A Brief Review on Sound Absorption Characteristics of Nonwoven
StructuresJagannath Sardar(2008TTZ8165)Department of Textile
TechnologyIndian Institute of Technology DelhiNew Delhi
110016December 11, 2008
1
Abstract:For a healthy and a pleasant environment, controlling
the sound hazards is an important issue.It is medical evidence,
that the human body will takes sound as pollution if the ambient
soundlevels exceed 65dB. This sound pollution leads to significant
health problems includinghypertension, dizziness, depression, and
most commonly, loss of hearing [1, 2].Noise control andits
principles play an important role in creating an acoustically
pleasing environment. This can beachieved when the intensity of
sound is brought down to a level that is not harmful to
humanears.Various techniques have been developed by using different
materials to make a pleasingenvironment. The sound absorbing
materials absorbs the sound energy and it converts to thethermal
energy when the sound wave strikes the fibers assembly. This
process is called an energyconversion process. Many research papers
revealed that the fibrous materials (textile) have agood affinity
to absorb the sound energy [3, 6, 8]. The porous materials can
reduce the acousticenergy of a sound wave as the wave passes
through it by the phenomenon of absorption.Acoustic porous
materials can have porosity greater than 90%. Common sound
absorptivematerials have open cells, which is called pores [4, 5,
7]. Foam and fibre assembly likenonwovens are basically known as
porous materials and it has been observed that those materialshave
good sound absorption property. In some cases wood and composite
materials are also beused as a sound absorptive and barrier
materials. For porous and fibrous materials, acousticperformance is
defined by a set of experimentally determined constants namely:
absorptioncoefficient, reflection coefficient, acoustic impedance,
propagation constant, normal reductioncoefficient and transmission
loss. These parameters are depends on some factors like
fibrediameter, fiber surface area, thickness, bulk density,
porosity, airflow resistivity, tortuosity andsurface impedance.In
this report we have measured the noise absorption coefficient (NAC)
and noise reductioncoefficient (NRC) of different nonwovens of
polypropylene fibre in different thickness. It hasbeen found that
the needle punched polypropylene has higher NAC value and it proves
that NACvalue is higher in case of higher thickness [8, 42, 45].In
acoustic engineering, the sound absorptive materials have an
important role according to itsapplications, such as aeronautical
industry, industrial noise control, room acoustics andautomotive
and acoustics [16, 21].
2
1. Introduction:A sound wave can be defined as the pattern of
disturbance caused by the movement of energytraveling through a
medium (such as air, water, or any other liquid or solid matter) as
itpropagates away from the source of the sound.The vibration can be
described as some object that causes disturbs the particles in
thesurrounding medium; those particles disturb those next to them,
and so on. Sound travels throughthe air (gas), water (liquid) or
brick (solid), as a pressurized longitudinal wave. In a
longitudinalwave the particle displacement is parallel to the
direction of wave propagation. And transversewave the particle
displacement is perpendicular to the direction of wave propagation
[9].The compressing and expanding of the air produces differences
in air pressure. The pressuredifferences in the air move away from
the drum surface like ripples in a pond, creating a soundwave. This
is how the drum produces a sound that we can hear.To generate
sound, it is necessary to have a vibrating source, such as the
tuning fork shown here.When the source vibrates, it displaces
adjacent particles and molecules in the medium, causingthem to
vibrate back and forth as well. Their vibrations cause more distant
particles to vibrate,and so on. The audible sound that we hear is
made up of tiny vibrations of air molecules, whichare transmitted
to our ears. This transmission of vibrations, starting from the
source andcontinuing from one molecule to the next, is how sound
travels through a medium [10].Sound intensity is defined as the
sound power per unit area. The usual context is themeasurement of
sound intensity in the air at a listener's location. The basic
units are watts/m
2
orwatts/cm2
. Many sound intensity measurements are made relative to a
standard threshold of hearing intensity I
0
[11]
Sound Intensity Level (dB) can be expressed by the intensity,
since intensity is nothing but theenergy. It is expressed by
[12]When this level exceeds the limit 65 dB then its called Noise
or Sound Hazards [1, 2].The final expression for the acoustic
intensity becomes [13],).(Energy(E)Area(a)Power(P)I)Intensity(t
atime Area
==
)log(10
0
I I
=
3where p = prms
. We will show that this same expression also applies for a
spherical sound waveand for a non-spherical sound wave. The human
ear can detect a wide range of sound intensities.The decibel scale
(dB) is commonly used to deal with the wide range in pressure,
intensity,power, and energy that are encountered in acoustics.
