ISO-10534-1-1996.pdf
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1、INTERNATIONAL STANDARD IS0 10534-l First edition 1996-l 2-l 5 Acoustics - Determination of sound absorption coefficient and impedance in impedance tubes - Part 1: Method using standing wave ratio Acoustique - D CO is the speed of sound in the medium. 3.10 normalized impedance, Z: Ratio of the imped-
2、 ance Z to the characteristic impedance Zo: 2 = z/z, 3.11 normalized admittance, g: Product of the admittance G and the characteristic impedance Z,: g = ZoG 3.12 standing wave ratio, s: Ratio of the sound pressure amplitude at a pressure maximum, lprnax 1, to that at an adjacent pressure minimum, (P
3、min 1 (if necessary after correction for varying values at the minima due to sound attenuation in the impedance tube): s= Pmax l/lPminI 3.13 standing wave ratio with attenuation, s,: Standing wave ratio of the rzth maximum to the rrth minimum. 3.14 free-field wave number, ko: k, = 4co = 27c$ co wher
4、e co is the angular frequency; f is the frequency; co is the speed of sound. In general the wave number is complex, so k. = k, - jko” where ko is the real component (ko = 2dAo); ko” is the imaginary component which is the at- tenuation constant in nepers per metre. 3.15 phase of reflection (factor),
5、 I Results from the representation of the complex reflection factor by magnitude and phase: I = r + jr” = jr/. ei = Irl (cos 0 + jsin ) =arctanc r r = Irl cos r” = Irl sin 1) To be published. (Revision of IS0 266: 1975) 2 Copyright International Organization for Standardization Provided by IHS under
6、 license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/19/2007 00:58:25 MDTNo reproduction or networking permitted without license from IHS -,-,- 0 IS0 3.16 working frequency range, f: Range within which measurements can be performed in a given impedance tube: A + 1 The
7、sound absorption coefficient a for plane waves is s = 1020 . (15) a = I- lr12 . . . (9) The sound absorption coefficient then follows from where I. 1 indicates the magnitude of a complex quantity. a= 4x lO 20 (1 OM 20 + 1)2 . . (16) Equations (7) to (9) are the inter-relationships between the quanti
8、ties which are determined according to this part of IS0 10534. If the reference plane is in the sur- face of a flat test object, these quantities are the sur- face impedance, the reflection factor (for normal sound incidence) and the absorption coefficient (for normal sound incidence) of the test ob
9、ject, respec- tively. If the reference plane is in front of the test ob- ject (x 01, the absorption coefficient remains unchanged; the reflection factor r and the impedance Z will change to quantities which are said to be “transformed to a distance”, namely the distance between the reference plane a
10、nd the object surface. This concept is used sometimes in connection with structured test objects (see 9.1 and clause IO). 5.6 Reflection factor The phase angle of the complex reflection factor r=lrl.eiQ . . . (17) follows from the phase condition for a pressure mini- mum in the standing wave + (2n -
11、 1 = 2k0Xmin,n . (18) for the .th minimum (n = 1, 2,.) in front of the refer- ence plane (towards the sound source). 5.4 Standing wave From this it follows that A pressure maximum in the standing wave occurs when pi and pr are in phase, i.e. =lr: t 4xrnin n A-22n+l a-0 1 . (19) lPrnaxI= lPoI(l+ Id)
12、. . . (IO) 4 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/19/2007 00:58:25 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 IS0 10534-1:1996(E) and for th
13、e first minimum (n = 1) =7L 4xmin 1 L 1 A-1 10 The test equipment shall be checked before use by a series of tests These help to exclude error sources and to secure the minimum requirements. Procedures . . . (20) for these tests are given in annex B. The complex reflection factor is then r = r + jr”
14、 (211 6.1 Impedance tube . . . r = Irl. COS 0 . . (22) r” = Irl. sin Qr (23) . . 5.7 Impedance From equation (7) one obtains the normalized imped- ante z = Z/Zo: 6.1 .l Construction The impedance tube shall be straight, with a constant cross-section (to within 0,2 %) and with rigid, smooth, non-poro
15、us walls without holes or slits in the test section. The walls shall be heavy and thick enough (preferably made from metal or, for tubes of larger cross-sections, from tight and smooth concrete) not to be excited to vibration by the sound signal, and not to show vibration resonances in the working f
16、re- quency range of the tube. For metal walls, a thickness of about 5 % or about 10 % of the cross-dimension is z=z + jz” . . . (24) recommended for circular or rectangular tubes, re- spectively. Tube walls made out of concrete shall be I- . 2 - y2 sealed by a smooth tight and highly adhesive finish
17、. z = (25) The same holds for tube walls made of wood. These (I- r )* + r”* . should be re-inforced and damped by an external coating of steel or lead sheets. ? = Cl _ rf)r+ ,q . (26) 5.8 Wavelength The shape of the cross-section of the tube is arbitrary, in principle. Circular or rectangular cross-
18、sections are recommended (if rectangular, then preferably square). If rectangular tubes are composed from plates, care shall be taken that there are no slits in the corners (e.g. by sealing with adhesives or with a finish). The wavelength il, at the frequency f of the sound signal follows either fro
19、m the equation a, = co/f . . (27) 6.1.2 Working frequency range where co is the sound velocity (for the determination of co see annex A), or from the distance between two pressure minima of the standing wave (with a rigid termination of the impedance tube) which are num- bered n and m, respectively
20、see equation (19)l The working frequency range 6 + ir”(cos* k,-,D -sin* bD)- (I - .z * - z”2),inkoDcos,$-,D (cos b) date of the test; cl name of the producer and tradename of the test object, if it is a commercial product; d) description of the test object including its acous- tically relevant chara
21、cteristics, i.e. 1) structural data such as e) f) 9) h) i) i) I) ml - - - - - - lateral dimensions and total thickness, flatness of the surface or characteristic profile height, if any, number, arrangement and thickness of layers, including air spaces, dimensions of structural units, such as resonat
22、ors, and their arrangement, positions of the cuts of the test sample relative to characteristic lines of test ob- jects with lateral structures, structure, thickness and porosity of covers such as grids, and perforated metal sheet; 2) material data such as - bulk density and, if available, air flow
23、re- sistivity of porous materials, - materials of the components of the test object; 3) construction characteristics such as - connection of layers to each other (glued or other), - partition walls normal to the surface in the test object; mounting conditions of the test object in the tube; number o
24、f test samples of the test object; inner dimensions of the impedance tube and its shape; material and thickness of the tube walls; type of probe microphone (i.e. with or without probe tube); maximum value and minimum value (in decibels) of the standing wave ratio in the test section and in the worki
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