best practices: a guide to making effective measurements of sound absorption … · 2018. 2. 6. ·...
TRANSCRIPT
BEST PRACTICES: A Guide to Making Effective Measurements of
Sound Absorption Coefficient
Contact us if questions arise: [email protected]
Copyright © 2018 TFAcoustics, LLC
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Table of Contents 1. INTRODUCTION ..................................................................................................................................... 2
2. SAC MEASUREMENT – OVERVIEW ........................................................................................................ 2
3. PREPARATION AND MOUNTING OF SAMPLES ...................................................................................... 3
3.1 Characteristics of Good Samples .................................................................................................. 3
3.2 Methods for Cutting Samples ....................................................................................................... 4
3.3 Selection of Material Sample Sheets ............................................................................................ 6
3.4 Cutting, Marking and Mounting of Samples ................................................................................. 6
3.5 Examples of Sound Absorption Measurement ............................................................................. 7
4. MICROPHONES AND CALIBRATION ........................................................................................................ 9
4.1 Microphone Type and Mounting .................................................................................................. 9
4.2 Calibration of Microphones ........................................................................................................ 11
5. DEVELOPING A QUALITY TESTING PROGRAM ...................................................................................... 11
6. PERIODIC MAINTENANCE OF TFAcoustics IMPEDANCE TUBE .......................................................... 12
7. REFERENCES ......................................................................................................................................... 12
APPENDIX 1: Sensitivity of Sound Absorption Coefficient to Environmental and Physical Parameters .... 13
APPENDIX 2: Frequency Limits of the TFAcoustics Impedance Tube ......................................................... 16
APPENDIX 3: Measurement of Microphone Acoustic Centers .................................................................. 20
APPENDIX 4: “Impedance Tube Specimen Preparation and Mounting Issues” by Dan R. Stanley,
Proceedings, Internoise 2012 ..................................................................................................................... 22
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1. INTRODUCTIONThe purpose of this manual is to help you obtain consistently reliable results of sound absorption coefficient (SAC) using the ACUPRO impedance tube. This manual is a
supplement to the standards1,2 that provide background information and describe the measurement procedure in detail.
Like all experimental methods, there is a certain amount of art or technique in making SAC measurements. We hope the information in this manual will help minimize the uncertainties in SAC measurements so that you may gain confidence in the results. If you need additional help or information, please do not hesitate to contact us:
2. SAC MEASUREMENT – OVERVIEW
The measurement of sound absorption of materials is complicated by the following considerations:
1. No “reference” sound absorbing material exists with which to compare results2. There is normally variation in the manufacture of sound absorbing materials3. There can be variation in sample preparation including sheet selection and sample
cutting4. Mounting of the sample in the sample holder must be done with care5. Not accounting for environmental factors such as temperature or physical parameters
such as microphone spacing (this is discussed in more detail in Appendix 1)6. Other factors, such as training of personnel, poor maintenance of records, etc.
The following suggestions will help remove much of the uncertainty in SAC measurements:
1. The user should become familiar with the process of SAC measurement by testing the samesample repeatedly over a number of days/weeks (items 4‐6 above)
2. The user should average the results from multiple samples (items 2‐4 above)3. The user should insure that materials selected for SAC measurements meet manufacturing
specifications and tolerances (item 2 above)
There are additional sources of uncertainty due to the SAC testing itself. These include:
Region 1: Uncertainty where the SAC is low, especially at low frequencies. Region 2: Uncertainty where the SAC has a resonance condition. Region 3: Uncertainty where the SAC approaches the high frequency limit of the tube.
Figure 1 shows these three regions for two common foam materials. As with most uncertainties, these uncertainties can be reduced by averaging the SAC results from multiple samples. These regions and the limitation of the impedance tube are discussed in detail in Appendix 2.
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When making SAC measurements, the value of practice and experience cannot be overstated. Users who make measurements on a frequent basis become skilled at noticing when data looks “good” and when it looks “bad.” Users also become practiced at cutting and preparing samples and learn how to mount samples in a repeatable manner.
3. PREPARATION AND MOUNTING OF SAMPLES
3.1 Characteristics of Good Samples
• Samples should be geometric circular cylinders• Both faces should be flat and parallel
– Roll test: place the sample on its side and push it gently. A good sample will rollin a straight line.
• Sides should be smooth (no bulges or cups)• Facings (coverings), if any, must not extend beyond the edge of the sample. It may be
necessary to trim the facing slightly around its edge. The facing should cover the samplebut not extend beyond it as this will cause the facing to bend when it is inserted into thesample holder.