Levels in decibels are defined using thepreferred SI reference
quantities for acoustics in Table 1.1 (ISO 1683); these reference
quantitiesare used for all figures in the book [14]. Table 1.1
Sound definitions of levels in decibelsSound hazard system can be
divided into three elements [13, 15], such as1. Noise Source: The
element which vibrates in a particular frequency and make noise
hazardsin the air.2. Noise Path: The medium through which the
acoustical energypropagates from one point to another and3. Noise
Receiver: The person who could potentially complain about
thequantity or level of noise as perceived at same pointNoise
control and its principles play an important role in creating an
acoustically pleasingenvironment. This can be achieved when the
intensity of sound is brought down to a level that isnot harmful to
human health [30]. It is medical evidence, that the human body will
takes soundas pollution if the ambient sound levels exceed 65dB.
This sound pollution leads to significant
health problems including hypertension, dizziness, depression,
and most commonly, loss of hearing [1, 2]. From the early 10
decades, lots of considerable research and developments havebeen
done for dampening the sound intensity levels to control sound
pollution. A variousapplication area of the noise reduction
techniques[16, 17, 5, 18, 8] are as AeronauticalEngineering,
interiors of cars and public transport, hospital rooms,
auditoriums, and laboratoriesetc. Multi-layered panels are widely
used in aircraft, automotive and building industries. Thesound
transmission loss (TL) provided by the panels is an important
factor in evaluating theacoustical performance of such panels
[19].It is obvious that various techniques used to reduce the noise
levels using different soundabsorbing materials [20, 21]. One
reliable technique is to absorb the sound energy and convertsto
thermal energy.Different fibrous material such as different
nonwoven textiles, porous foam, composite or othermaterials are
extensively used for the same aspects.Many literatures reveals that
nonwoven porous materials have a high impact characteristic
toabsorb the sound energy[3, 24, 25, 26], hence, nonwovens have
fibrous quantity and air. Due tothis combination, nonwovens absorb
the sound energy and convert it to heat by the mechanismof
thermodynamics and aerodynamics principle [6, 22, 23].
2. Materials for sound absorption:Sound absorptive materials can
be classified into three categories such as absorptive
materials,Barrier materials and damping material. [27]. These sound
absorptive materials can be includedrugs, carpet with felt pads,
heavy drapes etc. [28] The sound wave passes through the porous
andfibrous structural materials which transfer the aerodynamics
energy to thermodynamics by thephenomenon of absorption [27]. These
materials are mostly used to control the acousticenvironment by
dampening the sound energy of the resultant waves which is called
reflectivewave. If the incident wave is a plane wave, and the
structural properties of the slab do not changein the direction of
wave propagation, the transmitted wave will also be a plane wave
traveling inthe same direction as the incident wave [7]. Absorptive
materials are generally resistive innature, either fibrous, porous
or in rather special cases reactive resonators [27]. Classic
examplesof resistive material are nonwovens, fibrous glass, mineral
wools, felt and foams. Porousmaterials used for noise control are
generally categorized as fibrous medium or porous foam.
5Fibrous media usually consists of rock wool or glass, polyester
fibers and have high acousticabsorption. Sometimes fire resistant
fibers are also used in making acoustical products [29, 30].Kannan
Allampalayam Jayaraman [30] obtained his ms research preparing the
nonwovensamples, in needle punched and thermally bonded process,
using kenaf fibre and PET in differentblend percentage. He
explained and shows that the materials which he has used are
efficient fornoise absorption.Often sound barriers are confused
with sound absorbing materials. Generally materials thatprovide
good absorption are poor barriers. K.O.Ballagh [8] explained that
the acousticalproperties, i.e. Barriers and damping of the
materials, the mass of the material, do not dependstrongly on the
flow resistivity, and so, provided that it is within +20% of the
desired value, theacoustical properties should be maintained.no
direct effect on the performance of the absorptivematerials [8].
Some of the acoustical fibric which are available in the market,
has shown bellow[fig. 2.1(a), 2.1(b)].(a) (b)Figure 2.1 (a)
CrossPoint Acoustical Wall Fabric and b) EcoSorpt Recycled Cotton
PanelsMichael Coates and Marek Kierzkowsld [31] explained that,
bulk porous absorbers, such asfiberglass or mineral wool batts or
blankets, and needle punched, resin or thermally bondedfibrous
textiles, are well known and all qualify as rigid porous absorbers.
