The Negative Coefficient of Bruel & Kjaer Green Paint

Transcription

1 The Negative Coefficient of Bruel & Kjaer Green Paint Euronoise2018 Proceedings Steven Cooper The Acoustic Group Pty Ltd, Australia. Summary The typical expression of the absorption of blockwork can describe plain blockwork and painted blockwork. For certification of a reverberation room the reverberation times were obtained during the construction phase. As expected painting the blockwork increased the reverberation time. The application of multiple layers of a green paint identical to Bruel & Kjaer's green colour was subject to testing. Technically this acoustic treatment must be a negative absorption coefficient although such an expression would not normally be used. When applying the standard calculations for absorption is there a difference in the absorption coefficients? PACS no Architectural acoustics, Ev Sound Absorption properties of materials 1. Introduction 1 Most people attending this presentation (or reading the paper) and who are involved in the fascinating world of acoustics consider that their student days when learning acoustics occurred a long while ago. On the contrary, I suggest those persons who are dedicated, obsessed or fascinated with the area of acoustics (and love their work) never stop being a student of acoustics. They utilise their activities and experiences in the real world to develop different concepts or challenge the status quo of acoustics, whilst many acousticians simply accept what is contained in textbooks. The real challenges that exist for acousticians when they are faced with a situation in the real world that does not agree with the textbook concept is whether they take up the challenge in seeking to follow the principal question of Prof Julius Sumner Miller Why is it so?. Whilst working in the field as an acoustic engineer for just on 40 years I have had the privilege over the years on a few projects of being faced with situations that do not fit into the norm that has required alternative thinking (in some cases may take a few years) to fully investigate the problem. It is those unique challenges and an inquisitive mind that has given rise to technical satisfaction and enjoyment in my world of acoustics. For example, in the early 2000s, we were faced with real-world measurements of helicopter operations, for determining noise power distance curves for input into the Integrated Noise Model ( INM ). We found that the lateral attenuation formula contained in the INM was incorrect and did not agree with actual measurements. We set about conducting physical measurements of different helicopter types to determine the actual noise power distance curves and input them into INM. INM predicted levels did not agree with field measurements. For example, INM provided an increase in noise levels for an out of ground effect hover as the elevation of the helicopter was increased? From the methodology instilled into me from my supervisor for my post graduate research to go back and look at acoustic history, I went back to the INM references related to the lateral attenuation to find that the work was conducted with engines in the rear tail of aircraft and therefore the directivity patterns associated with the lateral attenuation algorithms were incorrect. With the results of field testing of 7 military helicopters that showed noise power distance curves different to that theoretically assumed in Copyright 2018 EAA HELINA ISSN: All rights reserved

2 INM that material was presented in 2003 to the US American Standard committee on aircraft noise (with the FAA present as part of committee SAE -21) [1]. Two years later the FAA modified INM for helicopter operations [2]. The same data when applied to fixed wing aircraft noise is also relevant, but the system would not change the lateral attenuation algorithm for aircraft because it could possibly be claimed that all the INM maps around the world were inaccurate [3], [4] & [5]. In relation to architectural acoustics, and particularly the concept of reverberation time in rooms, if a student of acoustics looks at Sabine s work and has the privilege of attending the Boston Symphony Hall (as I did as part of the Acoustical Society of America s conference in June 2017) they will see that the degree of diffusion in that hall applies to the internal surface of the hall and is not extended to the type of diffusion that we see in acoustic laboratories that are used for the determination of the absorption coefficient expressed in Sabines, in honour of the pioneering work of Wallace Clements Sabine [6]. Of interest in attendance to the Boston Symphony Hall was that when there were only one or two instruments performing on stage the sound in the hall was weak/thin and not, in my opinion, that of the great hall to which I had expected. However, when the organ was used the character and the quality of the sound of the organ blossomed into a full and wonderful experience of music only when the organ was playing at a moderate to loud level. The room reacted to that level of music and provided a full and rich sound that was dramatically different to when the organ was playing quietly and did not excite the hall to accompany the acoustic signal that was being generated. Similarly, the criteria to which we define dimensions for reverberation rooms has been developed on a trial and error basis. One can view the work by CW Kosten (in the 1960s) of a roundrobin comparison of rooms using a standard absorptive material [7] that lead to the development of a range of dimensions/proportions to which there was general agreement in the absorption coefficient of the sample material. A similar round robin was conducted in Australia [8] & [9]. The concepts for reverberation room as generic parameters (relative to the volume) may have different shapes and sizes. It is the concept of the title of this paper to which thought-provoking challenges (with respect to what is generally accepted as factual material) for absorption coefficients are raised. 2. Absorption coefficients Most acousticians in attendance will be aware that there are different types of absorption coefficients depending upon the situation upon which the acoustic absorption is assessed. With the impedance tube method, using a small size sample of acoustic material subject to sound waves travelling perpendicular to the surface of the sample (normal incidence) has limitations in terms of its performance, by reason of the size of the sample. The impedance tube method tends to be used for preliminary analysis of different materials as a cost-effective solution. The absorption coefficient determined by an impedance tube method is different to the absorption coefficient obtained by the random incidence method occurring in a reverberation room. The difference between two processes might be only slight at low frequencies, but with respect to higher frequencies, the impedance tube values can be both higher and lower than those measured in a reverberation room with the degree of difference being a function of the thickness of the test material [10]. The reverberation room method using either the Sabine equation or incorporating the modifications from Eyring utilise parameters obtained from the physical size of the room and a methodology for determining the sound absorption coefficient. The student of acoustics when following the US reverberation room absorption method (ASTM Standard C423) will see that there are several versions that have changed the size of the sample. If one has data for the same material tested under the different versions of the ASTM standard and the ISO 354 Standard, it can be seen there are variations in the different test results. Table 1 presents examples of the difference in published data for the Sonex 50mm Natural Foam (now Sonex Classic Acoustic Foam) update from reference [11]. The first two ASTM results [12] & [13] were contained on one data sheet, one set presented in a graphical format but different to the coefficients presented in a table of results!

