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3、ustical Society of America All rights reserved viiiIntroduction Experimental techniques to measure impedance include the use of an impedance tube, techniques that measure the sound pressure levels above a surface, and direct measurements of sound pressure and volume velocity. This Standard does not
4、consider the direct measurement of sound pressure and volume velocity. The impedance tube is in common use to measure the acoustic impedance of porous materials. It has the advantage of a straightforward theoretical framework that allows direct determination of both the real and imaginary parts of t
5、he impedance. However, its application in the field to obtain ground impedance suffers from two major disadvantages. First, it requires an accurate measurement of the distance from the first interference minimum to an ill-defined test surface, and, secondly, it is invasive. This Standard does not re
6、commend the use of an impedance tube for the measurement of the acoustic impedance of a ground surface. Techniques that use measurements of sound pressure levels above a surface include several variations based on the type of excitation, angle of incidence, number of microphones, and fitting methods
7、. All enjoy the advantage that the measurement is performed on the ground in its natural condition. However, because of the spherical wavefront, the theoretical framework is mathematically intricate. Annexes C and D detail the expressions and special functions used in the calculations in Clause 4. .
8、 AMERICAN NATIONAL STANDARD ANSI/ASA S1.18-2010 2010 Acoustical Society of America All rights reserved 1American National Standard Method for Determining the Acoustic Impedance of Ground Surfaces 1 Scope Outdoor sound close to the ground is influenced by the acoustical properties of the ground. This
9、 Standard describes recommended procedures to characterize, and the instruments to measure quantities that may be used to deduce, the acoustical properties of ground surfaces. Although this Standard is intended primarily for outdoor measurements, indoor measurement of undisturbed portions of a groun
10、d surface, such as sod, is within its scope also. The Standard yields the real and imaginary parts of the normalized specific acoustic impedance ratio of ground surfaces in the frequency range between 250 and 4000 Hz for outdoor sound propagation predictions. The Standard uses measurements of the in
11、terference between direct and ground-reflected sound to deduce both normalized specific acoustic impedance ratio and impedance model parameters. The impedance-ratio model parameters of effective flow resistivity and a porosity factor, determined from best fits to the templates of calculated level di
12、fferences, may be used to estimate the normalized specific acoustic impedance ratio at frequencies outside the specified range. The basic purpose of this Standard is to establish uniform procedures for obtaining the real and imaginary parts of the normalized specific acoustic impedance ratio of grou
13、nd surfaces outdoors. The method is applicable to all nominally flat, commonly occurring surfaces including grassland or snow-covered ground. The method is not applicable to rough grounds where the variation in height is greater than half of the shortest wavelength of interest. For the specified upp
14、er frequency of 4 kHz this limits the variation in height to about 5 cm. See also Clause 4.4. 2 Normative references The following referenced documents are indispensable for the application of this Standard. For dated references, only the edition cited applies. For undated references, the latest edi
15、tion of the referenced document (including any amendments) applies. ANSI S1.1, American National Standard Acoustical Terminology ANSI/ASA S1.11, American National Standard Specification for Octave-Band and Fractional-Octave-Band Analog and Digital Filters ANSI S1.40, American National Standard Speci
16、fications and Verification Procedures for Sound Calibrators IEC 61672-1, Electroacoustics Sound level meters Part 1: Specifications ANSI/ASA S1.18-2010 2010 Acoustical Society of America All rights reserved 2 3 Terms and definitions For the purposes of this Standard, the terms and definitions given
17、in ANSI S1.1 and the following apply: 3.1 specific acoustic impedance, Zsat a point in a sound field, quotient of sound pressure by particle velocity NOTE 1 Unit, pascal per (meter per second) Pa/(m/s). NOTE 2 The real part of the specific acoustic impedance is specific acoustic resistance; the imag
18、inary part is specific acoustic reactance. 3.2 specific acoustic admittance, reciprocal of the specific acoustic impedance NOTE 1 Unit, (meter per second) per pascal (m/s)/Pa. NOTE 2 The real part of specific acoustic admittance is specific acoustic conductance; the imaginary part is specific acoust
