SIST EN 25136:1997
(Main)Acoustics - Determination of sound power radiated into a duct by fans - In-duct method (ISO 5136:1990 and Technical Corrigendum 1:1993)
Acoustics - Determination of sound power radiated into a duct by fans - In-duct method (ISO 5136:1990 and Technical Corrigendum 1:1993)
This International Standard specifies a method for testing ducted fans to determine the sound power radiated into an anechoically-terminated duct on the inlet and/or outlet side of the equipment. It applies to fans which emit steady, broad-band, narrow-band and discrete-frequency sound. It applies to air temperatures between - 50 oC and + 70 oC. The test duct diameter range is from 0,15 to 2 m. The maximum flow velocity is 30 m/s and the maximum swirl angle is 15o. An example of a method for determining the angle of swirl is given in Annex F. The one-third octave band centre frequency range is from 50 Hz to 10 000 Hz. Note: The flow noise suppression of the sampling tube (see 6.2.1) may be insufficient at higher velocities and at higher angles of swirl.
Akustik - Ermittlung der von Ventilatoren in Kanäle abgestrahlten Schalleistung - Kanalverfahren (ISO 5136:1990 und Technisches Korrigendum 1:1993)
Diese internationale Norm beschreibt ein Verfahren zur Bestimmung der Schalleistung, die von Ventilatoren in einen reflexionsarm endenden Kanal ansaug- und/oder ausblasseitig abgestrahlt wird. Sie ist auf Ventilatoren anwendbar, die stationären, breitbandigen, schmalbandigen oder tonalen Schall aussenden. Sie gilt für Lufttemperaturen von -50°C bis +70°C.
Acoustique - Détermination de la puissance acoustique rayonnée dans un conduit par des ventilateurs - Méthode en conduit (ISO 5136:1990 et Rectificatif Technique 1:1993)
Akustika - Ugotavljanje zvočne moči, ki jo ventilator seva v kanal - Metoda za merjenje v kanalu (ISO 5136:1990 in tehnični popravek 1:1993)
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Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Akustik - Ermittlung der von Ventilatoren in Kanäle abgestrahlten Schalleistung - Kanalverfahren (ISO 5136:1990 und Technisches Korrigendum 1:1993)Acoustique - Détermination de la puissance acoustique rayonnée dans un conduit par des ventilateurs - Méthode en conduit (ISO 5136:1990 et Rectificatif Technique 1:1993)Acoustics - Determination of sound power radiated into a duct by fans - In-duct method (ISO 5136:1990 and Technical Corrigendum 1:1993)23.120QDSUDYHVentilators. Fans. Air-conditioners17.140.20Emisija hrupa naprav in opremeNoise emitted by machines and equipmentICS:Ta slovenski standard je istoveten z:EN 25136:1993SIST EN 25136:1997en01-april-1997SIST EN 25136:1997SLOVENSKI
STANDARD
SIST EN 25136:1997
SIST EN 25136:1997
SIST EN 25136:1997
INTERNATIONAL STANDARD ISO ’ 5136 First edition 1990-12-15 Acoustics - Determination of Sound power radiated into a duct by fans - In-duct method Acoustique - Determination de Ia puissance par des ven tifa teurs - Methode en conduit acoustique rayonnee dans un conduit Reference number ISO 5136 : 1990 (El SIST EN 25136:1997
ISO 5136 : 1990 (El Contents Page 1 Scope. 1 2 Normative references . 2 3 Definitions and Symbols. . 2 4 Test facilities and instrumentation . 3 5 Testarrangement . 8 6 Testprocedure . 9 7 Calculations . 10 8 Information to be recorded . 10 9 Information to be reported. . 10 Annexes Computational procedures for calculating A-weighted Sound power level from octave or one-third octave band power levels . 11 Determination of the signal-to-noise ratio of Sound to turbulente noise in thetestduct . 12 Guidelines for the design and construction of an anechoic termination . 13 Evaluation of Performance of anechoic terminations . 18 Example of a sampling tube. . 19 Measurement of the swirl component . 21 Bibliography . 22 0 ISO 1990 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronie or mechanical, including photocopying and microfilm, without Permission in writing from the publisher. International Organkation for Standardkation Case postale 56 l CH-1211 Geneve 20 l Switzerland Printed in Switzerland ii SIST EN 25136:1997
