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 DOI: 10.1177/0040517511402121

2012 82: 875 originally published online 7 March 2012Textile Research JournalParham Soltani and Mohammad Zerrebini

The analysis of acoustical characteristics and sound absorption coefficient of woven fabrics  

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Original article

The analysis of acoustical characteristicsand sound absorption coefficient ofwoven fabrics

Parham Soltani and Mohammad Zerrebini

Abstract

The amount of research conducted on sound absorption properties of woven fabrics in comparison to nonwoven and

spacer textile fabrics is very limited. The main aim of this research is to study acoustical and sound absorption properties

of woven fabrics. Normal incident sound absorption coefficient was determined via the impedance tube method. To

achieve the objective of the research, general acoustical properties of textiles together with existing standards were

thoroughly studied. Samples of woven fabrics having varying structural elements, such as weave type, weft yarn linear

density, thickness created by layering of test fabrics, yarn spinning system and depth of air space at the back of samples

were tested. Results showed that, for most samples while the maximum value of the sound absorption coefficient

occurred at a frequency of 1000 Hz, the minimum value was observed at frequencies of 250 and 2000 Hz. Results showed

that, the sound absorption coefficient of woven fabrics is influenced by both density and porosity of fabrics. It was found

that, plain fabric absorbed sound wave more than the other weave types. Results also indicated that, for a given weft

density, fabrics produced with weft yarn linear density of 24.5 tex has a higher absorption than fabrics woven with weft

yarns of other counts. It was found that, finer weft yarn and higher thickness of fabric causes the noise reduction

coefficient or NRC of the fabric to be increased. It was established that, fabrics woven using rotor-spun yarns exhibited

the highest absorption in comparison to samples woven using ring-spun or compact yarns.

Keywords

Normal incident sound absorption coefficient, noise reduction coefficient, impedance tube, weave type, weft yarn linear

density, thickness, spinning system

Introduction

The acoustic phenomenon and its consequences in envi-ronments, such as work place and residential homes,have gained paramount importance. The use of varioustypes of textiles in mass residential building units hasemphasized this importance. While extensive researchin the field of acoustic properties of nonwoven fabricshas been extensively undertaken, in the case of othertypes of textiles, namely woven and knitted fabrics,very limited research has been conducted. Prior to thepresent research, other researchers, such as Aso andKinoshita1,2 had investigated the relationship betweenthe flow resistance and the absorption characteristics ofcotton fabrics. In these researches the effects of thick-ness, weight and fabric cover have been investigated. Inanother research work the absorption properties offiber assemblies was studied and the importance ofporosity and total surface area of the fiber assemblyas pivotal factors was emphasized.3

Na et al.4 studied the sound absorption coefficient ofmicro-fiber made fabrics via the reverberation method.

It was found that these fabrics, with the exception ofmesh structures, absorb the complete range of thesound frequencies better than fabrics made of conven-tional fibers. Other studies in this field have ratherfocused on the absorption mechanism and methods ofsound absorption measurement.

Lee and Joo5 examined the sound absorption coeffi-cient of recycled polyester nonwovens. The relation-ships between acoustic absorption property andnonwoven parameters, such as fiber and web propertieswere established.

In this work acoustical characteristics of woven fab-rics including sound absorption coefficient and factorsaffecting sound absorption coefficient via the imped-ance tube method are determined. The sound

Isfahan University of Technology, Faculty of Textile Engineering, Iran.

Corresponding author:

Mohammad Zarrebini, Isfahan University of Technology, Faculty of Textile

Engineering, Isfahan 84156, Iran

Email: zarrebini@cc.iut.ac.ir

Textile Research Journal

82(9) 875–882

! The Author(s) 2012

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DOI: 10.1177/0040517511402121

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absorption coefficient of woven fabrics is significantlylower than that of nonwoven textiles. Despite this fact,woven fabrics are preferred in end-uses, where nonwo-ven textiles cannot be used economically. In these appli-cations, provided an optimal air space is allowed at theback of fabric then, woven textiles are a more effectivemeans of sound absorption. In order to measure soundabsorption properties of woven samples,Texsonicmeter� apparatus was used. Based on results,the effect of fabric structural parameters on the soundabsorption coefficient was established.