• Must be correct diameter
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Sound Absorption Coefficient
Frequency (Hz)
25 mm foam
12 mm foam
Figure 1. Three common regions of uncertainty in SAC measurements.
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– Cutters may cut different materials to a slightly different diameter due to thecompressibility of the material; user may need several cutters with differentdiameters
– Test for correct diameter: when the sample holder is held in the verticalposition, sample should remain inside, but just barely.
• Cutter should be sharpened after cutting several samples
3.2 Methods for Cutting Samples
There are three common methods to cut samples for SAC measurements. These are CNC waterjet cutters, rotating blade cutters, and die cutters. The advantages and disadvantages of each method are discussed below; more information is provided in Reference 3 which is reprinted in Appendix 4.
(1) CNC Waterjet Cutters
CNC waterjet cutters are the best overall method for preparing samples for a wide variety of materials, especially plastic foams. The size of the sample is accurate, and, most of all is repeatable. The major disadvantage of waterjet cutters is their very high cost which is difficult to justify unless SAC testing is done on a regular basis.
(2) Rotating Blade Cutters
Rotating blade cutters are very sharp, circular devices that are mounted in a drill press or similar device as shown in Figure 2. Rotating blade cutters are a good alternative to waterjet cutters because they are inexpensive. The problem with rotating blade cutters is they have a fixed diameter; depending on the compressibility of the material, a rotating blade cutter will cut samples having slightly different diameter.
The other problem with rotating blade cutters is they must be kept very sharp to produce high‐quality samples. Figure 3 shows how to sharpen a rotating blade cutter. It is suggested that rotating blade cutters be sharpened after several samples.
Figure 2. A rotating cutter (left); cutter mounted in a drill press (right).
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(3) Die Cutters
Die cutters are similar to rotating blade cutters but are used to shear cut a material using a mechanical press or a hammer. Like rotating blade cutters they have a fixed diameter and must be kept sharp. The main advantage of die cutters is that they can cut semi‐hard materials such as rubbers. They also work well in cutting low density fibrous materials. Figure 4 shows how to use a hand‐operated die cutter.
Figure 3. Sharpening a rotating blade cutter
Figure 4 Using a hand‐operated die cutter.
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3.3 Selection of Material Sample Sheets
An often overlooked factor in SAC measurements is the selection of the material sheets from which samples are cut. The following are important criteria in material sheet selection:
Material sheets should have uniform thickness and must not have tears, warps, bendsor other physical deformities – sheets must be flat and uniform in all directions
Material sheets must be clean – no dirt, moisture, or foreign materials
Material sheets that meet the above criteria must meet chemical and physicalspecifications of the manufacturer
3.4 Cutting, Marking and Mounting of Samples
The material sample sheets should be selected according to the criteria in Section 3.3. Once this has been done, use an appropriate sample cutting system (see Section 3.2) to prepare the samples.
Initially, three to five samples should be prepared. Subsequently, cut a quantity ofsamples needed to produce an acceptable uncertainty based on material variability,cutting variability, and other testing factors.
Give special attention to the fit of the samples inside the sample holder; use the“diameter test” in Section 3.1 for the correct fit.
Mark the side of the samples to be placed toward the sound source. All samples cutfrom a material sample sheet should be tested with the same side toward the soundsource. The properties of some materials are different depending which side is tested.
Set the correct piston position before inserting the sample into the sample holder, asshown in Figure 5. Inserting the sample into the holder before setting the pistonposition may compress the sample or create an air space behind it.
Attach a piece of thin “double‐sided” tape (such as “Scotch #665 Double‐Sided Tape”) tothe piston face. This will hold the sample to the piston.
Insert the sample with the marked side toward the sound source.
The sample should be located flush with the sample holder flange, as shown in Figure 6.
If the sample has a cover, it should not be larger than the sample diameter; it may benecessary to trim the cover slightly around its circumference to prevent the cover frombending when the sample is inserted into the sample holder.
Attach the sample holder to the impedance tube using the three screws supplied. Thesescrews are sufficient to hold the sample holder against the impedance tube. (Six holesare provided for the convenience of attaching the sample holder, but only three screwsare needed.) The screws should be spaced 120 degrees apart (every other hole) in thesample holder flange. Hand‐tighten the screws equally. Do not use a wrench.
When properly mounted, the O‐ring located in the sample holder flange will becompressed; a very small space may remain.