ISO 5136 : 1990 (EI Foreword ISO (the International Organization for Standardization) is a worldwide federation of national Standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Esch member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, govern- mental and non-governmental, in Iiaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. International Standard ISO 5136 was prepared by Technical Committee ISO/TC 43, Acous tics. Annexes A and B form an are for information only. part of this International Sta ndard. An C to G SIST EN 25136:1997
ISO 5136 :199O(E) Introduction The Sound power radiated into a duct by a fan depends to some extent on the type of duct, characterized by its acoustical impedance. For a measurement method, the duct has, therefore, to be clearly specified. In this International Standard, the duct is of cir- cular Cross-section and terminated nearly anechoically. Details of typical anechoic ter- minations are given in annex C. The Sound power obtained under these special con- ditions is a representative value for actual applications, as the anechoic termination forms an impedance about midway between the higher and lower impedances found in practice. The Sound power radiated in actual applications tan, in theory, be estimated from data on fans and duct impedances. Since this information is at present in- complete, these effects are not usually considered in acoustical calculations. In Order to suppress the turbulent pressure fluctuations at the microphone, the use of a long cylindrical Windscreen (“sampling tube”) is stipulated. The microphone, with the sampling tube, is mounted at a radial Position such that the Sound pressure is accept- ably well related to the Sound power by the plane wave formula, even in the frequency range in which radial standing waves (Cross-modes) are possible. The testing precision is given in terms of the Standard deviation to be measurements were repeated in many different laboratories. expected if the The procedures for measuring the operating conditions (Performance measurements) are not specified in detail in this International Standard. The operating conditions are intended to be specified in a separate code which will be the subject of a future Inter- national Standard. This International Standard is one of a mining the Sound power levels of fans. series specifying different methods for deter- iv SIST EN 25136:1997
INTERNATIONAL STANDARD ISO 5136 : 1990 (E) Acoustics - Determination of Sound power radiated into a duct by fans - In-duct method 1 Scope 1.1 Measurement conditions This International Standard specifies a method for testing ducted fans to determine the Sound power radiated into an anechoically-terminated duct on the inlet and/or outlet side of the equipment. lt applies to fans which emit steady, broad- band, narrow-band and discrete-frequency Sound. lt applies to air temperatures between - 50 OC and + 70 OC. The test duct diameter range is from 0,15 m to 2 m. The maxi- mum flow velocity is 30 m/s and the maximum swirl angle is 15O. An example of a method for determining the angle of swirl is given in annex F. The one-third octave band centre frequency range is from 50 Hz to 10 000 Hz. NOTE - The flow noise suppression of the sampling tube (sec 6.2.1) may be insufficient at higher velocities and at higher angles of swirl. 1.2 Types of Source The method applies to a Sound Source in which a fan is usually connected to ducts on at least one side. Examples of the ducted fans and fan equipment covered by this International Standard are - ducted centrifugal fans; - ducted axial flow fans; - ducted mixed flow fans. This International Standard may also apply to other aero- dynamic sources, such as boxes, dampers and throttle devices. This International Standard does not apply to non-ducted fans or non-ducted fan equipment. 