Sound absorption mechanisms

Aso3 has defined three general mechanisms for absorp-tion of sound by fibrous assemblies. The first mecha-nism deals with absorption characteristics of fibrousmaterials, while the sound absorption is very small atlow frequencies, at high-pitched tones absorption isquite considerable. This type of absorption mecha-nism is known as viscosity resistance absorption.When a sound wave strikes a fibrous assembly, thesound pressure causes the entrapped air in narrowair spaces of the assembly to vibrate. To overcomethe frictional resistance of the fibers and the vibratingair, energy must be taken out from the sound wave.The relative movement of air on fibers, converts soundenergy into heat by air viscosity. Thus, this type ofabsorption is also known as viscosity resistance. Inthe second mechanism, a peak occurs at a low fre-quency. At the high-frequency range, the absorptioncoefficient is improved with increases in frequency.This mechanism differs from the absorption character-istics of resonance absorption obtained when airtightmaterials, such as a pulp broad or plywood, are placedat a distance from a solid wall. The peak at a lowfrequency is presumed to be due to the resonance ofthe sample. The third mechanism behaves somewherebetween the first and second mechanisms. The thirdmechanism shows the resonance absorption withouta peak. This is because at frequencies higher than res-onance frequency, the absorption coefficient is morethan that of the viscosity resistance type.3

Method of measuring sound absorption coefficient

This research uses the impedance tube method which isnot only faster and generally reproducible, but alsorequires 35 mm diameter circular test samples. Sincein this method a sound wave strikes the materialperpendicularly, the measured sound absorption coeffi-cient is known as the normal incidence sound absorp-tion coefficient.

In the impedance tube method, sound waves are con-fined within the tube. Thus, the size of the test sample

needs only to be large enough just to cover the smallcircular cross-section area of the impedance tube. Thus,the elimination of a large test sample with lateraldimensions several times the acoustical wave length.Traces of reflected sound wave appear on the screenof an oscilloscope which is connected in parallel to adigital voltmeter. The proportion of sound absorbed bytest fabric is calculated using equations 1 and 2.

� ¼IiIr¼

pi�� ��2� pr

�� ��2pi�� ��2 ¼ 1� Rj j2¼ 1�

n� 1

nþ 1

� �2

¼4n

1þ nð Þ2

ð1Þ

n ¼ Pmax=Pmin ð2Þ

Where a¼ sound absorption coefficient; Ii & Ir¼ inten-sities of incident and reflected waves respectively; pi &pr¼ pressure of incident and reflected waves respec-tively; R¼ reflectance factor; n¼ standing wave ratiowhich is the ratio of the maximum to minimum pres-sure of the sound wave;1,6 pmax & pmax¼maximum andminimum values of sound wave pressure respectively asshown on oscilloscope screen.

Since accurate reading of pressure on the oscillo-scope cannot be achieved, a digital voltmeter wasplaced in parallel with the oscilloscope. Maximumand minimum voltage corresponding to maximumand minimum pressures as appearing on the oscillo-scope screen was noted. Hence, instead of pmax/pmin

the value of Vmax/Vmin is used in calculations. TheTexsonicmeter� set up is shown in Figure 1.

Experimental

A ring spinning frame and weaving loom were used forthe production of yarns and fabrics. Fabrics were made

Figure 1. Texsonicmeter�.

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under standard conditions. Circular test samples werecut from each fabric piece. Test samples with a diame-ter of 35 mm were cut randomly at a considerable dis-tance from fabric selvedges. Fabric thickness wasmeasured at a pressure of 5 g/cm2, using a standardcompression tester. The sound absorption coefficientof each test sample was determined using theTexsonicmeter�. The air space at the back of the sam-ples was set at 4 cm. Each sample was tested 10 times.In the absence of no peripheral sound, data were takenat an ambient temperature of 23�C and relative humid-ity of 55%. A MATLAB 7 based image analysis com-puter program was developed. The ratio of total voidspace to the nominal total surface area of the testsample was calculated using the above computerprogram.

Results and discussion

Effect of weave type on sound absorption coefficient

Variation of the sound absorption coefficient in relationto fabric weave was determined using weave type such

as plain, 2/1 twill, 3/1 twill, 2/2 twill, rips and satin.Fabrics were woven with equal warp and weft densityof 27 threads per cm. Polyester spun yarns with lineardensities of 29.5 and 24 tex and tpm of 557 and 600were used as warp and weft respectively. The fabricstructural parameters are shown in Table 1.