Flow resistive screenscan provide similar performance to the
high-loft materials, without the bulk. Thin lightweightacoustic
textiles, such as INC Engineered Materials Deci-Tex range, act as
flexible porousscreens. They also said, for porous fibrous sound
absorbers, it has been demonstrated that theflow resistance is a
function of density. Fibre packing density decreases the air
permeability,with a resultant increase in pressure drop and hence
flow resistance. For increased soundabsorption at a given
thickness, a higher-density fibrous material is used. [31]
6An absorber, when backed by a barrier, reduces the energy in a
sound wave by converting themechanical motion of the air particles
into low grade heat. This action prevents a buildup of sound in
enclosed spaces and reduces the strength of reflected noise
[27].David Frankovich [32] has shown that the porous nature of
absorptive materials renders themsusceptible to contamination,
moisture retention and deterioration due to physical abuse. Toavoid
these problems, facings may be attached to at least one side of the
absorber.Figure 2.2 Performance of Various 1-inch Acoustical Foams
with Surface TreatmentsThe addition of a facing to acoustical foam
has the effect of increasing the lower frequencyabsorption at the
expense of the higher frequencies [32]. Later on we will discus
regarding theperformance of absorptive materials which depends on
some parameters of the used samples.
3. Influence of different factors for Sound absorption
characteristics of fibrous materials:Many literatures have revealed
that how the different factors influenced to the characteristics of
sound absorption of the fibrous assembly [33, 3, 8, 6, 34]. A
porous material with a non-porousbarrier bonded to the face of the
material carries the sound energy in the form of the
structure-borne wave. The factors that have a strong influence on
the structure-borne wave are the bulk stiffness and the structural
loss factor. For most porous materials, noise absorption
coefficientgenerally depends on such three factors as: flow
resistance, porosity, morphology of pores, etc.[35]. Summary from
some literatures are cited below.
3.1. Fibre diameter:Young Joo Na, Jeff Lancaster, John Casali
and Gilsoo Cho [36] has explained that the microfiberfabric has
fine fibres and a high surface area and it has been used in such
applications as wipers,thermal insulator, filters or breathable
layers. It can be also used for sound absorption. They havetaken
five microfibre fabrics of polyester and nylon in different blend
percentage and one regularfibre fabric of 100% polyester for the
reverberation room method. The results showed that themicro-fibre
fabrics sound absorption is superior to that of conventional fabric
with the samethickness or weight, and the micro-fibre fabrics
structure was found to be important forcontrolling sound absorption
according to sound frequency. In the given table (3.1) shows
theNRC(Noise Reduction Coefficient) changes with frequency.Table
3.1 Sound absorption coefficients of micro-fiber fabrics and
fleece.From the table we can see that the NRC is higher in case of
microfibre fabrics than the regularfabric (fleece fabric).Youneung
Lee and Changwhan Joo [33] explained that the NAC of the sample is
proportional tothe in the fine fibre contents upto a certain
frequency range[37]. Increasing the frequency beyond1500 Hz, NAC
curve shows no clear tendency with fine fibre content. Youneung Lee
et al. haveused 3 different parametric recyled polyester fibres
like 1.2, 2, 7 denier and 38mm length and forbond purpose 6 denier,
42 mm low melting polyester fibre in different percentage.
8Figure 3.1 Effect of fine fibre contents on sound absorption
propertiesK. A. Jayaraman [30] has observed that the finer size PET
absorbs more sound than other fibers.This is because finer linear
density allows more fibers per volume (fig. 3.1, fig. 3.2),
morecontact area and more tortuous channels allowing more
absorption. Moreover fine fibers moverelatively more easily than
coarser fibers which causes finer fibers to convert acoustic
energyinto heat more easily than coarser fibers.Figure 3.2 Sound
absorption of fabric made from100% PET fibers of varying cross
sectionsFrom the above fundamentals, fine denier fibres have better
sound absorbing properties thancoarse denier fibers. Super fine
fibres have good sound absorption characteristics [8]. Absorptionof
the energy of plane acoustic waves is different in the low and high
frequency bands [38].