1.3 Precision of the method of measurement The precision of the method of measurement is given in terms of the Standard deviation of the Sound power level. lt includes the effects of end reflections, transitions, the possible errors in computing Sound power from pressure measurements, and the tolerante of the instrument calibration. The estimated Standard deviations are given in table 1. Table 1 - Precision of the method of measurement One-third octave band centre frequency Standard deviation Hz dB 50 315 63 3 80; 100 2,5 125to4000 2 5000 23 6 300 3 8000 315 10 000 4 The Standard deviations given in table 1 reflect the cumulative effects of all Causes of measurement uncertainty, excluding variations in the Sound power from machine to machine or from test to test which may be caused, for example, by changes in the mounting or operating conditions of the Source. NOTES 1 The Standard deviations given in table 1 are derived from infor- mation in [3], [51 and [191. 2 The precision data will increase in the presence of swirling flows. 3 If discrete frequency components are present or if measurements are not averaged over a sufficiently long period, the precision will be less than that indicated. 4 At high frequencies, particularly above about 4 000 Hz, the precision data quoted in table 1 may increase when the noise spectrum being measured decreases rapidly with frequency. Under these con- ditions, the high-frequency Sound pressure levels sensed by the microphone tan be of small magnitude compared with those at lower frequencies, and electrical noise, pat-ticularly from the frequency analyser, tan interfere with the Sound Signal at these high frequencies. In Order to achieve correct determinations of Sound power it may be necessary to repeat the high-frequency Sound measurement by passing the microphone Signal through a high pass filter before it is analysed by the frequency analyser. 1 SIST EN 25136:1997
ISO 5136 : 1990 (E) 2 Normative references =,,,=p21 =p31 are the Sound pressure levels measurement positions in the test duct. at each of the three The following Standards contain provisions which, through reference in this text, constitute provisions of this International Standard. At the time of publication, the editions indicated were valid. All Standards are subject to revision, and Parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the Standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards. ISO 266 : 1975, Acoustics - Preferred frequencies for measurements. ISO 5221 : 1904, Air distribution and air diffusion - Rules to me thods of measuring air flow rate in an air handling duct. ISO 7235 : - 11, Acoustics - Measurement procedures for due ted silencers - Insertion Ioss, flow noise and total pressure Ioss. IEC 225 : 1966, Octave, half-octave and third-octave band filters intended for the analysis of Sounds and vibrations. IEC 651 : 1979, Sound level meters. 3 Definitions and Symbols For the purposes of this Interna definitions and Symbols apply. tional Standard, the following 3.1 fan inlet [outletl area, Sf: The area of the fan fitting provided for connection to attached ductwork. 3.2 ducts: Any of the airways defined in 3.2.1, 3.2.2 and 3.2.3. 3.2.1 test duct : The duct in which the measu red. lt has an anechoic termina tion. fan Sound power is 3.2.2 terminating duct: The duct opposite to the test duct, if both sides of the fan are ducted. lt has an anechoic termin- ation. 3.2.3 intermediate duct: The duct fitted on the intake side and on the discharge side of the fan to ensure desired flow con- ditions. lt connects to the test duct or the terminating duct, if necessary by a transition section (sec figure 1). 33 measurement p Ichich the microphone Ilane: The radial plane diaphragm is located. in the test duct in 3.4 Sound pressure level, L,, in decibels: Ten times the Iogarithm to the base 10 of the ratio of the mean-Square Sound pressure of a Sound to the Square of the reference Sound pressure. The width of a restricted frequency band shall be in- dicated, for example one-third octave band pressure level, A-weighted Sound pressure level, etc. The reference Sound pressure is 20 FPa. L pm is the spatially averaged Sound pressure level obtained from averaging over the measurement positions in the test duct. lt may also be obtained from a continuous circumferential traverse (see 6.2.4). q is the spatially averaged Sound pressure level at the measure- ment plane, corrected for the combined free-field response C (sec 3.9 and 7.1). 