t# : 0^. j TEST REPORT CO SOUND ABSORPTION TESTS ON POLYURETHANE AND LIQUID SOAP FOAMS FOR ANECHOIC ENCLOSURES by G. KRISHNAPPA, G.G. LEVY, AND G.A. MAv MJLAY E.P. Cckshutt, Head Engine Labratry D.C. MacPhail Directr
SUMMARY An investigatin was made f the nrmal and randm incidence absrptin f samples f 2-in thick plyurethane fam selected fr use in the cnstructin f an anechic enclsure fr lifting-fan nise studies. Impedance tube measurements gave nrmal incidence absrptin cefficients ranging frm 0.38 at 500 c/s t 0.91 at 1500 c/s. Reverberatin time measurements carried ut at the NAE Acustic Test Facility indicated a randm incidence absrptin cefficient f 0.79 at 500 c/s, increasing t almst 1.0 at 1250 c/s, after crrectin fr diffractin effects. Reverberatin time measurements n 1-in, 2-in, and 4-in thick liquid sap fam shwed high sund absrptin. A 4-in thick fam gave a randm incidence cefficient f 0.72 ac 300 c/s t 1.0 at 800 c/s (nt crrected fr the effects f diffractin). (")
TABLE OF CONTENTS Page SUMMARY (ii) TABLE (iii) ILLUSTRATIONS (iv) 1.0 INI RODUCTION 1 2.0 STANDARD TEST METHODS 1 2.1 Impedance and Absrptin f Plyurethane Fam by the Tube Methd 1 2.1.1 Definitins 1 2.1.2 Impedance Tube and Methd f Measurement 2 2.1.3 Cmputatin Methd 2 2 2 Reverberatin Methd f Estimating Sund Absrptin... 3 2.2.1 Reverberatin Time Measurements 3 2.2.2 Diffractin Effects 4 3.0 LIQUID SOAP FOAM TESTS 5 4.0 TEST RESULTS 5 5.0 DISCUSSION OF RESULTS 6 6.0 CONCLUSIONS 6 7.0 REFERENCES 7 TABLE Table Page 1 Impedance Measurements f 2-inch Thick Plyurethane Fam 8 (iii)
ILLUSTRATIONS Figure Page 1 Impedance Tube 9 2 Draining Rate f 1:1 Water: Liquid Sap Fams 10 Absrptin Cefficient f Liquid Fams by Reverberatin Time Measurements 11 Absrptin Cefficient f 2-inch Plyurethane Fam 12 Cmparisn f Absrptin Cefficient Measurements 13 (iv)
-1 SOUND ABSORPTION TESTS ON POLYURETHANE AND LIQUID SOAP FOAMS FOR ANECHOIC ENCLOSURES 1.0 INTRODUCTION The Engine Labratry f the DME has undertaken a preliminary prgram f fan-in-wing nise studies. T btain meaningful results, the basic facility fr nise studies required an anechic chamber f suitable dimensins with a gd anechic terminatin fr the high intensity sund. As the primary interest was in btaining reliable results in the frequency range f 500 c/s t 10 Kc/s, a structure lined with 2-in thick, 80 ppi plyurethane fam was expected t give gd absrptin in the desired frequency range. The results f tests n separated layers f etched plyurethane fam (Ref. 1) als suggested favurable absrptin qualities and the ability t survive high intensity sund fr lnger perids. The impedance tube measurements (Ref. 2) n the liquid sap fam had shwn highly prmising sund absrptin qualities. It seemed wrthwhile t further^ explre the randm absrptin qualities f the liquid sap fam. 2.0 STANDARD TEST METHODS The standard acustic tests cnsist f finding the nrmal and randm absrptin cefficients f the material. The nrmal incidence absrptin is fund by munting the specimen in an impedance tube and explring the standing wave pattern excited by a single tne f a selected frequency. The randm absrptin cefficients were explred by measuring the decay time f sund with a knwn area f the material, in the reverberatin rm f the NAE Acustic Test Facility. 2.1 Impedance and Absrptin f Plyurethane Fam by the Tube Methd 2.1.1 Definitins The nrmal incidence sund absrptin cefficient, a n, f a material is the fractin f nrmally incident sund energy absrbed by the material. The absrptin cefficient will, in general, depend n bth the structure f the material and its methd f munting. The rati between pressure and nrmal fluid velcity at a pint n the surface is called the acustic impedance f the surface. The impedance may depend n the frequency f the incident wave and it may be cmplex, in which case the nrmal velcity is nt in phase with the pressure. The nrmal absrptin cefficient i? related t the impedance f the material by where Z is the impedance f the material p is the density f air c is the velcity f sund in air.