19、ic susceptance. 3.3 normalized specific acoustic impedance ratio ratio of specific acoustic impedance of a ground surface to the characteristic impedance of air at specified atmospheric conditions 3.4 complex sound pressure ratio ratio of magnitudes and relative phase of the pressures measured by tw
20、o spatially separated microphones subjected to sound emitted by the same point source NOTE This definition is specific to this Standard and is not within the scope of ANSI S1.1-1999 (R 2004). 4 Measurement method 4.1 Recommended geometries The measurement of level difference spectra shall be conduct
21、ed using both of the following two geometries (see Figure 1): Geometry A Source height (hs) = 0.325 m Upper microphone height (ht) = 0.46 m Lower microphone height (hb) = 0.23 m Horizontal separation (d) = 1.75 m ANSI/ASA S1.18-2010 2010 Acoustical Society of America All rights reserved 3Geometry B
22、Source height (hs) = 0.20 m Upper microphone height (ht) = 0.20 m Lower microphone height (hb) = 0.05 m Horizontal separation (d) = 1.0 m Note that these geometries will not yield satisfactory results if the ground impedance is high. But, in this case, accurate values for ground impedance should not
23、 be necessary and it is recommended that Steps 2 to 4 be omitted from the procedure detailed in 4.5.1. Geometry A covers the broadest range of frequencies. Geometry B emphasizes ground effect at frequencies above 1000 Hz and may be better suited for hard grounds. As long as the horizontal separation
24、 of the source and the receiver is not greater than 3.0 m or not less than 1.0 m and the angles of incidence are broadly the same as those for geometries A and B, the user has discretion in the choice of the geometry. However, it is essential that the source and receiver positions be measured as acc
25、urately as possible (i.e., to within 0.01 m), and the test report shall include information on the geometry used for measurements. If Figures 3a through 4d are used, then the geometry must correspond to those specified above to within 0.01 m. SourceTop microphoneBottommicrophonehshthbR1R2dFigure 1 G
26、eometrical definitions: hs= source height, ht= top microphone height, hb= bottom microphone height, d = source/receiver horizontal separation 4.2 Time convention A harmonic time dependence of tieis assumed throughout this Standard. However, certain spectrum analyzers assume an alternative harmonic t
27、ime convention of tie. The user shall ascertain the time dependence used by the analysis equipment. If the alternative dependence is used, the complex sound pressure ratio, T(f), as measured and recorded by such equipment, shall be converted to its conjugate before carrying out the procedure set out
28、 in this clause. ANSI/ASA S1.18-2010 2010 Acoustical Society of America All rights reserved 4 4.3 Measurement procedure The test signal shall have frequency content between 250 Hz and 4 kHz. Signals may be continuous broad band, continuous discrete tones, stepped tones, tone bursts or impulses. The
29、minimum recommended frequency resolution is a 1/3 octave band such as might be achieved by use of sound level meters with a band-pass filter. The use of a spectrum analyzer should lead to a higher frequency resolution (see 5.4). The sound source shall generate a sound pressure level that is at least
30、 10 dB higher than the ambient sound pressure level in each 1/3 octave band in the frequency range of interest from 250 Hz to 4 kHz. The sound source shall have a smooth free-field frequency response with no significant maxima or minima over the principal frequency range of interest. It is to be exp
31、ected that the acoustic impedance of the ground surface will vary with location. Consequently, a sufficient number of measurements shall be taken to establish the variability with position. The source and the receiver shall be translated or rotated, or by both techniques, moved to an adjacent piece
32、of nominally identical ground. A minimum of four independent measurements shall be made for every geometry and location. The total signal complex sound pressure ratio, T(f) see Equation (C.1) in Annex C, is obtained by the same procedure for each test location and each geometry. Ideally, microphones
33、 should be selected that have nominally identical pressure sensitivity, nominally identical frequency response, and nominally identical phase response. These characteristics will require extensive calibrations. Despite calibration of the microphones, there will be some relative phase response. It is
34、 essential to take account of this for the complex sound pressure ratio method (see 4.5.1, Step 2) by averaging the complex pressure ratios obtained before and after reversing the positions of the upper and lower microphones. This procedure is advisable also when the template method (see 4.5.1, Step
