ISO-10534-1-1996.pdf

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- 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, (Pmin 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 where 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), 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 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 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 quantities 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 object, 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 and 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 - 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) . . . (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 the 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” (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-porous 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 fre- 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. 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-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 from 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 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 characteristics, 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 resonators, 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 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 of 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 working frequency range from the tests in annex B; distance of the reference plane from the surface of the test object, if larger than zero, and if so, indication whether the results are corrected to the object surface or not; statement as to whether or not a correction for tube attenuation has been applied: representation of the test results in tabular and/or in graphical form; 12 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 10534-1:1996(E) n) temperature and atmospheric pressure; 01 statement that the tests were performed in agreement with this part of IS0 10534, if this is true in all details. tf more than one test sample is tested for one test object, both the individual results for each test sample as well as the average shall be given. If different im- pedance tubes were used for a wider frequency range of tests, the working frequency ranges of the tubes shall overlap by about one octave. Test results from different impedance tubes shall not be averaged at the overlap frequencies but shall be given (and plot- ted) separately. If the sound absorption coefficient is determined and represented in graphs over frequency, the abscissa shall be logarithmic with a scale of 100 mm per fre- quency decade, and the ordinate shall be linear with 100 mm for the range between 0 and 1, or otherwise in the same proportions as the coordinates (see IS0 354). If the sound absorption coefficient a has values higher than 0,9, it is recommended to plot (on the same scale as rxI the magnitude 1 rl of the reflection factor. The reflection factor r can either be represented by its components r and J or by its magnitude 1 rl and phase CD. The unit for (in degrees or radians) shall be indicated. If impedances and/or admittances are determined. they shall preferably be represented as a normalized impedance (z = Z/Z,) or as a normalized admittance (g = GZo). If the results are presented in tabular form, the values for a shall be rounded to two significant figures, and the values for z and g shall be rounded to three signifi- cant figures. 13 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 10534-1:1996(E) Annex A (normative) Q IS0 Preliminary measurements A.1 Determination of the acoustic centre of the probe microphone The position of sound pressure minima must be de- termined for the evaluation of equation (I 9) or (20) and equation (28). Since the acoustic centre of the pick-up opening of the probe tube or microphone may be dif- ferent from their geometrical centres (especially with measurements of sound pressure minima), the acoustic centre of the probe microphone must be de- termined. This shall be performed at frequencies over all the working frequency range with mutual distances not larger than one-third octave and with the sample holder empty (rigid termination). Then the oressure minima are at distances Xmin,n = (2n - I) f is the frequency, in hertz. This estimation, however, does not consider sources of attenuation such as porous walls and objects in the tube. Thus it can be considered as a lower limit. If one is not sure whether such additional contribu- tions of attenuation do exist, it is recommended to de- termine the attenuation using equation (19) or (20) at mid and upper frequencies of the working frequency range, and to extrapolate to the lower frequencies. so = Jdxmax.l )I lpol (A.211 Figure A.2 - Correction for test tube attenuation 16 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 Annex B (normative) IS0 10534-1:1996(E) Verification of the test equipment B.l Verification of the standing wave ratio The standing wave patterns in the impedance tube shall be recorded (preferably by a slow continuous movement of the probe microphone through the test section) with an empty sample holder (rigid termin- ation by the back plate of the sample holder). The recordings shall be repeated at frequencies in the working frequency range with frequency steps not larger than one-third octave. Intermediate frequencies shall be added if strong variations of the recorded patterns are observed for adjacent one-third-octave frequencies. The standing wave ratios of the recorded patterns shall not be less than 45 dB (this allows for measure- ments of absorption coefficient CT as low as 0,041. The envelope of the pressure minima shall be either hori- zontal or have a monotonic increase towards the loud- speaker end of the impedance tube (see figure B.l). An increase between succeeding minima of 1 dB is permissible (its influence on the measuring results can be corrected for; see annex A). There shall be no rip- ples on the standing wave patterns (see figure B.2). If these requirements are not fulfilled, the reasons may be as follows. a) b) There is too strong an increase in the lower en- velope with standing wave ratios which are too small. Reason: too large an attenuation in the imped- ance tube (rough, porous, vibrating walls, leaks in the corners, cables and ropes in the test section). The standing wave ratios are too small. Reasons: -the signal processing equipment has too small a dynamic range (electronic and/or acoustic noise, too high an at- tenuation in the probe tube); - airborne or structure-borne cross-talk (i;ziFEent, insulation of the loud- , vrbrations of the probe tube). Figure B.l - Regular standing wave pattern with test tube attenuation Figure B.2 - Ripples on the standing wave pattern 17 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 10534-1:1996(E) c) There is a non-monotonic increase of the lower envelope (see figure 8.3). Reasons: - higher modes in the test section (too strong an excitation of higher modes by the loudspeaker; higher modes generated by the probe tube or the microphone or the supports or other objects in the test section; vibrating tube walls; leaks in the tube walls); - structure-borne sound in the imped- ance tube and/or in the probe tube. d) There are ripples on the standing wave patterns. Reason: higher harmonics in the signal (insuf- ficient filtering; nonlinearity of a signal- generating component; rattling of an object in the test section, such as the probe tube or supports). e) The minima are rounded off. Reason: signal in the minima is below electronic or acoustic noise level (too high an at- tenuation in the probe tube; too small a signal amplitude, etc.). Several phenomena and reasons may be combined. Resonant excitation to vibrations of the probe tube usually not only depends on the frequency but also on the position of the probe. B.2 Dynamic range of the probe microphone In a first check the (electronic and acoustic) back- ground noise levels shall be determined at the same frequencies as in B.l with the signal