3.2 Fiber surface area:One of the important factor which
influence the sound absorption characteristics of the materialsis
fibre surface area. More finer fibres means more surface area.The
relation between the total surface area S (cm2) of fibers
constituting a fiber assembly of porosity P
e
(%) and T (cm) is shown as follows:S = a T
b
x 10
4
where a and b are constants and T is the thickness. A fiber
assembly which meets this equationhas the maximum sound absorption
coefficient at a certain frequency, if it has no back air spaceor
at an optional frequency if it has a back air space suited to the
frequency [39]. If samples areuniform in thickness, the total
surface area of fiber at P
e
is constant, irrespective of the finenessof fibers. This means
that the relation between the fineness of fibers d (denier) and
P
e
(%) for asample of uniform thickness is shown thus,(100-P
e
) d
-1/2
= constantP
e
for a sample made up of fibers of differing in denier is easily
calculable by using thisequation. In a porosity range higher than
P
e
, the maximum absorption coefficients of samplescomposed of
fibers differing in fineness but arranged to be the same in total
surface area do notagree completely [39].Kyoichi et al. and Narang
et al. [40, 41] indicated a direct correlation between sound
absorptionand fiber surface area. Their study explained the fact
that friction between fibers and airincreases with fiber surface
area resulting in a higher sound absorption. Kyoichi et al.
observedthat the sound absorption coefficient rises as the fibre
surface area of the sound-absorbingmaterials increases (fig. 3.3(a)
& fig. 3.3(b)).
10(a) (b)Figure 3.3 (a) Sound absorption comparison for various
fibres with frequency.(b) surface area for various fibre-based
sound-absorbing materials.This can be explained by the fact that
friction between the fibres and the air increases with alarger
fibre surface area, resulting in a higher sound absorption
coefficient. Moreover it has beensaid that, in the frequency range
1125 Hz 5000 Hz, fibers with serrated cross sections absorbmore
sound compared to ones with round cross sectional area.The fabric
weight would then become less important than fabric thickness as
fabric lightness canbe achieved by using a micro-fiber fabric,
which has less weight due to its large surface area.Therefore these
possibilities of micro-fiber fabrics were tested for their
application as sound-absorbing materials. As a result, micro-fiber
fabrics (except those with a mesh structure)absorbed all sound
frequencies better than a conventional fabric, and also better than
the datafrom other studies of absorbing materials. Micro-fiber
fabrics absorb sound better because theirfibers have a higher
surface area than those of regular fiber fabrics, resulting in
higher flowresistance [36].
3.3 Thickness:Many literature have cited that sound absorption
in porous materials have concluded that lowfrequency sound
absorption has direct relationship with thickness. The
effectiveness of absorption is directly related to the thickness of
the material [32]; absorbers are most effectivewhen their thickness
is between one-fourth and one-half the wavelength of the sound,
with themaximum performance where the thickness is one-fourth the
wavelength [5]. MasatakaHakamada et al. [42] shows that the sound
absorption coefficient increased with increasing
specimen thickness at all frequencies (fig. 3.4 a). Not only
that, the air gap, from specimen to therigid wall, has an
importance to sound absorption too (fig. 3.3 b).Fig. 3.4 (a) Effect
of specimen thickness on sound absorption coefficient(b) Effect of
air-gap interval on the sound absorption coefficient [42]A study by
M.A. Ibrahim et al [43] showed the increase of sound absorption
only at lowfrequencies, as the material gets thicker. However, at
higher frequencies thickness hasinsignificant effect on sound
absorption. When there is air space inside and behind the
material,the maximum value of the sound absorption coefficient
moves from the high to the lowfrequency range [42]. Another work
has done by Kazuhiko Kosuge et al [44]. They also showsfor the
lower frequency of the normal incidence wave, sound absorption
increased by increasingthe thickness of the nonwoven (fig.
3.5).Figure 3.5 Nonwoven thicknesses vs. normal incidence sound
absorption
12
3.4 Bulk density:Numerous works has been done to study the
influence of the bulk density on the sound absorbingproperties of
fibrous materials. K.O.Ballagh [8] has shown the effect on NAC is
quite significantfor bulk density, thickness and flow resistivity
in the frequency range of 500 - 2000Hz for 25mm thickness and
250-1000Hz for 100 mm thickness. He explains that within the
frequencyrange of 500-2000Hz, the NAC is proportionally higher with
the higher bulk density, thicknessand higher flow resistivity (fig.