As can be seen in Figure 2, the maximum soundabsorption coefficient for all samples occurs at a fre-quency of 1000Hz. The results show that, the soundabsorption coefficient varies in the range of 0.107 to0.530. The minimum value of the sound absorptioncoefficient for all samples with the exception of satinweave occurred at a frequency of 250Hz. The minimumvalue of the sound absorption coefficient of satin fabricwhich has the minimum value of noise absorption coef-ficient or NAC value was found to occur at a frequencyof 2000Hz. This is due to the greater thickness of thisfabric in comparison to other weaves. It must be notedthat, at a frequency of 2000Hz, which is an earsplittingsound, manipulation of sound absorption by textilesirrespective of their structures is rather an arduous task.

The reduction of NAC at a frequency of 2000Hzcan be due to the coincidence dip phenomenon.

0

0.1

0.2

0.3

0.4

0.5

0.6

0

NA

C

500

Frequency

1000 1500 2000

plain

twill 2/1

twill 3/1

twill 2/2

rips

satin

Figure 2. Effect of weave type on NAC.

Table 1. Fabric structural parameters

Weave type Fabric weight (g/m2) Fabric thickness (mm) Fabric density (g/cm3) Porosity (%)

Plain 160 0.51 0.31 0.77

Twill 2/1 154 0.53 0.29 0.79

Twill 3/1 150 0.53 0.28 0.80

Twill 2/2 152 0.54 0.28 0.80

Rips 151 0.56 0.26 0.81

Satin 148 0.59 0.25 0.82

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This phenomenon is commonly known as the criticalfrequency which can severely limit sound absorptionability of the sample. The coincidence dip occurswhen the incident sound wave is in phase with thereflected wave from the test sample. This system ofsound wave is known as standing wave.

The sound absorption coefficient can be affected byfabric structure. Figure 2 shows that, at low frequen-cies, fabrics exhibit poor sound absorption. At low fre-quencies complete sound wave transmission takes placethrough the test fabric. This is due to the tissue-likethickness of the test samples. In contrast to thickness,fabric weight and density have a profound effect onsound absorption behavior of the fabrics. Fabric den-sity is defined as the ratio of fabric weight to fabricvolume. This is confirmed by thicker satin weavewhich exhibits a lower NAC than the plain weavefabric. This is due to a lower weight and density ofsatin weave in comparison to those of plain weave.Results show that, plain fabric density and porosityare 0.31 g/cm3 and 0.77, respectively. This fabricabsorbs sound waves more than the other fabrics.The results show that, both the fabric density andporosity influence the sound absorption coefficient ofwoven fabrics. However, it should be noted that, theeffect of fabric porosity on the sound absorption prop-erty of thicker textiles such as nonwoven or spacer fab-rics is by far more than that in tissue-like woven fabrics.

Similarity of general form of diagrams can beregarded as the confirmation of the accuracy of theresults. In certain cases, when other materials than tex-tiles are used as acoustic barriers, another index knownas noise reduction coefficient or NRC is used instead ofthe NAC. The former is the mean of absorption coef-ficient values at frequencies of 250, 500, 1000 and2000Hz.

NRC values for the weaves are shown in Figure 3.The results show that, NRC values vary from 0.21 to0.32. The maximum and minimum NRC values wereobtained for plain and satin fabrics. This is due to dif-ferent interlacing of yarn in these two weave types. Theplain weave has the greatest number of yarn inter-sec-tions, which results in increases in weight and density ofthis fabric. Additionally, shorter free float length causesmore air to be entrapped within the thread spaces in thefabric. Thus, the NAC of the fabric against soundwaves is increased. Therefore, absorption occurs dueto energy loss as the sound wave passes through thefabric and the frictional resistances offered by thefibers and entrapped air in the fabric is overcome.Accordingly, the severe crimping of the yarns in theplain weave offers more resistance to sound waves,thus the increase in observed value of NRC in thecase of the plain fabric which has the highest yarninter-sections. The length of free float of the yarns in

satin fabric is an additional contributing factor inreflection of large proportion of the incident soundwave by the fabric. A similar trend is also seen in thecase of weaves, such as 2/1 twill, 3/1 twill and 2/2 twill,where the NRC value is reduced due to a reduction inthe resistance of the fabrics to sound wave.

