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GB/T 18696.1-2004: Acoustics -- Determination of sound absorption coefficient and impedance in impedance tubes -- Part 1: Method using standing wave ratio Delivery: 9 seconds. True-PDF full-copy in English & invoice will be downloaded + auto-delivered via email. See step-by-step procedure Status: Valid
Similar standardsGB/T 18696.1-2004: Acoustics -- Determination of sound absorption coefficient and impedance in impedance tubes -- Part 1: Method using standing wave ratio---This is an excerpt. Full copy of true-PDF in English version (including equations, symbols, images, flow-chart, tables, and figures etc.), auto-downloaded/delivered in 9 seconds, can be purchased online: https://www.ChineseStandard.net/PDF.aspx/GBT18696.1-2004 GB NATIONAL STANDARD OF THE PEOPLE’S REPUBLIC OF CHINA ICS 17.140.01 A 59 Acoustics – Determination of Sound Absorption Coefficient and Impedance in Impedance Tubes – Part 1: Method Using Standing Wave Ratio (ISO 10534-1:1996, MOD) ISSUED ON: MAY 13, 2004 IMPLEMENTED ON: DECEMBER 01, 2004 Issued by: General Administration of Quality Supervision, Inspection and Quarantine; Standardization Administration of the People’s Republic of China. Table of ContentsForeword ... 3 Introduction ... 5 1 Scope ... 6 2 Normative References ... 7 3 Definitions and Symbols ... 7 4 Principle ... 10 5 Fundamentals ... 11 6 Test Equipment ... 15 7 Preliminary Tests and Measurements ... 22 8 Mounting of the Test Sample ... 22 9 Test Methods ... 23 10 Transformation of Reflection Factor and Impedance ... 25 11 Test Report ... 26 Annex (Normative) Preliminary Measurements ... 28 Annex B (Normative) Verification of the Test Equipment ... 33 Annex C (Normative) Pressure-Release Termination of Test Sample ... 36 Annex D (Informative) Determination of Diffuse Sound Absorption Coefficient αst of Locally Reacting Absorbers ... 38ForewordGB/T 18696 Acoustics – Determination of Sound Absorption Coefficient and Impedance in Impedance Tubes consists of the following 2 parts: --- Part 1: Method Using Standing Wave Ratio; --- Part 2: Transfer Function Method. This Part is Part 1 of GB/T 18696; it modifies and adopts the international standard ISO 10534- 1:1996 Acoustics – Determination of Sound Absorption Coefficient and Impedance in Impedance Tubes – Part 1: Method Using Standing Wave Ratio. During the modification process, some necessary modifications are made to the ISO original text as follows: --- According to the definition, in the text of the standard, change "acoustic impedance " to "specific acoustic impedance", but in order to correspond to ISO, the name of the standard remains unchanged; --- Deleted the original 3.5 "impedance in the reference plane" and 3.8 "surface admittance", because these two terms will not appear again; --- Appendix A.2 "Measurement of sound velocity, wavelength and characteristic impedance" of this Part adopts the method in subclause 7.2 of ISO10534-2:1998 (i.e., subclause 8.2 in GB/T 18696.2-2002); --- The constant 250 in formula (29) is derived from 0.75c0, and it is better to use 0.75c0; --- The original subclause 6.5 related to the sound level meter. The old and new editions of sound level meter standards are in the stage of replacement; and the provisions on 0-level sound level meters are no longer used; but this subclause only mentions sound level meters incidentally, so it is deleted. --- Add Introduction. This Part’s Annexes A, B, and C are normative. This Part’s Annex D is informative. This Part was proposed by China Academy of Sciences. This Part shall be under the jurisdiction of National Technical Committee on Acoustics of Standardization Administration of China (CSBTS/TC 17). Chief drafting organizations of this Part: Institute of Acoustics, Chinese Academy of Sciences; Acoustics – Determination of Sound Absorption Coefficient and Impedance in Impedance Tubes – Part 1: Method Using Standing Wave Ratio1 Scope1.1 This Part of GB/T 18696 specifies a method for the determination of the sound absorption coefficient, reflection factor and surface impedance or surface admittance of materials and objects. The values are determined for normal sound incidence by evaluation of the standing wave pattern of a plane wave in a tube, which is generated by the superposition of an incident sinusoidal plane wave with the plane wave reflected from the test object. This method can be used for the determination of the sound absorption coefficient of sound absorbers for normal sound incidence. It can further be used for the determination of the acoustical surface impedance or surface admittance of sound-absorbing materials. It is well suited for parameter studies and for the design of sound absorbers, because only small samples of the absorber material are needed. 