3.6).Figure 3.6 absorption coefficients showing the effect of
density and flow resistivity.There is a close relationship between
flow resistivity [45, 5], density and fibre diameter. It can beseen
that the flow resistivity generally increases with increasing
density [46]. Additional testshave done on a single sample with a
particular fibre diameter which was compressed to variousdegrees,
and the flow resistivity can be measured over a range of
different.K.O.Ballagh [8] explained that the flow resistivity is
inversely proportional to the fibre diameterand proportional to the
density of the sample.Energy loss increases as the surface friction
increases, thus the sound absorption coefficientincreases.3.5
Porosity:Porosity is relatively important factors which prominently
influenced to the Sound absorptioncharacteristics of porous
materials [47]. The fig. 3.6 shows the influence of porosity along
withthe bulk density on sound absorption coefficient of the porous
materials.
13Figure 3.6 Sound absorption characteristics of 2.5 cm thick
sampleD
a
: Observed apparent density and P: PorosityAlready we have seen
that many factors have the influence to sound absorption properties
of theporous materials. One of the important factor is porosity. To
allow sound dissipation by friction,the sound wave has to enter the
porous material. This means, there should be enough pores onthe
surface of the material for the sound to pass through and get
dampened. The porosity of aporous material is defined as the ratio
of the volume of the air in the material to its total
volume.Definition of the porosity (
) [48, 49], we can write as,
= 1-
=
t a
vv
where,
is the fibre volume fraction and v
a
and v
t
are the volume of the air (void volume) andtotal volume of the
sample respectively.A porous material such as nonwovens with an
open face carries most of the sound energy in theform of the
airborne wave. The exception is a porous material that has a
structural stiffness lessthan that of air. In this case, the
material behaves as a fluid. In either case, the sound energy canbe
thought of as being carried by the airborne wave. There are several
factors that have a stronginfluence on the airborne wave, but
usually the most important influence is due to the flowresistivity
of the material. Most of the materials tested in this study were
porous materials withan open or scrim covered face, so the airborne
wave is dominant [5].Shoshani et al. [50] considered that, four
functional forms of the porosity: linear, quadratic,exponential and
logarithmic. He assume that, layer can be approximately thought of
as acombination of several thin layers; each of which having a
constant porosity. Therefore, it seems
14to us that our generalized theory can be used as a tool for
assessing the noise absorption capacityof multilayer nonwoven
structure.According to the functional form of porosity, they
reveals numerical configuration as,a)
Linear:b)
Quadratic:c) Exponential:andc)
Logarithmic:Where, each of these forms depends on two parameters
P
1
and P
2
satisfying 0< P
1
, P
2
800 Hz) so thatthis wave can be thought of as a plane wave
propagating along the axis of the tube. The normalincidence NAC of
the specimen, designated by
, is defined bywhere Io and I, are-the energy flux of the
incident and reflected waves, respectively. If P
min
. isthe minimal sound pressure level in the tube and P
max
is its maximal value, a is given bywhere n is the ratio between
maximum pressure leve to minimum (P
max
/P
min
)The amplitude or loudness of a sound wave is expressed by its
sound pressure level. Soundshaving the same wavelength (equal
frequency) may have differing loudness because the soundpressure of
a sound wave may vary over a wide rangea change in magnitude of ten
million toonesound pressure is expressed using a logarithmic scale.
This is the basis of the decibel scale,which compresses the range
of sound pressure into a scale from 0 to 150. The decibel (dB)
isnot an actual measure of amplitude or loudness, but expresses the
ratio between a given soundpressure and a reference sound pressure.
This relationship is expressed by the followingequation:(L
p
) = 10 log (P/P
re
)
2
where, L
p
is the Sound Pressure Level, P is the Sound Pressure (Pa), P
re
is the sound pressure atthe threshold of hearing (0.00002 Pa)
[69].
28
6.1. Methods for acoustic measurements:Impedance tube methoduses
plane sound waves that strike the material straight and so thesound
absorption coefficient is called normal incidence sound absorption
coefficient, NAC (fig.6.1) [70]. Figure 6.1 Impedance Tube for
Sound AbsorptionThe impedance tube consists of a speaker, tube, two
microphones and material sample holder. Aspecial sound called white
noise is generated in the speaker. The white noise is composed of
sound contributions from all frequency bands in the audible range.
The sound travels straightdown the tube and strikes the material.
Some of the sound is absorbed and some is reflectedback. The two
microphones measure the reflected sound. From the two microphone's
signals, thesound absorption can be calculated [70].