Do not make any changes to the piston position while the sample holder is attached tothe impedance tube; the change in pressure created by moving the piston will alsomove the sample because the system is very well sealed.
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If you want to create an air space behind the sample for testing purposes, remove thesample holder, set the piston position, and insert the sample to the required position.
3.5 Examples of Sound Absorption Measurement
Four samples of 25 mm foam and four samples of 12 mm foam were prepared according to the procedures in Sections 3.3 and 3.4. All of these samples were cut using a rotating blade cutter as described in Section 3.2. Figures 7 and 8 show the SAC for each of the samples. Figure 9 shows the averages of the samples.
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Sound Absorption Coefficient
Frequency (Hz)
Sample 1
Sample 2
Sample 3
Sample 4
Figure 5. Setting the piston position. Figure 6. A correctly mounted sample.
Figure 7. SAC of four "identical" samples of 25 mm foam.
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Sound Absorption Coefficient
Frequency (Hz)
Sample1
Sample 2
Sample 3
Sample 4
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Sound Absorption Coefficient
Frequency (Hz)
Averages of Foam Samples
25 mm foam
12 mm foam
Figure 8. SAC of four "identical" samples of 12 mm foam.
Figure 9. Averages of four samples of 25 mm and 12 mm foam.
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4. MICROPHONES AND CALIBRATION
4.1 Microphone Type and Mounting
It is recommended that a pair of microphones be dedicated to SAC measurements. The use, calibration, and behavior of a pair of dedicated microphones can be recorded over time to note any changes in microphone response or sensitivity. The microphones should remain in the microphone holders without changing their orientation (i.e., do not rotate the microphones in their holders).
The standards1,2 recommend that “pressure” (pressure response) microphones be used for SAC testing. Either ½ inch or ¼ inch microphones may be used. Microphones may be either “pre‐polarized” or externally polarized. It is preferred to use so‐called “back‐vented” microphones as back venting improves the phase response at low frequency. It is not necessary to use so‐called “phase matched” microphones because the standards require a phase calibration in any case, as will be discussed in Section 4.2.
One‐half inch microphones have the advantage that they may be used for testing at relatively low sound levels compared to ¼ inch microphones. This is because ½ inch microphones have a nominal sensitivity of approximately 50 mV/Pa compared to 1 mV/Pa for ¼ inch microphones. However, for precision results above 5 kHz use ¼ inch microphones.
For both ½ and ¼ inch microphones sealing is accomplished by an O‐ring inside of the microphone holder.
The preferred method of mounting microphones is shown in Figure 10. Using the finger or a small tool, apply a very small amount of petroleum jelly or similar lubricant to the O‐ring inside of the microphone holder. This will prevent the O‐ring from being damaged by the sharp edges on some preamplifiers. (Almost certainly there will be some minor damage to the O‐rings, but this will not affect the O‐rings’ ability to seal around the microphones.) Also apply a small amount of lubricant to the edge of the preamplifier to help the preamplifier stretch the O‐ring as it is pushed through the microphone holder.
Warning: Do not remove the protective grid from the microphone.
For ½ inch microphones, insert the microphone from the curved surface side of the microphone holder as shown in Figure 10. Pull the connector end of the preamplifier through the microphone holder until the microphone stops. When properly installed the microphone grid will be flush with the curved surface of the microphone holder.
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One‐quarter inch microphones are mounted in a similar way except that a brass insert is used to hold the microphone inside the microphone holder, as seen in Figure 11. Insert the microphone into the brass insert, pull the connector end of the preamplifier through the brass insert until the microphone stops, and lightly tighten the nylon set screw using a 5/64” Allen wrench (supplied). Apply a small amount of lubricant to the rounded end of the brass insert and carefully push the insert into the microphone holder until it stops.
Figure 10. Preferred method of mounting ½ inch microphones.
Figure 11. Preferred method of mounting ¼ inch microphones.
Do not over‐tighten set screw
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4.2 Calibration of Microphones
It is not necessary to perform an amplitude calibration of the microphones using a sound calibrator or pistonphone device. However, the standards1,2 require a relative phase and amplitude calibration of the microphone pair. This procedure is described in some detail in the standards and is summarized here.
Insert the sample holders/microphones into the impedance tube. It is suggested that the microphones be identified in some way so that the same microphone/preamplifier combination always occupies the same position in the tube when testing takes place. The microphone position nearer to the sound source is always designated Position 1, and the microphone position nearer to the sample is always designated Position 2. This is called the “Standard Position” of the microphones/preamplifiers.