35、 1) only is used. 4.4 Ground and environmental conditions No acoustical data shall be measured when the average wind velocity exceeds 5 m/s when measured at a height of 2.0 m above the ground. A wind speed of 5 m/s or less is desirable for level difference measurements using two microphones simultan
36、eously. Although use of a single microphone for successive measurements at the two positions is not recommended, it might give reasonable results by the template method (see 4.5.1, Step 1) as long as a more restrictive wind speed limit of 2.5 m/s is observed. No measurement shall be made during any
37、form of precipitation. The ground shall be in the condition desired for the measurements. It can be dry, wet, snow covered, frozen, or in any other condition provided that there is an accurate description of it in the test report. If the ground of interest is frequently wet and ground impedance valu
38、es are needed for this common condition, soil moisture content shall be determined from a moisture-sealed soil sample taken in a representative ground location immediately after the ground impedance measurements. The moisture content may be determined by measuring the percent change in weight of the
39、 soil sample, relative to the dry weight, before and after drying it completely in a suitable drying oven. This moisture content shall be included in the test report. ANSI/ASA S1.18-2010 2010 Acoustical Society of America All rights reserved 5A ground profile that is convex or concave over the test
40、site is not covered by this Standard. Over the distance d (see Figure 1), the test site shall be flat within 0.05 m and there should not be any reflecting objects within ten times the separation distance between source and receiver. Rough ground surfaces with a variation in height of less than 0.05
41、m may be treated as flat ground for the purpose of this Standard. In the case of sloping ground, the source and microphone heights shall be measured perpendicular to the surface. NOTE 1 For the purposes of this Standard, reference meteorological conditions are: Static air pressure 101.325 kPa Air te
42、mperature 288.15 K (15 C) Relative humidity 50 % For these values 00416.45 Pa/(m/s) c . The uncertainty in the specific acoustic (characteristic) impedance, c of air introduced by differences between the conditions prevailing at the time of measurement and the reference meteorological conditions is
43、not important in comparison to the other uncertainties in determining the normalized specific ground impedance ratio. NOTE 2 The specific acoustic (characteristic) impedance, c of air for any normal range of temperatures, T in Kelvin and Relative Humidity, RH, in %, and at a sea level atmospheric pr
44、essure of 1.01325 kPa, can be defined, to an accuracy of better than 0.2%, by: c = 428.0/(T/273.15)1/2(1+1.9510-5RH(%) Pa/(m/s). 4.5 Determination of the normalized specific acoustic impedance ratio 4.5.1 Steps for obtaining real and imaginary parts of normalized specific acoustic impedance ratio Th
45、e real and imaginary parts of the normalized specific acoustic impedance ratio 00cZs of the ground, henceforth abbreviated to normalized surface impedance, shall be obtained by best fit between measured and calculated level differences using the following four steps: Step 1 - Template method The mag
46、nitude of the measured level difference spectrum, LD(f) see Equation (C.3), shall be compared with that predicted using either (or both) of the two models specified in 4.5.2 and 4.5.3. This Standard provides a set of predicted level difference magnitudes and tables for each of the recommended geomet
47、ries and for each normalized surface impedance model for use as templates to yield best-fit values of model parameters. Equations (C.1) through (C.9) in Annex C enable the user to calculate level difference spectra for other normalized surface impedance models. The measured level difference spectrum
48、 shall be the average of those measured before and after switching the microphone locations. The difference given by Equation (8) between the measured and calculated LD(f), based on the templates (Figures 3 through 5) or use of Tables 1 through 6 in Clause 9, shall be determined and the parameters f
49、or which this difference is minimum shall be found using the procedure set out in 4.6. The impedance spectrum corresponding to the best-fit parameters shall be determined using the appropriate formulae in 4.5.2 and 4.5.3. ANSI/ASA S1.18-2010 2010 Acoustical Society of America All rights reserved 6 Step 2 - Complex sound pressure ratio method The measured complex sound pressure ratio, T(f) Equation (C.1), shall be the arithmetic average of the magnitudes (in dB) and the phases of averaged complex pressure ratios obtai