In an ITM (Impedance Tube Method) measurement (fig.6.2), the
acoustic waves are confined within the impedance tube, which is
typically a fewcentimeters in diameter, and the size of the
materials sample need only be large enough to fill thecross-section
of the tube.[71].
29Figure 6.2
Schematic Sketch of an Impedance Tube Set-Up [30]
Thus this method avoids the need to fabricate large test sample
with lateral dimensions severaltimes the acoustical wavelength. The
impedance tube method employs two techniques todetermine NAC,
namely:1. Movable microphone which is one-third octave frequencies
technique (ASTM C 384) isbased on the standing wave ratio principle
and uses an audio frequency spectrometer to measurethe absorption
coefficients at various centre frequencies of the one-third octave
bands.2. Two-fixed microphone impedance tube or transfer function
method (ASTM E 1050), which isrelatively recent development. In
this technique, a broadband random signal is used as a soundsource.
The normal incidence absorption coefficients and the impedance
ratios of the testmaterials can be measured much faster and easier
compared with the first technique [72]. Thefinal method of
measuring the sound absorption coefficient is known as,
Steady state method. This method is mostly used when the other
will not work.This particular method is described in ASTM E336-71.
To measure the transmission coefficientof the materials, a third
microphone or even a second pair of microphone can be placed
behindthe test sample in a second impedance tube.
Reverberant field methodfor measuring sound absorption is
concerned with the performanceof a material exposed to a randomly
incident sound wave, which technically occurs when thematerial is
in diffusive field [69]. However creation of a diffusive sound
field requires a large
30and costly reverberation room. A completely diffuse sound
field can be achieved only rarely.Moreover, an accurate value of
complex impedance cannot be derived from the absorptioncoefficient
alone [73]. Since sound is allowed to strike the material from all
directions, theabsorption coefficient determined is called random
incidence sound absorption coefficient, RAC.This method is clearly
explained in ASTM C 423 72.
Two Microphone Impedance Tube Technique (Transfer Function
Method)The transfer function method (ASTM E 1050) covers the use of
an impedance tube, with twomicrophone locations and a digital
frequency analysis system for the determination of normalincidence
sound absorption coefficients (NAC) and normal specific acoustic
impedance ratios of materials. This test method is similar to Test
Method (ASTM C 384) in that it also uses animpedance tube with a
sound source connected to one end and the test sample mounted at
theother end. The measurement techniques for the two methods are
fundamentally different,however. First microphone tube method
(standing wave method) is quite cumbersome since aprobing of the
sound field has to be carried for each frequency.The usable
frequency range depends on the diameter of the tube and the spacing
between themicrophone positions. An extended frequency range may be
obtained by using tubes withvarious diameters and microphones
spacing. By this method acoustical parameters likeabsorption
coefficient, reflection coefficient and surface admittance for a
small samples exposedto plane waves can be determined [74]. In the
fig. 6.2 (a) shows the wave propagation throughthe sample and fig.
6.2(b) shows the measuring system.(a) (b)Fig: 6.2 (a) Sound wave
propagation and (b) Measuring system configuration.
31The major parameters to be measured are the corrected transfer
function
H
broken down into thereal partH
r
and the imaginary part
H
i
, the complex reflection coefficient
R
determined by thereal partR
r
and the imaginary part
Ri
, and the normal incidence sound absorption coefficient
(taking values between 0 and 1). These parameters are described
below [6]:where:H
= measured transfer function;
c
H
= microphone calibrated factor;
j
=1
, indicating an imaginary unit in the equation;
c
= speed of sound (m/s);
= density of air (kg/m3);
f
= sound frequency (Hz);
k
= 2
f
/
c
(m
1
); (wabe no.)l
= distance from the test specimen to the center of the nearest
microphone (m);
s
= center-to-center spacing between the two microphones (m);
r
/
c
=
/[2(1
R
r
)
], acoustic resistance ratio;
x
/
c
= 2
R
i
[2(1
R
r
)
], acoustic reactance ratio;
z
/
c
= acoustic impedance ratio [6, 74].
32
7. Materials and methods and Experimental Results:For getting an
experimental experience we have taken three types of nonwoven
fabrics whichmade by polypropylene. In the Department of Textile
Technology, IIT Delhi, we have NormalImpedance Tube instrument. All
the results, we have got experimentally by the above
mentionedinstrument. The instrument has been designed followed
by
ASTM C 384-98standard. The bandfrequency has taken as
one-third-octave band. The lower cut-off frequency has been kept at
250Hz. The experiment obtained up to 2000 Hz.Details parameters of
the sample has been shown in the table 7a.