Select a very absorptive material (such as 25 mm foam) and insert it into the sample holder. Attach the sample holder to the impedance tube. Perform a frequency response function (FRF) measurement with the microphones in the Standard Position. Interchange the positions of the microphone holders so that the microphone holder from Position 2 is now located in Position 1, and vice versa. Perform a second FRF measurement. The software will divide the two FRF’s to yield the microphone calibration function Hc. Return the microphone holders to the Standard Position.
Do not rotate the microphones in their holders or rotate the holders after calibration is finished. This is because a microphone has an “acoustic” center that may not coincide with its geometric center. (To correct for the microphone acoustic centers, please refer to Appendix 3.)
It is recommended that the calibration be performed at the beginning of each series of SAC tests on a given day. A copy of the microphone calibration Hc should be kept in order to observe the behavior of the microphones over time.
5. DEVELOPING A QUALITY TESTING PROGRAM
There is no “reference” test sample with which to compare one’s SAC results. To develop confidences in SAC testing it is recommended that the user (1) develop a detailed test protocol that is followed whenever testing is conducted, (2) maintain records of microphone calibration functions Hc, and (3) select a group of “reference” samples with which to compare results from time‐to‐time.
The reference samples should be materials that are typically bought (or sold) by the company on a regular basis. The samples should be prepared in a rigorous way as described in Sections 3.3 and 3.4. Approximately five samples of each of several materials should be prepared and
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tested. After testing, the samples should be kept in a clean, safe place and not exposed to heat or sunlight. Protect the samples so they are not crushed or misshapen during storage.
All samples have a “shelf life” so they must be replaced periodically, such as every 12‐18 months depending on sample material.
6. PERIODIC MAINTENANCE OF THE IMPEDANCE TUBE
Very little periodic maintenance is required to keep the impedance tube in good working order.
If it is noticed that the microphone holders become difficult to remove or insert into the impedance tube, wipe with a cloth the interior surfaces of the impedance tube where the microphone holders are inserted and coat these surfaces with a very small amount of petroleum jelly. Insert the microphone holders and move them up and down until they move
smoothly. If the microphone holders are too loose, remove them and wipe excess petroleum
jelly from the surfaces.
If the piston becomes difficult to move in the sample holder, remove the piston by first removing the black knob at the end of the piston shaft and pushing the shaft through the shaft
collar at the rear of the sample holder. At this point you should be able to remove the piston/shaft from the front of the sample holder where the sample holder attaches to the impedance tube.
7. REFERENCES1. Acoustics – Determination of sound absorption coefficient and impedance in impedance tubes –
Part 2: Transfer‐function method; ISO 10534‐2:1998, International Organization forStandardization, Geneva.
2. Standard Test Method for Impedance and Absorption of Acoustical Materials Using a Tube, TwoMicrophones and a Digital Frequency Analysis System; ASTM E1050‐10, American NationalStandards Institute, New York City.
3. Impedance Tube Specimen Preparation and Mounting Issues, Stanley, Dan R., Proceedings,Internoise 2012, New York City, 19‐22 August, 2012.
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APPENDIX 1: Sensitivity of Sound Absorption Coefficient to Environmental
and Physical Parameters
The primary environmental and physical parameters for impedance tube measurements are shown below. These include the ambient temperature T, the ambient pressure P, the distances x1 and x2 from the sample to the microphones, the microphone spacing s, and the tube diameter d.
The sound absorption coefficient depends only on the microphone spacing s and the ambient temperature T. The acoustical impedance (z) depends on the ambient temperature and ambient pressure P (unless z is normalized by the characteristic impedance) as well as the distance x1 (or x2) and the spacing s. All measured quantities are independent of the impedance tube diameter d. Figures A1‐A4 show the sensitivity of T and s on the sound absorption coefficient for two thicknesses of foam.
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Sound Absorption Coefficient
Frequency (Hz)
Temperature Sensitivity, 12 mm Foam
T = 20 C; mic. Spacing = 29.21 mm
T= 15 C; mic. Spacing = 29.21 mm
T = 25 C; mic. Spacing = 29.21 mm
Figure A1. Temperature sensitivity of 12 mm foam.