2.1.2 Impedance Tube and Methd f Measurement The nrmal absrptin cefficients were measured in an impedance tube. The measuring technique emplyed here was that stipulated in ASTM Standards (C.384) (Ref. 3). A schematic diagram f the impedance tube is shwn in Figure 1. The impedance tube was f 3-7/8-in diameter and 100 cm in length, with rigid walls that transmitted r absrbed negligible sund energy. A ludspeaker was munted at ne end f the tube and the specimen munted, with rigid backing, at the ther. The speaker, driven by a beat frequency scillatr, prvided a surce f lngitudinal sinusidal plane waves. The standing wave pattern was explred in the tube by a mvable micrphne. The utput frm the micrphne was fed t a cnstant percentage bandwidth frequency analyzer. The quantities measured were the maximum and minimum amplitudes f the standing wave pattern, Dj, the distance frm the face f the specimen t the first minimum, and D2, the distance between tw successive minima, which was equal t the half wavelength, as shwn in Figure 1. Test frequencies chsen were 250, 400, 500, 750, 1000, 1250, 1500, 1750, and 2000 c/s. The high frequency limit was determined by the size f the impedance tube. The inside diameter f the tube shuld nt be greater than the value 7, where ^ax fmax is the highest frequency fr which measurements are desired. Larger diameter tubes wuld give rise t crss mdes preventing the frmatin f plane waves. 2.1.3 Cmputatin Methd The nrmal incidence absrptin cefficient was calculated by a n = -73 & (1+X) where x is the rati f minimum t maximum amplitude f the standing wave. By using the standing wave rati and jr-, the resistive and reactive cmpnents f the impedance were fund by applicatin f the well-knwn "Smith chart". Nrmal absrptin cefficients were cnverted int mre useful randm incidence absrptin cefficients by the empirical graph given by Zwikker and Ksten (Ref. 4). The impedance measurements are presented in Table 1. cefficients are pltted in Figure 4 against the frequency. The absrptin
2.2 Reverberatin Methd f Estimating Sund Absrptin In this methd the absrptin cefficient f a test specimen is determined by measuring the change in the decay rate f sund in the presence f a test specimen in a reverberatin rm. Decay rate d is related t the ttal sund absrptin A in a rm by A = 0.9210 (3) where v is the vlume f air in the rm d is the rate f decay f sund pressure level in db/sec c is the speed f sund in ft/sec. If subscripts 1 and 2 are used t dente the cnditins befre and after the specimen has been placed in the reverberatin rm, the increase in the absrptin cefficient is given by the difference... 0.9210. A A. /A. A 2 " A l = TT- ( v 2 d 2 " v l d l) ( 4 ) If the test specimen has an absrptin cefficient f ;, and cvers an area S f a reverberatin rm having an absrptin cefficient a, then the absrptin cefficient f the specimen is given by A 2 - A 1 «1 ^ 5 + % (5) 2.2.1 Reverberatin Time Measurements Reverberatin time measurements were cnducted in the reverberatin rm f the NAE Acustic Test Facility. The details f the reverberatin rm and instrumentatin are reprted in Reference 1. The sund field in the reverberatin rm was prduced by tw 15-in diameter ludspeakers driven by a beat frequency scillatr. The sund pressure levels were measured by tw 1/2-in cndenser micrphnes crnected t.1 cnstant percentage bandwidth frequency analyzer. The utput frm the analyzer was cnnected t a nise level recrder. Tw electrical cntacts n the recrder were separated by 20 db and were energized by a wiper attached t the arm f the pen. The cntacts were cnnected t the start and stp inputs f an electrnic time cunter. >HmmmMitm»*«r-'
-4 Samples f the fcam were placed in the centre prtin f the flr f the reverberatin rm. The plyurethane fam sample tested was 6 ft x 4 ft x 2 in thick. The samples f the liquid fam tested had an area f 6 ft x 6 ft with the thickness f the fam varying frm 1 t 4 in. Decay times were measured at three micrphne statins fr each ne f a set f 1/3 ctave band randm nise, with midfrequencies ranging frm 250 c/s t 5000 c/s. The sund pressure levels in the reverberatin rm, befre switching ff the ludspeaker, were f the rder f 95 t 115 db, depending n frequency. In ASTM Standards C.423-60T (Ref. 3) it has been recmmended that the number f samples f the reverberatin time shuld be large enugh t yield a mean reverberatin time whse errr is less than 1%. As the present tests were f a verifying nature, nly a limited number f readings were taken. The absrptin cefficients were calculated by using equatins (3) t (5). 