Effect of weft yarn linear density and tissue thicknesson the sound absorption coefficient

In order to analyze the effect of weft yarn linear densityon the sound absorption coefficient, six samples of 3/1twill fabrics with identical warp and weft density of 27threads per cm were produced. Polyester spun warpyarns of 557 tpm were used. Weft yarns with identicaltwist factor and linear densities of 59, 39, 29.5, 24.5,19.7 and 14.8 tex were used. Fabrics structural param-eters are shown in Table 2.

As can be seen in Figure 4, for all samples, while themaximum sound absorption coefficient occurs at a fre-quency of 1000Hz, the minimum sound absorptioncoefficient occurs at frequencies of 250 and 2000Hz.

The results indicate that, as yarn fineness is reduced,the sound absorption coefficient at low frequenciestends to be higher. However, this trend changes as thesound wave frequency is increased. This point is verifiedwhen considering 24.5 tex yarn which has a highersound absorption coefficient at most frequencies incomparison to other yarns.

Figure 5 shows a variation of NRC with yarn fine-ness. As can be seen, NRC increases up to a yarn lineardensity of 24.5 tex, beyond which it reduces.Considering the results, it can be said that, at constantweft density, fabrics woven with yarns in the range of59 to 39 tex, reflect a greater portion of the incidentsound wave. This is due to an increase in density ofthese samples, which in turn is due to a reduction inthe voids in the fabrics. However, decreasing the yarnlinear density not only increases pores and hollow spacein fabric, but also facilitates the passage of sound wavesthrough the fabric, thus reducing the sound absorptioncoefficient of the fabric, particularly at high frequencies.

0

0.1

0.2

0.3

0.4

plain twill2/1

twill3/1

twill2/2

rips satin

NR

C

Weave

Figure 3. Effect of weave type on NRC.

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The results show that, a sound wave at high frequencyis absorbed more in dense fabrics. As can be seen in thecase of 19.7 and 14.8 tex yarns, as frequency isincreased, the absorption coefficient reduces sharply.Sound absorption coefficients at frequencies of 1500and 2000Hz are small and almost identical. Thesudden reduction in the value of NAC is due to thecoincidence dip phenomenon. Fabrics woven withyarns of linear densities of 19.7 and 14.8 tex showed apoor NRC. This is because these yarns are deficient, asfar as sound absorption is concerned, at both high andlow frequencies.

Additionally, for yarn linear densities up to 24.5 tex,provided fabric thread density of the fabrics are identi-cal, then fabric porosity and density become the dom-inant factors. It was found that, NRC follows anincreasing rate with increase in fabric density. NRCpeaks at fabric density of 0.32 g/cm3. This correspondsto a fabric porosity of 0.77, beyond which NRCdecreases. This is in total agreement with the findingsof Na4 who showed that, there is a certain fabric den-sity at which, micro-fiber made fabrics are most efficientin absorbing sound waves. Therefore, a fabric densityof 0.32 g/cm3 is the optimum density, at which wovenfabrics most efficiently absorb the entire range of soundfrequencies. This indicates the importance of fabric

porosity and density as pivotal factors in NAC deter-mination of fabrics.

In order to establish the effect of fabric thickness andto widen the effect of yarn linear density on the soundabsorption coefficient, thicker test samples were pre-pared by a layering technique. The effect of numberof layers or fabric thickness, created in this manner,on NAC was investigated for fabrics woven with weftyarns of 59 and 14.8 tex. The orientation of warp yarnsin each layer of the layered test sample was the same.Figures 6 and 7 show the results for three and six lay-ered test samples.

The sound absorption coefficient in three layeredsamples lies in the range of 0.566 to 0.750. This canbe attributed to an increase in porosity of three layeredtest samples. The value of the sound absorption coeffi-cient for three layered test samples woven with a weftyarn of 14.8 tex is less than the corresponding value ofidentical test samples woven with a weft yarn of 59 tex.This is despite of the fact that the difference in soundabsorption coefficient for the two sets of sample is notsignificant.