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Sound Absorption Coefficient
Frequency (Hz)
Microphone Spacing Sensitivity, 12 mm Foam
T = 20 C; mic. Spacing = 29.21 mm
T = 20 C; mic. Spacing = 28.91 mm
T = 20 C; mic. Spacing = 29.51 mm
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Sound Absorption Coefficient
Frequency (Hz)
Temperature Sensitivity, 25 mm Foam
T = 20 C; mic. spacing = 29.21 mm
T = 15 C; mic. Spacing = 29.21 mm
T = 25 C; mic. Spacing = 29.21 mm
Figure A2. Microphone spacing sensitivity of 12 mm foam.
Figure A3. Temperature sensitivity of 25 mm foam.
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From Figures A1‐A4 it may be seen that the uncertainty increases above 5 kHz due to uncertainty in the ambient temperature and/or microphones spacing. However, the sensitivity to microphone spacing is much greater than that due to ambient temperature.
It should be possible to measure the ambient temperature to an uncertainty less than one degree Celsius, so that the uncertainty due to ambient temperature is quite small and can be neglected in most cases.
The physical microphone spacing s is known with high certainty to be 29.21 mm. However, the uncertainty due to microphone spacing is complicated by the fact that the acoustic center of a microphone does not coincide with its geometric center. This is explained further in Appendix 3. To reduce the uncertainty due to microphone spacing above 5 kHz, it is necessary in mostcases to replace s with its value based on knowledge of the acoustic centers of the microphonesused in the SAC measurement.
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Sound Absorption Coefficient
Frequency (Hz)
Microphone Spacing Sensitivity, 25 mm Foam
T = 20 C; mic. spacing = 29.21 mm
T = 20 C; mic. Spacing = 28.91 mm
T = 20 C; mic. Spacing = 29.51 mm
Figure A4. Microphone spacing sensitivity of 25 mm foam.
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APPENDIX 2: Frequency Limits of the TFAcoustics Impedance Tube
The TFAcoustics impedance tube may be used to obtain SAC data from 50 Hz to 5650 Hz. However, the useful frequency range will be somewhat less depending on the uncertainty in the SAC data that the user is willing to accept and the additional calibration (see Appendix 3) that the user is willing to conduct. The following examples will illustrate the limits of the impedance tube.
Low Frequency Uncertainty
The uncertainty in Region 1 of Figure 1 manifests itself as a random error superimposed on the
mean SAC result, as seen in the figure below for a sample of 25 mm foam. The results in this
figure were obtained using microphone spacings of 29.2 and 76.2 mm. Except for a small amount of random error on the 29.2 mm data, the results are identical for both spacings. If the
random error is undesirable, a frequency smoothing of the SAC data can be performed.
Figure A5. Low‐frequency response of impedance tube for two microphone spacings.
High Frequency Uncertainty
The uncertainty in Region 3 is due to the proximity of the measured frequency to the maximum
frequency of the impedance tube. The maximum frequency is determined by the first cross mode frequency of the impedance tube or by the limitation due to the microphone spacing, whichever is smaller. Table A1 shows these frequencies for the TFAcoustics’ tube at 20° C.
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Sound Absorption Coefficient
Frequency (Hz)
Microphone spacing = 29.2 mm
Microphone spacing = 76.2 mm
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Table A1. Maximum Frequency of Tube as Recommended by Standards and from Theory.
Standard (or Theory) Based on tube diameter Based on microphone spacing
ISO 10534‐2 5717 Hz 5284 Hz
ASTM E‐1050 5776 Hz 4697 Hz
Theory 5776 Hz 5871 Hz
The standards1,2 are somewhat conservative in their recommendation of a maximum
frequency, especially ASTM E‐1050, as seen from Table A1.
The standards also point out that there is no “reference” absorption material with which to
compare one’s results. However, there is another option just as effective: measure the
acoustical impedance of a closed, rigid tube and compare it to acoustic theory. Both the sound
absorption coefficient and the acoustical impedance are calculated from the pressure reflection
coefficient, so we would expect one to be a good substitute for the other. A measurement of
the acoustical impedance of a closed tube will show the frequency limits of the impedance tube
and serve as a check on the effect of parameters such as ambient temperature and the location
of the microphones. The idea is shown in Figure A6.
Figure A6. Acoustical Impedance of a closed tube of length L.
The normalized acoustical impedance of a closed, rigid tube of length L is z = ‐jcot(kL) where k is
the wave number 2πf/c, f is the frequency in hertz, and c is the speed of sound. This
experiment must be done with a careful measurement of L, an accurate value of the ambient
temperature with which to determine the speed of sound, and correction for microphone
acoustic centers (see Appendix 3).