2.2.2 Diffractin Effects When the patch f specimen tested is relatively small, the effects f diffractin are likely t creep in, resulting in the measured absrptin cefficients being dependent upn the size f the specimen. Fr small samples the measured absrptin wuld be higher than the absrptin cefficient f a sample ccupying the whle area. Njrthwd (Ref. 5) hus discussed these effects and has cmputed the effects f diffractin fr a range f values f cnductance rati g, susceptance rati b, and 2n Ka, which is a measure f the wavelength and size f the material (K = -r-, and cd a = -q-j, where c and d are the dimensins f the rectangular patch). The cn- ductance rati g and susceptance rati b can be btained by impedance tube measurements and are given by e R i_ X,_, ' * r 7j b - im? {6) where R and X are the resistive and reactive cmpnents, respectively. Nrthwd's curves shw that the absrptin cefficient is strngly dependent n the values f Ka. In particular, the diffractin effects are dminant at lwer values f Ka. The diffractin crrectins applied were calculated as the rati f the absrptin cefficient fr Ka = «t the value f Ka in the desired frequency range. The curves presented are limited t the values f b = -0.5 t +0.5 and g = 0 t 1.0. The results were extraplated fr the values f b and g fr a small variatin beynd the abve range. The diffractin crractins were applied nly t the reverberatin absrptin cefficient f the 2-in thick plyurethane fam.
-5 3.0 LIQUID SOAP FOAM TESTS The liquid fam was made by whipping a knwn vlume f equal parts f water and a synthetic liquid hand sap in a 45-gal drum until sufficient air had been incrprated t give an eight-fld vlume increase and a fam density f apprximately 0.125 g/cm"*. The beater was a 9-in diameter circular disk f 1/4-in mesh heavy wire screen, munted radially at ne end f a steel shaft, the ther end being chucked in a 3/4-in heavy duty 375 rpm prtable electric drill. After the fam had been prepared it was pured t the required depth int a 6-ft square trugn, with S-in high 16-gauge sheet metal sides, placed n the ^r f the reverberatin rm. It was nticed that in the curse f a tt^t, which tk frm 15 t 20 minutes, cnsiderable draining f fam had ccurred. The fam height fell, typically, abut 1 in during the test using 4 in f fam, but it was als apparent that the fam density at the end f the test was much less than at the start. T get sme idea f the amunt f change in density that had ccurred, sme drainage»ime experiments were made in a 4-in diameter beaker, using the same liquid as in the reverberatin tests. The results are shwn in Figure 2. The indicatin is that at the end f a 20-minute reverberatin test the density f the liquid fam was prbably nly abut ne-sixth f its riginal value. This suggests that in any future wrk alng these lines sme effrt shuld be made t reduce this rate f draining. It will be nted in Figure 2 that fr the liquid sap used in the impedance tube experiment (Ref. 2), which was nt available in quantity at the time the reverberatin tests were made, the drainage rate was very much less than fr the sap that was used in the reverberatin tests. 4.0 TEST RESULTS The nrmal and randm absrptin cefficients f the 2-in thick plyurethane fam are shwn in Figure 4. Reverberatin time measurements shw (with diffractin effect crrectins) that the absrptin cefficient increases frm 0.67 at 250 c/s t almst 1.0 beynd 1200 c/s. Nrmal absrptin cefficients crrected t randm incidence are generally lwer than absrptin cefficients cmputed frm reverberatin time measurements by mre than 15% at 500 c/s t 10% at 1250 c/s. The results f the liquid sap fam tests are presented in Figure 3. The randm incidence absrptin cefficient a is pltted against the frequency. The absrptin cefficient f 1-in thick sap fam increases frm almst 0 at 200 c/s t 0.55 at 600 c/s, decreases t 0.25 at 800 c/s, and again increases. Tw-inch and 4-in thick fams d nt shw such marked peaks and trughs. The absrptin cefficieiit increases as the thickness f the fam is increased frm 1 t 4 in. The 4-in thick fam shws an absrptin cefficient greater than 1.0 beynd 800 c/s, which is presumably due t the effects f diffractin. A 2-in thick liquid sap fam t Uws a substantial reductin in the absrptin cefficient with a 1/2-hr drainage. This reductin is particularly high at the middle frequency range (600-1300 c/s). The reductin in absrptin cefficient is nearly 50% arund a frequency f 800 c/s.