3.5 Sound power level, LW, in decibels: Ten times the logarithm to the base 10 of the ratio of a given Sound power to the reference Sound power. The width of a restricted frequency band shall be indicated, for example one-third octave band power level, A-weighted Sound power level, etc. The reference Sound power is 1 pW. 36 ist fan Sound power duct by the fan. : The Sound power radiated into the 3.7 frequency range of interest: For general purposes, the frequency range of interest includes the one-third octave bands . with centre frequencies between 100 Hz and 10 000 Hz. For special purposes, the frequency range of interest may be ex- tended down to 50 Hz. For fans which radiate predominantly high- or low-frequency Sound, the frequency range of interest may be limited in Order to reduce the costs of the test facilities and procedures. The limits of the restricted frequency range shall be given in the test report. 3.8 sampling tube: A tubular Windscreen to be attached to a Standard microphone designed to minimize its sensitivity to flow noise. 3.9 Further Symbols C, correction supplied by the manufacturer to be added to the calibrated microphone response to obtain the free field response, expressed in decibels C2 frequency response correction of the sampling tube of nor- mal incidence, expressed in decibels, to be added to the calibrated microphone response Esee 4.3.3 c)] C3 flow velocity correction for the frequency response required by the use of the sampling tube, expressed in decibels (see table 5) C4 modal correction for the frequency response required by the use of the sampling tube, expressed in decibels (sec table 6) c=c,+ C2 + C3 + C4 Combi rection expressed in decibels ned frequency response cor- c Speed of Sound in the test duct Q fluid density in the test duct 1) To be published. 2 SIST EN 25136:1997
ISO 5136 : 1990’ (El d diameter of the fan inlet, fan outlet, test duct, intermediate ducts, terminating ducts (sec figure 1) I length of the ducts and transitions (see figure 1) r radial distance from the test duct centreline to the sampling tube centreline ra pressure reflection coefficient defined as the ratio of the Sound pressure amplitude of the Sound wave reflected from the anechoic termination to the Sound pressure amplitude of the incident wave b, h Cross-dimensions of the rectangular fan inlet or fan ou tlet 4 Test facilities and instrumentation 4.1 General requirements The test arrangement shall consist of the fan to be tested, an intermediate duct, the test duct with anechoic termination, and the instrumentation (see figure 1). If a fan usually used with duct work on both sides is to be tested, a termination duct with anechoic termination plus an intermediate duct shall be connected opposite to the side on which the Sound power is determined. All connections between the fan and the ducts shall be firm, unless a Vibration-isolating coupling is an inherent patt of the fan. The test ducts shall include provisions for mounting the microphone and sampling tube at the locations specified in 5.2. Suitable provisions shall also be made for controlling the desired fan operating conditions. NOTES 1 Examples of designs of anechoic terminations and throttling devices are given in annex C. 2 Measurement of mass flow is the preferred method of controliing the fan operating Point fsee ISO 5221); an alternative method is to measure the fan pressure rise. 3 The aerodynamic Performance characteristics of the fan may be measured using a different test arrangement. 4.2 Dutt specifications 4.2.1 Construction of ducts and transitions The ducts shall be straight, coaxial with the fan inlet or outlet, and of uniformly circular Cross-section. The ducts and tran- sitions shall be manufactured either from steel having a mini- mum thickness 1 mm or from a material of equivalent mass per unit area and rigidity which ensures an acoustically hard and smooth interior surface. The ducts and transitions should preferably be treated with a Vibration-damping material on the outside. NOTE - This International Standard prescribes ducts with circular Cross-sections. Future International Standards may involve ducts with rectangular Cross-sections. 