3 Test Sound Absorption Coefficients k 2k 4k ASTM ASTM ISO ASTM C423-90a Table 1. Sonex 50 mm Natural Foam If an absorption coefficient of 1.0 is expressed in acoustic textbooks as 100% absorption, then should the student of acoustics question how we can get absorption coefficients greater than 1.0? For example, last year in dealing with the refurbishment of an upmarket restaurant, I specified lagging around air-conditioning plant that was giving rise to unacceptable noise levels in the restaurant and that below the lagging (and above the perforated timber panels) I required a 50 mm thick polyester insulation for maintaining the ambience of the restaurant. However, the building contractor responsible for the ceiling work substituted the polyester with a plastic material described as a membrane with a microporous design having small air cavities to provide exceptional acoustic absorption. The basis of the replacement (apart from the contractor being a distributor of this alternative product) was a test result from a well-respected acoustic laboratory in Australia, giving sound absorption coefficients above 500 Hz significantly greater that a value of 1.0. The problem was when this material was installed in the restaurant it was obvious to many people that the acoustic characteristics of the restaurant had deteriorated dramatically and that there was far too much reverberant energy in the room. Replacement of a sample of the exceptional absorbing product with the polyester material that had been specified (that technically had lower absorption characteristics than the material installed by the ceiling contractor) rectified the problem. Taking account of the above discussion I now move to the original topic of the paper. 3. Test chamber the B & K Green Paint The general concept for the determination of absorption coefficients in a reverberation room is to start with an empty room of a reasonable volume and reflective surfaces. One measures the reverberation time at multiple locations in the room and then determines the mean reverberation time in 1/3 octave bands. One then installs a nominal m 2 of absorption on the floor of the chamber (noting that for larger chambers the sample area is proportionally increased. One then repeats the reverberation testing several times at multiple locations. There is an expectation of lower reverberation times (because of the absorptive properties of the sample) from which comparison of the different reverberation times by way of standard formula set out in the relevant Standard leads to the determination of the absorption coefficient of that test sample. On building a set of reverberation chambers a few years ago (that have been used primarily for acoustic research in relation to wind turbine noise) we adapted an existing space in the basement of our building to provide a source room with a volume of 128 m³. The room has 200 mm thick concrete slabs for the floor and roof, and a 400 mm thick concrete wall to the receiving chamber. Existing brickwork with negligible setback in the joints for the wall opposite the aperture to the test chamber and one side wall. The other side wall is core filled blockwork (untreated surface). Figure 1: Flanking Testing of receive room showing blockwork wall For this discussion we consider the blockwork wall as the test sample. Rather than provide render to the walls, that may have potential resonant components by reason of the thickness of the render it was decided to paint the walls of the room with multiple layers of an acrylic paint noting that for the blockwork wall there would need to be multiple applications of