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Figure A7 shows the results of the closed tube test for the 34.8 mm TFAcoustics impedance tube where L is approximately 51 mm. Figure A7 shows good agreement between the theoretical impedance and the impedance measured by the impedance tube.
The upper limit of the impedance tube occurs at approximately 5,700 Hz which is due to the
onset of the first tube cross mode. The frequency of this mode at 20° C is
As the frequency approaches fmax the uncertainty in the SAC results will increase as can be seen
in many of the examples in this report.
Figure A7. Acoustical impedance of a closed tube.
Uncertainty Near Resonances
Like all acoustic systems, sound absorbing materials exhibit acoustic/structural resonances. The
resonance frequencies depend on the properties of the material (e.g., flow resistivity) and
material thickness and occur where the imaginary part (reactance) of the acoustical impedance
is zero:
0⁄
The acoustical impedance is part of the SAC measurement procedure. Figure A8 shows the
acoustical impedance of the 25 mm foam shown in Figure 1. Figure A8 shows two resonance
frequencies between 1‐2 kHz that are responsible for the peak in the SAC in Figure 1. The
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0
1
2
3
4
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Imaginary Part of Im
pedan
ce
Frequency (Hz)
Measured
Theory: ‐ cot(kL)
Hz 5776586.0max dcf
5,650 Hz
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location of these resonance frequencies and indeed the position of the entire reactance curve
in Figure A8 is quite sensitive to the factors mentioned above as well as to sample cutting and
preparation. This “resonance” sensitivity will increase the uncertainty in SAC measurements of
such samples.
Figure A8. Acoustical impedance of a 25 mm foam sample.
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Norm
alized Im
pedan
ce
Frequency (Hz)
Reactance
Resistance
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APPENDIX 3: Measurement of Microphone Acoustic Centers
To reduce the uncertainty of sound absorption measurements at high frequency it may be necessary to know the spacing between the acoustic centers of the microphones. This spacing should be used in place of the “geometric” spacing s to determine more precisely the sound absorption coefficient. The following is a procedure for determining the microphone acoustic centers.
1. Select a frequency range 0 – 2 kHz and at least 1600 measurement points.2. Measure the microphone calibration function Hc for this frequency range using a highly
absorptive sample such as 25 mm foam.3. Determine the frequency response function (FRF) between the microphones with no
sample and with the backing piston positioned flush with the flange of the sampleholder, as shown in Figure A9 below.
4. Correct the FRF of Step 3 by dividing by Hc to obtain a result similar to Figure A10.
Figure A9. Determining the microphone acoustic centers.
Figure A10. Corrected FRF (magnitude).
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5. Obtain the distance s between the acoustic centers using the following equations:
2501 1
mm
331.6 1273
m/s
Example: T = 20 C f1 = 1055.6 Hz f2 = 1657.4 Hz
331.6 120
273343.53 m/s
250 · 343.531
1055.61
1657.429.54 mm
6. Use 29.54 mm in place of the geometric spacing 29.21 mm. See example below.
Note: changing the orientation of the microphones in their holders or rotating the holders will require re‐measurement of the microphone acoustic centers.
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Sound Absorption Coefficient
Frequency (Hz)
Correction for Microphone Acoustic Centers
After correction, s = 29.54 mm
Before correction, s = 29.21 mm
Figure A11. SAC of 12 mm foam corrected for microphone acoustic centers.
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APPENDIX 4: “Impedance Tube Specimen Preparation and Mounting Issues” by
Dan R. Stanley, Proceedings, Internoise 2012
Impedance Tube Specimen Preparation And Mounting Issues
Dan R. Stanley a)
E-A-RTM
Thermal/Acoustic Systems
Aearo technologies LLC, a 3M Company
Indianapolis, IN 46268
1 INTRODUCTION
The collection of impedance data by ASTM E10501 to characterize the acoustic absorption
of a material is a process that has been precisely addressed with regards to the theory,
mathematics, and physical apparatus. In actual laboratory practice, a critical part of obtaining
reliable measurement data rests with the preparation and fitting of measurement specimens in an
impedance tube test cell. This is a component of the process that is typically either sketchy, or
largely undefined in references on the subject of impedance measurement. The quality of a test
specimen and the way that it fits into a test cell can spell the difference between very good and
poor measurement results. This paper will outline and address a range of issues and concerns
that revolve around specimen selection, preparation, and mounting to insure accurate test results.
