 db, and it is proved that the engine room is a quite inadequate environment of measurement with extremely large degree of reverberation. However, the K value calculated by the conventional evaluation method seldom exceeds 7 db, and the power determination is found to be larger as a result. This may February (5 )

6 6 Minoru Kamata, Hideo Ikeda, Nobuhiro Baba be attributable to the effect of the structure-borne noise on board the ship, but it is not investigated as yet to separate this component and to determine its power level, and no further discussion are made here. In conclusion, as for the sound power determination by the sound pressure free sound field method, evaluation of the error level below 3 db is possible in the measurement at the test shop of the manufacturer. However, the measurement in the in-board engine room shows the slightly larger error, and a larger evaluation value. This is because the value of the environmental correction term is evaluated to be smaller, and it is necessary to improve the evaluation method. This method has another disadvantage that the additional measurement of reverberation time is necessary to estimate the measurement room which are directly effective to the accuracy. 4.3 Sound Pressure Diffuse Sound Field Method Then, the diffuse sound field method is examined. This method is applied to ME-C, F, G where the reverberation time is measured and the sound source is 1arger relative to the sound field, and the results are found to be approximately same. In the case of the in-board engine room where the ratio of the volume of the sound source to the volume of the measurement room is large, a trend is seen that the evaluation value is slightly improved compared with the free sound filed method, but the measurement data is too few to have conclusion by the strict comparison. Because the measuring position in this calculation is same as that of the free sound field method, it seems that the accuracy becomes worse if there is a part where the direct sound affects much. But it seems that the accuracy is slightly improved in the environment with fairly high degree of reverberation in ME-C. However, the difference from the true value is stil1large. In applying this method, it is necessary to determine the condition of the sound field (variation of the sound pressure) is reasonable for this method, or, in other words, the requirements to regard the condition of the sound field as the diffuse sound field, and it is also necessary to cope with the case when the object to be measured is extremely large, such as a marine engine, where the conditions of C-method9) in JIS Z8733 are not satisfied. 4.4 Comparative Method The comparative method is evaluated using the data obtained in the experiment which was carried out to check the validity of the mesh setting at the power determination of the reference sound source by the intensity method. The measuring points are same as those in the diffuse sound field method. Though all requirements of JIS Me not satisfied, it was examined to find the effectiveness of the method. The results show the equivalent level to those of other sound pressure methods except that the values are abnormally too smal1 with ME-C. This may be because the measurement space in the in-board engine room is devided into several decks or the back ground noise level is different between when the engine is in operation and stopped, but details are unknown. In this comparative method, it is necessary to prepare the reference sound source whose sound power is known, but it can be a simple and useful method for the engine which is not so large, because various numerical values of the room need not be known. 4.5 Hybrid Method Finally, the "Hybrid method" proposed in this paper is examined. From the table of the results, it is found that the deviation from the true value is smaller compared with that by any other methods, and the variance is also smal1. From these findings, it can be concluded that the fundamental idea of this method is very effective. Referring to the objects to be measured and the difference in the environment, the true value of the environmental correction factor (the value converted from the power level obtained from the intensity and the mean sound pressure level) is 3-4 db for DG, and 6-8 db for ME in the test shop of the manufacturer, while 5-6 db for DG, and 9-12 db for ME in the in-board engine room, and the value corresponding to the respective places can be evaluated using the PI index. Examination is further advanced on the evaluation method of the mean value of the PI index based on the data of ME-A. In obtaining the mean value of the PI index to be used as the environmental correction factor, the effect on the accuracy becomes a problem by using the representative 2-6 points for each surface. Because the PI index becomes extremely large and greatly affects the mean value if the intensity happens to be extremely smal1 or negative, the values exceeding 20 db are omitted from calculating the mean value in the hybrid method. The fol1owing evaluation is made so as to examine the appropriateness of this point. Table 3 shows the PI index in A-weighted overal1 with 99 points on the left side measurement surface. (The part whose index value is over 100 means that the intensity is negative.) When the mean value is calculated from the absolute value of these 8p1, the mean value is 13.0 while the standard deviation is 3.9. In this case, it can be much affected by the excessive values and risky to estimate the total mean level from the sampling of several points. Thus, if the parts of the negative (6) Bulletin of the M.E.S.J., Vol. 25, No.1