4.2.2 Dutt lengths Dutt lengths shall be as specified in figure 1. 4.2.3 Dutt Cross-sectional area The duct Cross-sectional areas shall be as specified in table 2, where the inlet or outlet area Sf is the area on the side to which the respective duct is connected. Table 2 - Cross-sectional areas of ducts Dutt Cross-sectional area min. I max. Fan inlet side Fan outlet side ~~~~~~~~ 1 ~~~~ 1 21~~ 4.2.4 Transitions All transitions, including any transitions from rectangular fan outlets or inlets to the circular ducts, shall be coaxial and shall meet the following criteria : a) the maximum enclosed angle of the sides shall be 15O; b) the minimum length, /min, shall be calculated from z min larger area -= - 1 ZO smaller area with l. = 1 m 4.2.5 Anechoic termination The pressure reflection coefficient, rar of the anechoic termin- ation when installed and when a throttling device is fitted shall not exceed the values specified in table 3. Table 3 - Maximum pressure reflection coefficients One-third octave band centre frequency HZ 50 014 63 0,35 80 O,3 100 0,25 > 125 0,15 Maximum pressure ref lection coefficient NOTE - Guidelines for the design of the anechoic terminations and a method for measuring the pressure reflection coefficient of the ter- mination are given in annexes C and D. 3 SIST EN 25136:1997
ISO 5136 : 1990 (E) Dimensions in metres Anechoic termination / (schematic) Conical or rectangular Measurement, r to round transition Stepped anechoic termination and throttle (schematic) Measurement r Plane Flow measurement (schematic) For circular fan inlet, diameter dl 1 < ld3/d,l 2 < 2,1 2d, G 1, < 5d, 13, a 3,8 ld3 - d,) and > (d3/dl) 2 - 1 For rectangular fan inlet, b, x h, l< nd; - < 2,1 4wl 0,7 < ld61d2> 2 Q 2,1 12d > 3,8 ld4 - d2) and > td4/d21 2 - 1 for dJ > d2 124 > 3,8 ld2 - da) and > (d2/d4, 2 - 1 for d2 > d4 For rectangular fan outlet, b2 x h2 0,95 < nd; - < 1,07 4b2h2 0,7 < nd; - < 2,1 4b2h2 nd; b31 > 3,81dm - d31 and > - - 1 4Wl 2d4 < ld < 5d4 le > 3,8 ld6 - d4) and > for d6 > da lM > 3,8 (da - d6) and > for da > d6 d6/d4) 2 - 1 d4/d6) 2 - 1 For circular fan outlet, diameter d2 0,95 < (d4/d2> 2 < 1,07 nd2 * 124 > 3,81Jm - dJ1 and > z - 1 3 3 forLdz > b2h2 4 402h2 124 a 3,81d-+h2 - d4 and > x - 1 4 for b2h2 ’ 4 -d; a) Simultaneous measurement of inlet and outlet in-duct noise Figure 1 - Test arrangement and limiting dimensions of test ducts, intermediate ducts and transitions 4 SIST EN 25136:1997
ISO 5136 :1990 (EI Dimensions in metres Conical or rectangular round transition Flow measurement Fans with ducted (schematic) - outlet All dimensions as for figure 1 a) except for l~ 16 > da and > 1 Flow measurement and Fans with unducted control (schematic) - ou tlet L Anechoic termination (schematic) All dimensions as for figure 1 a) b) Measurement of inlet in-duct noise only Figure 1 - (contkwed) Dimensions in metres Fans with ducted inlet Flow measurement --+ (schematic) Conical or rectangular to round transition Flow measurement control (schematic) and Stepped anechoic termination -/ (schematic) All dimensions as for figure 1 a) except for 13 13 > 44 and > 1 Fans with unducted inlet Flow measurement and control (schematic) Anechoic termination -/ All dimensions as for figure 1 a) Fan Transition Intermediate Test duct Terminating duct duct duct c) Measurement of outlet in-duct noise only Figure 1 - (concluded) SIST EN 25136:1997
ISO5136:1990 (E) 4.2.6 Throttling device a) The turbulente noise shall be suppressed by at least 10 dB in the frequency range of interest as compared with a nose cone. The actual values of turbulente noise sup- Pression as a function of frequency and flow velocity shall be known in Order to determine the signal-to-noise ratio as specified in 6.2.1 (see also annex B and table E.l). An adjustable throttling device, if necessary, shall be provided at the end of the anechoic termination remote from the fan. No other throttle shall be placed between the fan and the anechoic termination. The throttling section shall provide control to adjust the operating conditions under which it is desired to determine the Sound power of the fan. b) The maximum diameter of the sampling tube shall be 22 mm. The