5.0 DISCUSSION OF RESULTS One may ntice cnsiderable difference in the randm incidence sund absrptin cefficients estimated directly frm the reverberatin time measurements and the cefficients cmputed frm nrmal incidence measurements. This is because the estimatin f randm incidence frm nrmal incidence is based n a direct empirical apprach, by measuring a n and af ran and relating these tw parameters fr a number f materials with similar prperties. This methd f apprach seems t give very lw values f «ran at l w frequencies and appraches «rev at very high frequencies. The maximum numerical difference culd be f the rder f 0.15 t 0.20, as has been reprted in ASTM Standards. The randm absrptin cefficient, as reprted by the manufacturer, it repltted in cmparisn with the present tests in Figure 5. Bth the results wer. cmputed frm nrmal incidence measurements. The present results shw n an average 10-15% lwer absrptin cefficients than thse reprted by the manufacturer. Hwever, as nly limited tests were cnducted, a firm cnclusin cannt be established. As the present (6-ft x 4-ft) sample f plyurethane fam tested was smaller than the standard sample (9-ft x 8-ft), ne may expect significant diffractin effects. Nrthwd's curves fr crrecting fr diffractin effects were limited t a relatively small range f values f b, g, and Ka. The curves had t be extraplated fr estimating these effects beynd the range presented. The liquid sap fam results shw highly prmising acustical absrptin qualities. The 1-in thick fam shws a wavy pattern f absrptin with a marked peak at 6C0 c/s. One pssible explanatin fr this marked peak is quarter wavelength resnance. The impedance tube measurements n the liquid sap fam f different densities (Ref. 2) supprt this view. One may (Fig. 3) see a significant change in the absrptin qualities f 2-in thick liquid sap fam after a 1/2-hr drainage. This is presumably due t the reductin in the density f the sap fam, resulting in a lnger wavelength f the sund inside the fam. The effects f diffractin n the randm absrptin cefficient f 4-in thir!: liquid sap fam is nt expected t be very serius, as bigger samples f the fam were tested. One may reasnably guess a value f 8-107c increase in the absrptin cefficu its in the intermediate frequency range. 6.0 CONCLUSIONS 1) The 2-in thick plyurethane fam had a nrmal incidence sund absrptin cefficient f 0.39 at 500 c/s, increasing t 0.91 at 1750 c/s, 2) Reverberatin time measurements shwed a randm incidence absrptin cefficient f G.79 at 500 c/s t 1.0 at 1250 c/s with crrectins fr the effects f diffractin.
3) Randm incidence absrptin cefficients cmputed frm impedance tube measurements shwed marked differences with reverberatin time measurements. 4) Liquid sap fams shwed high sund-absrbing capabilities. A 4-in thick liquid sap fam layer had a randm incidence cefficient ranging frm 0.72 at 300 c/s t i.o at 800 c/s (withut diffractin crrectins). 5) A 1/2-hr drainage shwed a marked reductin in the sund-absrptin capacity with 2 in f liquid sap fam. In view f the relatively small number f tests carried ut, the abve results may nt be regarded as cnclusive. Further tests wuld be necessary t quantitatively establish the absrptin cefficients. 7.0 REFERENCES 1. Westley, R. Wlley, J.H. Sund Absrptin Studies f an Anechic Treatment fr Nise Terminatin Chambers. NRC, NAE, Aer. Reprt LR-466, Natinal Research Cuncil f Canada, January 1967. 2. Macaulay, G.A. Sme Experiments n Acustic Absrptin in Liquid Fams. NRC, Mech. Eng. Reprt (t be published). 3. Tentative Methd f Test fr Sund Absrptin f Acustical Materials in Reverberatin Rms. ASTM Standards, American Sciety f Testing f Materials, Nvember 1966. 4. Zwikker, C.S. Ksten, C.W. Sund Absrbing Materials. Elsevier Press Inc., 1949. 5. Nrthwd, T.D. Absrptin f Diffuse Sund by a Strip r Rectangular Patch f Absrptive Materials. NRC, DBR N. 44, Natinal Research Cuncil f Canada, April 1904.
- 8 TABLE 1 IMPEDANCE MEASUREMENTS OF 2-INCH THICK POLYURETHANE FOAM Frequency Resistance R Reactance X 250 0.2-1.3 400 0.65-0.9 500 0.60-0.8 750 0.60-0.60 1000 0.60-0.50 1250 0.60-0.30 1500 0.60 0 1750 0.70 0 2000 0.60 0
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