4 paint to provide a fully sealed surface. In a general concept the paint to the existing slabs and painted brickwork is not considered to affect the absorption of those surfaces. The paint used was not an official Bruel and Kjaer paint (supplied by B & K) but was simply a paint that was colour matched to agree with the Brüel and Kjaer colour on their instruments. The colour has caused great amusement to acousticians (and B & K representatives) attending the chamber. Figure 3: Reverberation Times As expected the reverberation times for each test increase as the absorption of the blockwork is decreased. Clause of AS ISO provides the formula for determination of the equivalent sound absorption area of the test specimen identified as equation 8: A T = A 2 A 1 = 55.3V ( 1 c 2 T 2 1 c 1 T 1 ) 4V(m 2 m 1 ) Figure 2: Source Room showing painted blockwork wall As part of a series of tests that would form the calibration of the room (originally conducted several years ago) we sought to conduct measurements of what started as the empty room and then retested as three layers of paint were applied to the wall surfaces. Testing then occurred with diffusors and later testing with absorption to all walls and roof, to convert the room to a listening room for the subjective testing of wind turbine noise [14] & [15]. The determination of reverberation time utilised a dodecahedron speaker located in one corner of the room with nine test positions in the room. and measurements being conducted using a Brüel and a Kjaer Modular Sound Level Meter Type 2260 with Building Acoustics Package BZ 7204 to ISO 354 [16]. The analysis of the recorded data utilised the Brüel and Kjaer Qualifier Program Type 7230 to derive the results used in this assessment. We now use multiple channels with B & K LANXI modules. Figure 3 presents the results of four separate reverberation tests. The first test has the unpainted blockwork wall, then one coat of paint, followed by the second coat of paint then the third coat of paint. where c 1 is the propagation speed of sound in air at the temperature t 1 c 2 is the propagation speed of sound in air at the temperature t 2 A 1 is the equivalent sound absorption area of the empty reverberation room, in square metres A 2 is the equivalent sound absorption area of the reverberation room containing the test V specimen, in square metres is the volume, in cubic metres, of the empty reverberation room T 1 is the reverberation time, in seconds, of the empty reverberation room T 2 is the reverberation time, in seconds, of the reverberation room after the test specimen has been introduced m 1 m 2 is the power attenuation coefficient, in reciprocal metres, calculated according to ISO using the climatic conditions that have been present in the empty reverberation room during the measurement. is the power attenuation coefficient, in reciprocal metres, calculated according to ISO using the climatic conditions that have been present in the empty reverberation room during the measurement. Using the unpainted blockwork as the starting point as the empty room (T 1) the equivalent sound absorption area of the blockwork is calculated for

5 each layer of paint. From equation 9 the sound absorption coefficient is the equivalent sound absorption of the test specimen divided by the area of the specimen. Figure 4 presents the absorption coefficients for the different layers of paint. Because the absorption is reduced the Standard s calculation procedure automatically provides negative absorption coefficients. Figure 4: Absorption Coefficients If the testing procedure commenced with painted blockwork and was able to progressively remove the layers of paint to end up with bare blockwork one would achieve positive absorption coefficients. Using the reverberation times and equations 8 & 9 of ISO 354 leads to absorption coefficients versus the painted blockwork that are same as the original testing for the comparison of the unpainted blockwork versus the three coats. The intermediate removal of coats gave different absorption coefficients because of the different starting surfaces as shown in Figure 5 Figure 5: Absorption Coefficients if the Testing Could be Carried Out in Reverse Therefore, it is possible under the standard calculations for reverberation chambers to obtain negative coefficients if one can remove absorption from a panel. 4. Appropriate Sample Size The measurements associated with the green paint described above were simply a series of measurements undertaken as part of documenting the changes that occurred during the development of the chambers that may be relevant to future calibration, e.g. identification of flanking transmission loss, vibration measurements and modes of the room prior to and after an additional internal wall. The testing was not specifically undertaken for the purpose of the preparation of this paper, because if that had been the case the area of the painting of the blockwork may have been undertaken in sections rather than the entire area of the wall which when one excludes the single door shown in the centre of Figure 2 results in an area of blockwork just on 12 m². The relevance of identifying the area of the blockwork (and in turn the sample size used for absorption measurements) is that depending upon the volume of the room and the sample size of the material under test, there can be a significant difference in the measurement results. Referring to Table 1, that provides four sets of results for the same material when measured under three versions of the ASTM C423 code and one version of ISO 354 there are significant differences. As the absorption coefficients are based upon the original work of Sabine then the formula that are available for calculation of the absorption are based upon a diffuse environment, which was not defined by Sabine. The diffuse environment suggested by Sabine is a different environment to that in a laboratory where there are multiple panels in the upper space of the laboratory to improve the diffusion. If one takes a moderately diffuse room (rather than reverberation chamber) and introduces absorption on a progressive basis, similar to Sabine introducing cushions into his test environment, there is a progressive reduction of the reverberation time. In the late 1980s I conducted testing in the reverberation room at Sydney University on a moderate diffuse environment with only a few reflecting diffusion panels where I progressively introduced acoustic absorption placed on the floor of the chamber and then derived the absorption coefficients. The absorption coefficient decreased as one increased the amount of absorption in the room until a point was reached where the absorption coefficient remained relatively constant. Under ISO 354, clause identifies that the volume of the reverberation room should be at least 150 m 3 and for new construction, the volume