2 MATERIAL SAMPLE SHEET SELECTION
Material samples used to cut out impedance test specimens should meet the following
criteria where ever possible.
1. Material sample sheets should have the prescribed thickness.
2. Material samples should have even thickness over the cutting area.
3. Material samples should be without warps, bends, creases, wrinkles, or blemishes.
4. Material samples should be free of moisture, dirt, or other foreign materials on the
surface, or internal to the sheet.
5. Material samples with a facing present should have no unwanted perforations or tears in
the facing material, and preferably no wrinkles.
________________________ a)
email: [email protected]
6. Material samples should meet any engineering specified chemical and physical standards
that apply.
3 TEST SPECIMEN CUTTING
The cutting of test specimens is a critically important step in obtaining good, consistent test
results. A variety of cutting methods can be used, each with particular advantages and
disadvantages. There are three cutting systems that are well suited for obtaining specimens for
impedance testing, as follows:
1. Rotating circular steel blades mounted in a drill press or milling machine.
2. High pressure water jet stream with computer numeric control of cutting.
3. Circular die blades used with a stamping press.
Rotating circular blades work best with smaller sample number runs for most materials.
With good blades and proper technique, samples are usually of very good to excellent geometry.
Of the three cutting approaches, the rotating blade system is the most cost effective to obtaining
good test specimens due to the cost of a set of blades and a drill press being considerably lower
than that of a water jet or stamping press. This approach is not a good choice for some items
with a tough, fibrous nature, or a flimsy nature. Materials cut by this process must be carefully
stabilized on a faced cutting base to insure that friction does not rip the material apart when
cutting thicker samples. The process is also labor intensive due to a need for fairly frequent
honing of the edge of the blade. And there is no way to make very fine specimen diameter
adjustments within a standard test diameter for unusual or variable material characteristics.
To be effective, a rotating blade must be very smooth, true, accurate, and sharp edged.
Such a blade would be of a steel alloy with good strength and ease of sharpening. It would also
have a wall thickness as thin as reasonably possible, to minimize friction with the sample
material during the cutting process. A good blade will have a tapering wall profile with a
maximum thickness of no more that 0.045” to 0.050”, concentricity of 0.002” or less, and a very
high surface polish inside and out. The front edge will also be thin and sharp. Photos of a high
performance cutting blade are shown in Figures 1 and 2.
The water jet cutting system works best for larger sample cutting runs, where the labor time
with circular blade cutting may be prohibitive. The O.D. of a sample material can easily be
precisely adjusted with this method for special test or material circumstances. A fine water jet
operating with a slow feed rate can produce excellent test samples from most materials in a fairly
short period. This approach also can have problems cutting some materials of a tough, fibrous
nature, or a very flimsy nature. The sides of specimens over 1 inch thick can sometimes become
slightly ragged due to a loss of precision in the water jet when making deeper cuts. Also, time
must be allotted for the specimens to dry, assuming that post cut shrinkage of the material is not
a problem for test cell fit.
The die blade / stamping press method is used for materials that are unsuitable for the first
two methods. This includes materials of a tough fibrous, or a very flimsy nature. A sharp die
blade can produce good specimens from such materials. The drawbacks include the possibility of
concave edges on some materials, particularly some flexible foams, and a possible lack of
precision in controlling the O.D. compared with the other two methods. This method is
frequently very good for cutting low density fiberglass materials of reasonable thickness, where
edge shape distortion is less of a problem.
4 TEST SPECIMEN CRITERIA
The test specimens that are cut from a material sample sheet should meet the following
specifications:
1. The specimen geometry should be that of a uniform concentric cylinder.
2. The front and rear faces of a specimen should be as flat and parallel as possible.
3. The test specimen should have very straight, smooth sides without any rough
edges or protrusions.
4. Test specimens with a facing should be cut such that the facing does not in any way
protrude beyond the edge of the specimen body.
5. Test specimens with an adhesive backing should not have any protrusions of the adhesive
that will hinder a very smooth insertion of the specimen into the test cell.
6. The specimen should have a diameter such that the sample fits into the test cell with only the
slightest physical drag. Specimens that fit tightly or loosely can tend to produce errors or
inconsistencies in measurement, particularly with facings. The ideal specimen would have
only enough friction in the cell to just avoid moving if the specimen holder were turned
upside down.
Figure 3 shows a comparison between die punching and rotating blade cutting of the same 1
inch acoustic foam material, illustrating the undesirable geometry issues that can occur with a
poorly chosen and applied cutting technique.