As far as six layered test samples are concerned, thesound absorption coefficient tends to lie in the range of0.708 to 0.861. Additionally, fabric woven with a weftyarn of 14.8 tex absorbs sound more efficiently thanfabric woven with a weft yarn of 59 tex. This is becausethe weft densities are identical so the latter samplesreflect a greater portion of incident sound wave while

Table 2. Fabric structural parameters using various weft yarn linear density

Weft yarn linear density (tex) Fabric weight (g/m2) Fabric thickness (mm) Fabric density (g/cm3) Porosity (%)

59 238 0.60 0.39 0.71

39 186 0.51 0.36 0.74

29.5 158 0.47 0.33 0.76

24.5 145 0.45 0.32 0.77

19.7 127 0.43 0.29 0.79

14.8 114 0.40 0.28 0.80

0

0.1

0.2

0.3

0.4

0.5

0.6

0 500 1000 1500 2000

NA

C

Frequency

59

39

29.5

24.5

19.7

14.8

Figure 4. Effect of weft yarn linear density on NAC.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

59 39 29.5 24.5 19.7 14.8

NR

C

Yarn Linear Density- Tex

Figure 5. Effect of weft yarn linear density on NRC.

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in the former samples, fabric porosity together withvolume of entrapped air within the sample are respon-sible for the observed increase in the value of NAC. Ingeneral, NAC of the fabrics woven with a weft yarn of14.8 tex is higher than fabrics woven with a weft yarnof 59 tex. Therefore, the sound absorbing capability oflayered samples is directly in line with the noticeableincrease in porosity of these samples.

The NRC value of layered fabrics woven with weftyarns of linear densities of 14.8 and 59 tex is shown inFigure 8. The results show that the maximum NRCoccurs when fabric is woven with finer weft yarn andits thickness is greater.

Effect of yarn spinning system on soundabsorption coefficient

In order to analyze the effect of spinning system on thesound absorption coefficient of woven fabrics, threecotton plain woven fabrics with identical weft densitywere used. Test samples were woven, using yarns spunby three different spinning systems, namely open-endrotor, ring and compact. Figure 9 shows microscopicimages of the yarns.

The results are shown in Figure 10. As can be seen,the sound absorption coefficient varies in the range0.090 to 0.490. Maximum sound absorption occurs ata frequency of 1000Hz. At all frequencies, fabric wovenwith rotor spun yarn exhibits a higher absorption coef-ficient than fabric woven with yarns spun by the othertwo spinning systems. The rotor spun yarns have cer-tain characteristics which differentiate them from ring-spun or compact-spun yarns. This is due to generaldifferences in the structure of yarns. The presence of

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

3lay/14.8Tex 3lay/59Tex 6lay/14.8Tex 6lay/59Tex

NR

C

Thickness and Yarn Linear Density

Figure 8. Effect of weft yarn linear density and fabric thickness on NRC.

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0 500 1000 1500 2000

NA

C

Frequency

14.859

Figure 6. Effect of weft yarn linear density on NAC of three

layered samples.

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0 500 1000 1500 2000

NA

C

Frequency

5914.8

Figure 7. Effect of weft yarn linear density on NAC of six lay-

ered samples.

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wrapper fibers on the yarn surface is the marked featureof the rotor spun yarns. Bulkiness of rotor spun yarnsdue to the presence of wrapper fibers and the twistmanner causes the sound energy to be converted toheat by the action of air viscosity. The ring-spunyarns are not only less bulky than rotor spun yarns,but also there is less resonance in these yarns. In com-pact spinning, individual fibers are straightened andarranged parallel with each other by means of aerody-namic forces. Thus, yarn hairiness is reduced. Thistogether with a high degree of compactness leads to alower sound absorption by the fabric woven with com-pact yarns.

The NRC values obtained using various spinningsystems are shown in Figure 11. As can be seen, theNRC value is highest for the rotor spun yarns andlowest for the compact-spun yarns. It must be pointed

out that, at higher frequencies ring-spun yarns exhibit ahigher sound absorption than compact-spun yarns.However, the NRC value for both yarns is almost iden-tical. This can be explained by considering the fact that,although weft thread density and weft yarn count intest samples woven with yarns spun by the two afore-mentioned spinning systems were identical, due tostructural differences the yarn diameters are not neces-sary identical. Therefore, each yarn spinning systemyields to a different fabric cover. Thus, the observedvariation in sound absorption of the test sampleswoven with yarns of different structures.