6 is strongly recommended to be at least 200m 3. In clause of ISO 354 there is a requirement for the test specimen to have an area between 10 m² and 12 m² and that if the volume of the room is greater than 200 m³, the upper limit for the test specimen area shall be increased by the factor (V/200 m 3 ) 2/3. If the absorption coefficient varies by reason of selecting different size samples in a room, then the requirement for an area of between 10 and 12 m 2 up until 200 m³ would not accord with the upper limit test specimen factor provided in the Standard. Figure 6 provides a graph that identifies the upper limit test specimen as a solid blue line for a room volume of 200 m³ and greater with the dotted blue line representing the same factor to multiply against the specimen size for volumes less than 200 m³. Figure 6: Sample size from ISO 354 It would be not unreasonable to consider, if one was seeking consistent results, the sample size to accord with the upper limit test specimen that has been provided. Superimposed on the graph is a requirement under clause for volumes below 200 m³ to have a sample area between 10 and 12 m² that would appear to be inappropriate for smaller rooms. The ASTM C423 90a results for the Sonex classic acoustic foam undertaken under a US standard occurred in a test room of 150 m³, but with a sample size slightly above 4m 2, which is identified as grey circle in Figure 6. On the right-hand side of Figure 6 are 3 orange dots that relate to a reverberation chamber with a volume of 343 m³ operated by Louis Challis and Associates (Challis Consulting) [8] & [9] of which there are many tests conducted in Australia from that laboratory, that is no longer in use. The Sonex testing under ASTM C423-90a from Table 1 has an absorption coefficient at 1 khz above 1.0. Then for the student of acoustics there is a question whether the sample size for that official test is too low. If the absorption calculations for a diffuse environment and a very diffuse environment are different, how does one account for the amount of diffusion in the derivation of the absorption coefficient? Acousticians rely upon accurate absorption coefficients for acoustic control in the real world. If the absorption coefficients that are obtained in an acoustic laboratory with a highly diffuse environment are not the same as the absorption coefficients measured in a room with minimal diffusion, then which set of data should be used if you must guarantee compliance with certain acoustic parameters? Testing at multiple locations in a reverberation room provide different results and the use of multiple diffusors is to keep adding diffusors to obtain a stable result. Table 2 presents the results of testing in the Challis Chambers (LAC) in the Australian roundrobin test using the sample absorption panels [8] & [9]. Of interest is the results show the absorption coefficients for three sample sizes with and without diffusors. Sd is the surface area of the diffusors (both sides) and Sf is the surface area of the floor, i.e. Sd/Sf = 0 means no diffusor panels. 1/3 Octave Band Centre Frequency (Hz) S 1 khz 2 khz 4 khz Sd/Sf 16 m m m m m m Table 2. Variation in Absorption Coefficients for LAC room with and without diffusers Use of Figure 6 indicates that under the current version of ISO 354, the sample size for the LAC testing should be 14 m 2. Under the no diffusors scenario, the smaller the sample size the higher the absorption coefficient. For the diffusors scenario, the coefficients for the same material are greater for the sample size. Why? RT (empty) 1/3 Octave Bands Diffusers 500 Hz 1 khz 2 khz 4 khz 0 m m Table 3. Reverberation Time LAC(1) Room