throttling device and the anechoic termination shall be designed so that the Sound pressure level generated in the test duct by the throttling device is at least 10 dB below the fan Sound pressure level in the test duct. c) The frequency response correction C2 of the sampling tube for each one-third octave band of interest shall be determined to within + 0,5 dB in a plane-wave field inci- dent axially from the front. If tests are carried out in a free field, a minimum distance of 3 m between the loudspeaker and the sampling tube being tested shall be maintained, and the reference microphone Position shall be at the mid-Point of the sampling tube length. It is essential that the fre- quency response correction curve be smooth. Alternatively, a manufacturer’s calibration cutve, obtained in compliance with the requirements for the frequency response correc- tion, shall be used. Suggested throttling arrangements are shown in figure C.5. 4.3 lnstrumentation 4.3.1 Measuring System d) The directivity of the sampling tube, when measured in a free field with broad-band noise of one-third octave band- width, shall be within the limits given in figure 2. 4.3.1 .l Microphone A microphone of a Sound level meter complying with the re- . quirements for a type 1 instrument as specified in IEC 651 shall be used. The dimensions shall be compatible with those of the sampling tube. Curves illustrated in figure 2 are given by the following equation : 1 AL = 20 Ig l+&xKx03 for 0 < 0 < 1,31 rad (75O) 4.3.1.2 Microphone cable where The microphone/cable System shall be such that the sensitivity does not Change with temperature in the range prevailing during the test. Cable flexing arising from either microphone traversing or from airflow across the cable shall not introduce noise which interferes with the measurements. AL is the reduction of sensitivity, in decibels, at an incidence angle 0 compared with incidence axially from front (0 = OO); K is the directivity constant; f. is the centre frequency of the one-third octave band, in hertz; 0 is the angle of incidence, in radians. 4.3.1.3 Sound level meter or other microphone amplifier The limiting values of the directivity constant K are given in table 4. The Sound level meter or other amplifier used to amplify the microphone Signal shall conform to the electrical requirement for a type 1 Sound level meter as specified in IEC 651. The fre- quency response characteristic designated Lin shall be used. Table 4 - Limiting values of the directivity constant K One-third octave band centre frequency K min K max Hz 1 000 0,35 x IO-3 1,5 x 10-3 2 000 0,35 x 10-3 1,5 x IO-3 4 000 0,35 x IO-3 2,2 x IO-3 8 000 0,35 x IO-3 2,2 x IO-3 4.3.2 Frequency analyser A one-third octave band filter set complying with the re- quirements of IEC 225 shall be used. The filter band centre fre- quencies shall be those tabulated in ISO 266. 2 A manufacturer’s Statement that the sampling tu within the limits specified by figure 2 may be used. directivity 4.3.3 Sampling tube e) Values for the taken from table 5. flow velocity correction, Cs, shall be The sampling tube reduces the turbulent at the measurement positions in Order to signal-to-noise ratio (see 6.2.1). pressure maintain fluctuations a suff icient NOTE - Sampling tubes are available commercially. See for example figure E.I. The sampling requ irements : tube and its use shall comply with the following f) Values for the modal correction, C& shall be taken from table 6. SIST EN 25136:1997
ISO 5136:1990 (El t 8 0 - dB -10 -30 * 0 JI 6 rad -3 0 f rad $ e- e-- Figure 2 - Limiting curves for the directivity of the sampling tube (broad-band noise of one-third octave bandwidth) Table 5 - Flow velocity correction, CS, in decibels, for the frequency response of the sampling tube , One-third Range of flow Mach numbers (flow velocity/speed of Sound) octave band 0,011 7 to 0,017 5 to 0,023 3 to 0,029 2 to 0,035 0 to 0,040 8 to 0,046 6 to 0,052 5 to 0,058 3 to 0,064 1 to 0,070 0 to 0,075 8 to 0,081 6 to centre < 0,017 5 <0,0233 < 0,0292 1 < 0,035O < 0,0408 < 0,0466 < 0,052 5 < 0,0583 <0,064 1 < 0,070O <0,0758 <0,081 6 <0,087 5 frequency Range of flow velocities, in metres per second, for measurements in air at 20 OC (i.e. Speed of Sound, c = 343 mk) Hz 4to<6 6to<8 8to