7 Engineering Method of Sound Power Determination for Marine Engines 7 intensity values are omitted, the standard deviation becomes 3.4 with 90 points, and if the parts over 20 db are further omitted, the standard deviation becomes 3.2 with 79 points and the variation becomes small in this procedure. Reduction of the variance of the population means the improvement of the evaluation of the mean value by several points. In this example, the final points of employment are 6 in number, and 4 if those to be omitted is excluded, and the mean value then is not so different from that with a number of points. In conclusion, if excessively large points are included in the calculation of the mean value, the mean value level may be deviated. But if the points over 20 db are omitted, the stable evaluation value can be obtained without much deviation of the mean value level. In addition, if the threshold is reduced to 15 db, the variance of the data is further reduced while the deviation of the mean value level is increased. Thus, it can be concluded to have approximately appropriate level evaluation by omitting the excessively large values above 20 db and taking the mean value from the rest of the points. Incidentally, there is no large difference between through the mean value in the numerical value in db and through the conversion to the linear scale because this is the calculation of the mean value with the data of this degree of variance, and the simple average in db easy in calculation is emp1oyed. Similar examination is achieved on other measurement surfaces of ME-A to obtain the similar trend. The mean value is obtained through the finally integrated average, and the method here can evaluate the extreme1y excellent environmental correction factor. The hybrid method proposed in this paper enables the sound power determination which is extremely stable and excellent in accuracy. Though this method needs the intensity measuring instruments, it ensures the excellent accuracy by partially employing the information on the intensity while keeping the simplicity of the sound pressure method, and can be concluded to be an excellent and practical method. The accuracy of measurement is estimated to be within 2 db in standard deviation from the authors' experiences. Different from the intensity method, this method is based on the sound pressure method, so the effect of extraneous sound sources can not be canceled, but the conventional correction procedure can be emp1oyed to solve this problem. Several examples of raw data on the evaluation by the hybrid method are shown in the appendix for information. 5. Sound Power Evaluation Method by Hybrid Method In this chapter, the procedures of the practical method for the sound power determination proposed in this paper are summarized. <Measuring points> The measurement surface is set at the distance of about lm (minimum 0.5m) from the surface of the sound source to be measured, and the reference rectangular parallelepiped surrounding the object is considered. The positions of 9, 12, 15, or 19 measuring points according to the size of the object, and the direction of the probes are determined in accordance with the figures of the DIN standards8). <Treatment of extraneous noise> As for the sound except the sound radiated from the object to be measured, the conventional correction such as adopted by ISO should be considered. <Measurement> The sound pressure is measured when the object to be measured is stopped to evaluate the extraneous noise, and then, the object is operated to start the main measurement. The sound pressure at two close points is measured by the pair microphones at each measuring point. The spacer interval of 12 mm is used to cover the normal range of 125 Hz and 4 khz, and may be changed as necessary. The measuring time is dependent on the recording method and analyzing method, but the standard one is 20 seconds/point. <Analysis> The sound pressure level, and the intensity level of l octave band or 1/3 octave band in the appropriate frequency range is calculated by the analyzer. <Calculation of mean sound pressure level and mean δ pl> The mean sound pressure level at 9-19 measuring points and the mean value of δ pl where the values over 20 db are excluded are obtained. <Calculation of sound power leve1> The mean value of δ pl is substituted in the environ-mental correction factor to calculate the sound power February 1997 (7)

8 8 Minoru Kamata, Hideo Ikeda, Nobuhiro Baba level by ( 1 ). For the detailed notes related to the measurement,refer to the reference 8). 6. Conclusion Examination is made on the method of the sound power determination of the marine engine from various aspects. As a result, it is proved that the accuracy can be improved by using the information on the intensity measurement at the environmental correction though the method of measurement is fundamentally based on the sound pressure free sound field method. This method is named to as the "Hybrid method", and its effectiveness is examined. The sound power determination can be realized by this method with excellent accuracy on the equivalent scale of measurement to that of the sound pressure method without achieving the evaluation of the environmental conditions by other method. The authors wish to express their appreciation to those of the shipping companies, the shipbuilding companies, and the manufacturer of marine engines who took care of the measurement, members of the second sub-committee of the Vibration and Acoustics Research Committee of MESJ who participated in the earnest discussions on the ideal way of the evaluation method, and Prof. Fahy of Southampton University, UK who gave useful suggestions on the method. The authors also wish to thank our co11pague in the Re-search Institute for their contribution at the measurement. References 1) ISO/DIS9614-1, Acoustics-Determination of sound power levels of noise sources using sound intensity-measurement at discrete points ( 1989) 2) ISO/DIS9614-2, Acoustics-Determination of sound power levels of noise sources using sound intensity-measurement by scanning ( 1994) 3) ISO3743, Acoustics-Determination of sound power levels of noise sources-engineering methods for special reverberation test rooms (1976) 4) ISO3744, Acoustics-Determination of sound power levels of noise sources-engineering methods for free-field conditions over a reflecting plane (1981) 5) ISO/CD3746, Acoustics-Determination of sound power levels of noise sources-survey method employing an enveloping measurement surface over a reflecting plane ( 199 1) 6) N. Baba, et. al, Sound Power Measurement of Diesel Generator Unit by Sound Intensity Technology, Journal of MESJ, ( 1992), ) M. Kamata, Sound Power Measurement Method of Marine Diesel Engines, Journal of MESJ, ( 1995), ) DIN , Measurement of noise emitted by machines Airborne noise emission, enveloping surface method-internal combustion engines (1987) 9) JIS Z8733 Engineering and Survey Methods for the Determination of Sound Power Level in General Sound Fields ( 1987) (8) Bulletin of the M.E.S.J., Vol. 25, No.1

9 Engineering Method of Sound Power Determination for Marine Engines 9 February 1997 (9)