7 Table 3 presents reverberation times for the empty state with and without diffusors. A significant difference. Consider the sound field above the absorptive treatment versus the sound field in the upper portion of the chamber without diffusors. Are the non-diffusor state reverberation times governed by the reflections in the upper region and the various wall reflections? With the diffusors, are the upper reflections reduced by the diffusors or is there a concentration of reflected energy back onto the floor? Hughes et al [17] suggest the occurrence of absorption coefficients greater than 1.0 are not attributed to just diffraction edge effect and examined the effects of the surface area, thickness and edge sealing conditions. That testing did find differences in sample size as discussed above and concluded that for highly absorptive materials the use of the Eyring correction is preferred to Sabine (see Figure 7) and that the Sabine Absorption Coefficient should be supplemented by the Energy Absorption Coefficient. chambers) utilising small sample areas, a higher absorption coefficients are derived that is the upper sample size limit from ISO 354 is used. Are coefficients derived from small samples appropriate for use in the real world? Should the acoustic standards for absorption measurements in a laboratory specify a sample size proportional to the volume to achieve consistency? Is the amount of diffusion excessive for use of the Sabine equation? Is the use of the Sabine equation for highly diffuse spaces appropriate? As it is unlikely the Standards will replace the Sabine equation with the Eyring adjustment, should the Energy Absorption Coefficient also be provided to address the presence of absorption coefficients greater than 1? Acknowledgement This project has been unfunded but is provided for those acousticians who think outside of the box. Figure 7: Relationship between Eyring and Sabine Absorption coefficients (ref [17]). 5. Conclusions The equivalent sound absorption area of the test specimen formulae in ISO 354 provides the ability to determine the absorption coefficient of the test sample, whether it is a positive or a negative absorption coefficient. Supplementary to the derivation of the negative absorption coefficient, and standard testing to derive the positive absorption coefficient, there are a series of questions to be considered: How one can obtain an absorption coefficient greater than 1.0? The area of the sample affects the resultant absorption coefficient. By manufacturers (or References [1] Cooper SE, The INM Program is a much better program than HNM for helicopter modelling, but., SAE A-21 Helicopter Noise Working Group Meeting, Las Vegas, March, 2004 [2] Cooper SE Problems with the INM: Part 1 Lateral Attenuation, Noise of Progress Acoustics Conference 2006, New Zealand [3] Cooper SE, Problems with the INM: Part 2 Atmospheric Attenuation, Noise of Progress Acoustics Conference 2006, New Zealand [4] Cooper SE, Problems with the INM: Part 3 Derivation of NPD Curves, Noise of Progress Acoustics Conference 2006, New Zealand [5] Cooper SE, Problems with the INM: Part 4 INM Inaccuracies, Noise of Progress Acoustics Conference, 2006, New Zealand [6] Sabine WC, Collected Papers on Acoustics, Dover Publications, New York, 1964 [7] Kosten CW, International Comparison Measurements in the Reverberation Rooms, Acoustica 10, 1960 [8] Davern WA & Dubout P, First report on Australian Comparison Measurements of Sound Absorption Coefficients, CSIRO Melbourne, 1980 [9] Davern WA & Dubout P, First report on Australian Comparison Measurements of Sound Absorption Coefficients, CSIRO Melbourne,

8 [10] McGory M, Cirac DC, Gaussen O & Caberra D, Sound absorption coefficient measurement: Reexamining the relationship between impedance tube and reverberation room methods, Proceedings of Acoustics 2012-Fremantle, Australia [11] Cooper SE, An investigation of the alternative to Sabine s equation in the determination of absorption coefficients using the room method, Master of science (Architecture) thesis, University of Sydney, March 1990 [12] ASTM C423-77, Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method, The American Society for Testing and Materials. [13] ASTM C423-84a, Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method, The American Society for Testing and Materials. [14] Threshold of hearing v threshold of sensation for low frequency and infrasound, Acoustical Society of America Meeting, Salt Lake City, May 2016, ASA POMA vol 26/ / [15] Subjective perception of wind turbine noise The stereo approach, Acoustical Society of America Meeting, New Orleans, December 2017, ASA POMA Vol 31/ / [16] Standards Australia, Acoustics Measurements of sound absorption in a reverberation room, AS ISO (ISO 354:2003), January 2006 [17] Hughes W, McNelis A, Nottoli C & Wolfram E, Examination of the Measurement of Absorption Using the Reverberant Room Method for Highly Absorptive Acoustic foam, 29 th Aerospace Testing Seminar, October 2015, nrts.nasa.gov/search.jsp?r= (last accessed April 2018)