5 TEST SPECIMEN MOUNTING
The actual mounting of test specimens in the test cell is a very important step in obtaining
good test results. Most impedance test cells are round, and have an adjustable rear plate to
accommodate various thicknesses. This plate should be adjusted to a level slightly deeper than
the specimen height before insertion of the specimen. Test samples usually are either plain, or
pressure sensitive adhesive backed. If a rear adhesive is present, then a light coat of WD40 (or
similar) on the rear cell plate will make it easier to remove the sample from the back plate after
testing. With either case, there is a simple method of inserting a specimen into the test cell.
For insertion, the cell tube should be vertical, and the specimen should be slowly and gently
pushed down into the cell, allowing the specimen to self center in the circular cell as it
approaches the back plate. When the specimen contacts the rear plate, only a slight amount of
pressure should be put on the face of the specimen to insure that there are no rear air gaps, and
that specimens with a rear adhesive have ample adhesion to the plate. When the sample is
properly placed in the cell, the back plate can be moved forward to a point where the specimen
face is flush with the front cell lip to complete the mounting. When the mounting is completed,
the way that the sample rests in the cell should be examined. The sample should have the
following characteristics as it sits in the cell:
1. The surface, or face, should be uniformly even and flush with the front cell lip.
2. In the case of a material with a face that has topographical variations (such as a waffle
pattern), the highest point of the sample should be flush with the cell lip.
3. The front surface, or facing, should have a relatively even, consistent, and very small gap
(at most) with the cell wall around it’s circumference.
4. If a film facing is present, it should not have any wrinkles that protrude or interact with the
cell wall, and it should not be convex or concave in any way.
6 SPECIAL MOUNTING ISSUES
While most materials can be successfully mounted by using the previously detailed steps,
there may be special situations due to several issues. Various materials will usually cut slightly
differently from one to another, and final diameters tend to vary very slightly. There may also be
specimen shrinkage issues, and rates of, which vary from material to material. The best way to
deal with shrinkage (unless it is immediate) is to test samples as soon as possible after cutting. In
cases of unavoidable shrinkage, gaps between the cell wall and the face of the specimen may
exist. While the introduction of a foreign material to the face of a test specimen is not desirable,
there are times where the use of petroleum jelly may be called for to fill edge gaps around the
circumference of the specimen. The jelly is best applied with the use of a small plastic
hypodermic needle. In this way, a very small, narrow bead of the material can be applied to the
edge gap to seal the air path between the specimen and the cell wall.
7 PREPARATION & MOUNTING VERIFICATION
A good indicator of quality preparation, correct diameter, and good mounting lies with the
resulting absorption curves of groups of specimens that have been cut from a small area of a
material. If the material is relatively uniform, the resulting test curves should be closely
comparable. If there is a significant outlying test result, there may be an anomaly with the
specimen, or the way that it was mounted in the test cell. Figure 4 shows the test results of a
series of five 99mm diameter 25.4mm (1.0 inch) thick acoustic foam samples that verify proper
preparation, size, and mounting technique. All of the curves are relatively smooth and regular,
and without significant response anomalies. Averaging of groups of curves will give results
having a strong degree of measurement confidence. How measurement variation can begin to
appear when even slightly non- optimal diameters or cutting techniques are used on the same
material is shown in Figure 5.
While the use of an impedance tube system to measure acoustic absorption is not an
extremely precise and repeatable process due to unavoidable variations of specimen cutting and
cell fit, the disciplined use of the guidelines stated in this paper will help to insure that test results
maintain a consistent level of accuracy and validity. The experience gained with repeated
preparation and testing will also contribute to a better feel for more subtle aspects of preparation
and specimen fitting for testing.
8 REFERENCES
1. ASTM E1050-10, Standard Test Method for Impedance and Absorption of Acoustical
Materials Using A Tube, Two Microphones and a Digital Frequency Analysis System,
ASTM International, December 2010
Fig. 1 - Cutting end of 29mm circular steel blade with plunger tool.
Fig. 2 - Circular steel blade and center mounted specimen removal plunger.
Fig. 3 - Specimen geometry - die blade cut on the left, and rotating blade cut on the right. Note
the curved vertical edges of the die blade sample, and the slightly off axis profile.
Fig. 4 - Absorption response of a group of five nominally 99mm diameter foam samples that
were cut by the water jet technique.
Fig. 5 - Absorption response of a group of five nominally 99mm diameter foam samples that
were cut by the water jet technique.