1.2 There are some characteristic differences between this method and the measurement of sound absorption in a reverberation room (see ISO 354). The impedance tube method can be used for the determination of the reflection factor and also the impedance or admittance. The sound is normally incident on the object surface. The reverberation room method will (under idealized conditions) determine the sound absorption coefficient for random sound incidence. The impedance tube method relies on the existence of a plane incident sound wave and gives exact values under this condition (measuring and mounting errors excluded). The evaluation of the sound absorption coefficient in a reverberation room is based on a number of simplifying and approximate assumptions concerning the sound field and the size of the absorber. Sound absorption coefficients exceeding the value 1 are therefore sometimes obtained. The impedance tube method requires samples of the test object which are the size of the cross- sectional area of the impedance tube. The reverberation room method requires test objects which are rather large and can also be applied to test objects with pronounced structures in the lateral and/or normal directions. Measurements with such objects in the impedance tube must be interpreted with care (see 9.1). For the computational transformation of the test results from the impedance tube method (normal incidence) to the situation of diffuse sound incidence, see annex D. 1.3 This Part gives preference to numerical methods of evaluation instead of graphical methods, from its front and rear ends; so that the position and flatness of the front surface of the test piece can be verified, and the position of the rear surface can also be verified. The back plate of the sample holder shall be rigid and shall be fixed tightly to the tube since it serves as a rigid termination in many measurements. A metal plate of thickness not less than 20mm is recommended. For some tests, a volume of air behind the test object, with a depth of λ0/4, acts as a pressure- release termination. Movable plugs in the sample holder are used sometimes as rigid terminations, which allow for a variable depth of this air gap. They should be used with great care, because even tiny leaks between the plug and the wall of the sample holder will lead to erroneous results (for corrections for distances other than λ0/4, see annex C). 6.3 Microphone A movable microphone registers the standing wave pattern in the impedance tube, for the localization of pressure minima and for the acquisition of sound pressure amplitudes (or levels) in the maxima and minima of the standing wave. Either the microphone moves outside the impedance tube, in which case it is connected to a probe tube with a sound pick-up opening in the impedance tube, or the microphone itself is placed (and is movable) in the impedance tube. The blockage of the cross-section of the impedance tube by the microphone and/or supports and/or other installations shall not be larger than 5 % in any cross-section of the test section. 6.3.1 Microphone with probe tube The probe tube shall be of metal with sufficient wall thickness to avoid cross-talk of the sound field into the probe tube through the walls. The boring of the tube should be relative to its length; a long probe tube of small diameter may have too high an internal attenuation (for a check, see annex B). In a horizontal impedance tube, a centrally mounted probe tube shall be supported to avoid flexion of the probe tube, as this might give rise to higher sound modes. The supports shall not be close to the sound pick-up opening. In a vertical impedance tube with the installation section at the lower end, the microphone or the probe tube may hang freely in the impedance tube. In a rectangular impedance tube, the tube may be rotated around its axis by about 45° (see figure 3) and the probe placed into the lower corner; then any supports can be avoided. A further advantage of this position is that structure-borne vibrations of the impedance tube are smallest in the corners (see annex B for a check of structure-borne cross-talk). In principle, the sound pick-up opening can be in a corner, too. For registration of the sound pressure on or near the axis of the impedance tube, an elbow termination with the pick-up opening can be used. Mechanical contacts between the probe and the impedance tube which can transmit vibrations to the probe tube shall be avoided, especially at the opening where the probe tube enters the ......Source: Above contents are excerpted from the full-copy PDF -- translated/reviewed by: www.ChineseStandard.net / Wayne Zheng et al. 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