Effect of air space at the back of the fabric

In order to establish the effect of air space at the backof the woven fabrics on the sound absorption property,identical samples of 3/1 twill were prepared. The airspace (d) at the back of test samples was set at 0, 4,8, and 12 cm. As can be seen in Figure 12, the absorp-tion coefficient for d¼ 0 cm is very small and no

Figure 9. Microscopic images of spun yarns: a) ring spun, b)

compact spun, c) rotor spun (Source: http://rieter.com).

0

0.1

0.2

0.3

0.4

0.5

0.6

0 500 1000 1500 2000

NA

C

Frequency

RotorCompactRing

Figure 10. Effect of yarn spinning system on NAC.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Rotor Ring Compact

NR

C

Spinning system

Figure 11. Effect of yarn spinning system on NRC.

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significant change occurs when frequency is varied.Thus, when the air space is zero, the absorption coeffi-cient can be considered to be independent of frequency.Additionally, an increase in air space does not neces-sarily maximize the sound absorption coefficient of thetest fabric. However, the maximum value of NRCoccurs at a lower frequency. Therefore, it can bestated that, provided the end-use condition or fre-quency range are defined, then it is possible to calculatethe desired air space that renders the fabric for theintended end-use. The results are in agreement withthe finding of Aso1 regarding dependence of soundabsorption on the extent of air space and particle veloc-ity in the impedance tube.

When a sound wave is transmitted through fabric, asa result of viscous resistances between the surfaces ofyarns, fabric and air, the sound wave energy is partiallyexhausted in the form of thermodynamic energy duringthe initial transmission of sound waves through thefabric. The transmitted sound wave looses moreenergy due to the vibration of the elastic system com-posed of test fabric and the air space.

Conclusion

In this research, a study on the effect of various fabricparameters, such as weave type, weft yarn linear den-sity, fabric thickness created by layering technique,spinning system and the extent of air space at theback of the fabrics, has been attempted. In order tomeasure the sound absorption coefficient of the sam-ples, extensive review of the relevant literature and

standards were carried out. An extensive use ofTexsonicmeter�, as a user friendly apparatus in thefield of textile acoustics, was made. This study hasopened the way for further investigations in the fieldof textile acoustics. The results illustrate that, soundabsorption coefficient in plain, 2/1 twill, 3/1 twill, 2/2twill, rips and satin fabrics tends to decrease in thenamed order. Plain fabrics absorb more sound thanother weaves. For a given weft density, yarn linear den-sity of 24.5 tex not only leads to maximization of soundabsorption by fabric, but also variation around thisyarn count results in the reduction of sound absorptionby fabric.

It is concluded that, six layered samples, woven withweft yarn of 14.8 tex have a higher NRC than similarfabrics woven with weft yarn of 59 tex. The rotor-spunyarns due to their substantial structural bulk and alsopresence of wrapper fibers exhibit higher absorptionthan their equivalent ring-spun or compact-spunyarns. Despite the fact that the maximum value ofsound absorption coefficient occurred at lower frequen-cies, increases in the air space at the back of fabric doesnot necessarily lead to maximization of the soundabsorption coefficient.

Funding

This research received no specific grant from any fundingagency in the public, commercial, or not-for-profit sectors.

References

1. Aso K and Kinoshita R. Absorption of sound wave by

fabrics, Part 1: Absorption mechanism. Textile Machinery

Society of Japan 1963; 8(1): 32–39.2. Aso K and Kinoshita R. Absorption of sound wave by

fabrics, Part 3: Flow resistance. Textile Machinery

Society of Japan 1964; 10(5): 236–241.3. Aso K and Kinoshita R. Sound absorption characteristics

of fiber assemblies. Textile Machinery Society of Japan

1964; 10(5): 209–217.4. Na YJ, Lancaster J, Casali J and Cho G. Sound absorp-

tion coefficients of micro-fiber fabrics by reverberation

room method. Textile Res J 2007; 77(5): 330–335.

5. Lee YE and Joo CW. Sound absorption properties of

recycled polyester fibrous assembly absorbers. AUTEX

Res J 2003; 3(2): 77–84.6. Kundt A. Acoustic experiments. Philosophical Magazine

and Journal of Science 1868; 35(4): 41–48.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 500 1000 1500 2000

NA

C

Frequency

d=0 cm

d=4 cm

d=8 cm

d=12 cm

Figure 12. Effect of air space on NAC.

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