ISO 5136 : 1990 (EI Table 6 - Modal correction, C4, for the frequency response of the sampling tube One-third Diameter range of the test duct, d, in metres octave band centre frequency Relative radial Position 2r/d = 0,8 Relative radial Position 2r/d = 0,65 Hz 0,15 < d < 0,2 0,2 < d < 0,3 0,3 d 4 d < 0,5 0,5 Q d < 0,8 0,8 < d < 1,25 l,25 < d < 2 250 315 1 400 Ir5 500 1 115 - 630 1 lt5 1,5 800 lt5 L5 Ir5 1000 2 2 2 1250 1 2 2 2 1600 1 1 L5 2,5 2,5 23 2000 1 2 2,5 3 3 3 2500 L5 2,5 3 3,5 315 3,5 3150 2,5 3,5 4 4 4 4 4000 3,5 4,5 5 5 5 5 5000 4,5 5,5 5,5 5,5 5,5 5,5 6300 5,5 6 6 6 6 6 8000 6 6 6 6 6 6 10 000 6 6 6 6 6 6 g) An example of a sampling tube is given in annex E. A typical reduction of the turbulent pressure fluctuation as compared with the nose cone is given in table E.1 as a func- tion of flow velocity for one-third octave bands. 5 Test arrangement 5.1 Sampling tube mounting NOTE - Sampling tubes are available commercially. See for example figure E.1. 4.3.4 Graphit Ievel recorder or other read-out devices Graphit level recorders and other read-out devices shall comply with the requirements for a type 1 instrument as specified in IEC 651. 4.3.5 Multiplexing System If the procedure outlined in 5.2.2b) is used, the multiplexing System shall be qualified such that the resulting Sound pressure level is within + 0,5 dB of the true energy-equivalent average of the individual Sound pressure levels throughout the fre- quency range of interest. 4.4 System calibration A stable acoustical calibrator shall be applied to the microphone without the sampling tube to check the calibration of the entire measuring System before and after each series of tests. The calibrator shall be recalibrated annually. The porous part of the sampling tube shall be clean and undamaged. The microphone with the sampling tube shall be mounted in the test duct in the measuring plane as shown in figure 1. The microphone with the sampling tube shall be mounted rigidly in an axial direction pointing towards the fan. For fan inlet measurements, the sampling tube shall Point towards the fan, but the microphone end of the tube shall be rounded. The mounting shall introduce a minimum of flow noise. NOTE - Schematic drawings of typical mountings are given in annex E. 5.2 Sampling tube Position 5.2.1 Radial Position The sampling tube shall be mounted at the radial positions given in table 7. See figure 3. Table 7 - Radial positions of the sampling tube I Diameter of the test duct, d Relative radial Position 2rid m 0,15 Q d < 0,5 W3 0,5 < d a 2 0,65 NOTE - The given radial positions ensure a good estimate of the Sound power from the measured Sound pressure. 8 SIST EN 25136:1997
ISO 5136 : 1990 (EI Possible positions of the slit of the sampling tube. The slit should face away from the direction of any swirl. 62 . Test duct / 6.2.1 General Sampling tu be Figure 3 - Radial Position of the sampling tube 5.2.2 Circumferential positions At the radial positions specified in 5.2.1, a circumferential mean value of the Sound pressure level shall be obtained by using one of the following procedures: a) A Single microphone moved sequentially to at least three microphone positions distributed equally on the cir- cumference. This may be achieved by fixing the microphone in a duct section which tan be rotated in equal intervals. short b) Three of more fixed microphones equally distributed on the circumference. If the Signals of these microphones are to be averaged using a multiplexing System, they shall have the same type of sampling tube fitted and their sensitivities shall be equalized to ensure that they have equal frequency response corrections to within 0,5 dB. c) One microphone with a continuous circumferential traverse at constant angular velocity through one complete revolution. If the porous part of the sampling tube consists of one slit only, this slit shall be located in the circumferential direction opposite . to the incidence of the swirl component. 53 . Operating condition control
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