Sample ID Mass (gsm) Thickness (mm) Density (gm/cc) PorosityNPP1
378 6.24 0.06 0.93NPP2 756 12.48 0.06 0.93NPP3 1134 18.72 0.06
0.93TPP1 275 0.69 0.40 0.57TPP2 550 1.38 0.40 0.57TPP3 825 2.07
0.40 0.57TLPP1 82.3 0.43 0.19 0.79TLPP2 164.6 0.86 0.19 0.79TLPP3
246.9 1.29 0.19 0.79
Table 7(a)NPP ---- Needlepunch PolypropyleneTPP ---- Thermalbond
PolypropyleneTLPP --- Thermalbond Low gsm Polypropylene (1, 2, 3
denotes the thickness increasing)Noise absorption coefficient has
been observed in one-third octave band frequency range for allthe
samples.
33Table 7.1 shows the values of NAC with respect to Frequency
(Hz) of the samples NPP1, NPP2and NPP3.Fig. 7.1 shows the relation
between NAC and Frequency (Hz) below.
NPP1 (
) NPP2 (
) NPP3 (
)0.63 0.65 0.690.69 0.71 0.730.72 0.74 0.750.75 0.77 0.790.8
0.84 0.860.83 0.88 0.90.85 0.9 0.930.87 0.91 0.940.82 0.9 0.920.8
0.87 0.900.20.40.60.81
2 5 0 3 1 5 4 0 0 5 0 0 6 3 0 8 0 0 1 0 0 0 1 2 6 0 1 6 0 0 2 0
0 0
1/3 Octave band frequency (Hz)N A C (
)
NPP1NPP2NPP3Table 7.1 Figure 7.1Table 7.2 shows the values of
NAC with respect to Frequency (Hz) of the samples TPP1, TPP2and
TPP3.Fig. 7.2 shows the relation between NAC and Frequency (Hz) of
the sample below.
TPP1 (
) TPP2 (
) TPP3 (
)0.48 0.52 0.580.5 0.55 0.620.53 0.57 0.660.58 0.62 0.690.6 0.64
0.730.64 0.68 0.770.69 0.73 0.790.77 0.79 0.830.77 0.76 0.810.74
0.76 0.7700.20.40.60.81
2 5 0 3 1 5 4 0 0 5 0 0 6 3 0 8 0 0 1 0 0 0 1 2 6 0 1 6 0 0 2 0
0 0
1/3 Octave band frequency (Hz)N A C (
)
TPP1TPP2TPP3Table 7.2 Figure 7.2
34Table 7.3 shows the values of NAC with respect to Frequency
(Hz) of the samples TLPP1,TLPP2 and TLPP3.Fig. 7.3 shows the
relation between NAC and Frequency (Hz) of the samples below.
TLPP1 (
) TLPP2 (
) TLPP3 (
)0.46 0.49 0.540.5 0.53 0.580.55 0.56 0.630.58 0.6 0.660.6 0.63
0.680.66 0.68 0.720.69 0.7 0.750.73 0.75 0.790.76 0.77 0.790.72
0.74 0.7700.10.20.30.40.50.60.70.80.9
2 5 0 3 1 5 4 0 0 5 0 0 6 3 0 8 0 0 1 0 0 0 1 2 6 0 1 6 0 0 2 0
0 0
1/3 Octave band frequency (Hz)N A C (
)
TLPP1TLPP2TLPP3Table 7.3 Figure 7.3
From the above all graphs, we can see that the values of
increasing with increasing thethickness of the nonwoven
samples.The noise reduction coefficient has been shown in the table
7.4 and the fig.7.4 is showing thetendency of NRC (%) with
increasing thickness.
Sample NRC (%)NPP1 77.6NPP2 81.6NPP3 84.1TPP1 63TPP2 66.2TPP3
72.5TLPP1 62.5TLPP2 64.5TLPP3
69.177.681.684.16366.272.562.564.569.1505560657075808590MinMidMaxThicknessN
R C ( % )
NPPTPPTLPP
Table 7.4 Figure 7.4
35
8. Aplication of sound absorptive materials:Now a days,
acoustical material plays a number of important roles in acoustic
engineering suchas the control of room acoustics, industrial noise
control, studio acoustics and automotiveacoustics. Sound absorptive
materials are generally used to counteract the undesirable effects
of sound reflection by hard, rigid and interior surfaces and thus
help to reduce the reverberant noiselevels. They are used as
interior lining for apartments, automotives, aircrafts, and
ducts,enclosures for noise equipments and insulations for
appliances. Automotive interior noise beundesirable for both the
passenger and driver; many author have studied that the textile
structureshave the potential to reduce interior noise in
automobiles [75]. Sound absorptive materials mayalso be used to
control the response of artistic performance spaces to steady and
transient soundsources, thereby affecting the character of the
aural environment, the intelligibility of unreinforced speech and
the quality of unreinforced musical sound. Combining
absorptivematerials with barriers produces composite products that
can be used to lag pipe or provideabsorptive curtain assemblies
[30]. All noise control problem starts with the spectra of
theemitting source. Therefore, sound absorbing materials are chosen
in terms of material types anddimension, and also based on the
frequency of sound to be controlled [16, 62].
Some application area:Buildings & Construction Industrial
PlantsAcoustic Ceiling Panel Automotive industriesEnclosable Noise
Sources Outdoor Noise SourcesPrinting Presses Public
TransportDefense Industries Aeronautical EngineeringHVAC
Applications Stamping PressesHospital application Electronic
IndustriesMarine Insulation Gallery & Auditoriums etc.
36
9. Conclusion:A sound wave is an obvious parametric feature
which helps us to hear something. Not only that,sound wave is an
important communicator for the daily life too. Some times the sound
wavemakes us unhappy and irritated, because its a noisy world.
Twenty-four hours a day, seven daysa week, we are exposed to sounds
we do not want, need, or benefit from. There are few places onthe
planet where in our daily lives we are free from unwanted sounds.
We can get a pleasantenvironment by controlling the noise hazards.
For this purpose many people have studied how tocontrol the noise
and make peaceful circumstances.Lot of researchers have served the
results on the sound absorption characteristics of fibrous aswell
as other materials. Fibrous materials have good sound absorption
characteristics. Soundabsorptive materials can be classified into
three categories such as absorptive materials, Barriermaterials and
damping material.The performance of the sound absorptive materials
depends on some important factors that arefibre diameter, fiber
surface area, thickness, bulk density, porosity, airflow
resistivity, tortuosityand surface impedance.We have seen that the
most important factor for sound absorption is the air flow
resistivity of thefibrous materials. Several times, researchers
have found that the sound absorption coefficient (
)increasing with increasing the airflow resistivity. Because of
that, the airflow resistivity dependson the materials porosity and
bulk density. We know that, if the fibre volume fraction
decreasing,the porosity is increasing. So, porosity increasing that
means the bulkiness of the materials isincreasing and airflow
resistivity is decreasing. For a certain range of frequency, the
soundabsorption coefficient is increasing with increasing the flow
resistivity.In this report, we have seen that the above noted
factors have direct relation to the soundabsorption properties of
the materials and out of that, some secondary factors also
affectindirectly. Some researchers have reported that the sound
absorption coefficient is increasingwith increasing the thickness
as well as bulk density and airflow resistivity.The attenuation or
dissipation of acoustic energy as a sound wave moves through a
medium maybe attributed to three basic mechanisms that are,
frictional losses, momentum losses andtemperature fluctuations.
37A number of models have been established by several
researchers to find out the soundabsorption characteristics of the
fibrous materials. They have given some important equationsfrom
which we can easily calculate the sound absorption coefficient of
the tested materials.Different techniques have been developed to
measure the sound absorption properties of thematerials such as
impedance tube methods, steady state methods and reverberant field
methodsThese three methods have been briefly discussed in the
report.From the experiment result and discussion, we can see that
the noise absorption coefficient aswell as noise reduction
coefficient is increasing with increasing thickness.In the present
society, we have seen that the sound absorptive materials have
crucial demand.Application of the sound absorbing materials to
various fields is necessary. For the purpose of the specific
application of the materials, manufacturers consider the following
criteria as for thevarieties of products which should be
economical, durable, good aesthetic property, easyprocessibility
and obviously beneficial. Depending on those factors, huge
applications of thesound absorptive materials have been found in
automobile industries, aeronautical industries,building
construction, hospital application, and so on.
38
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