Download development of an instrument to measure sound absorption

Niresh et al., International Journal of Advanced Engineering Technology
E-ISSN 0976-3945
Research Paper
DEVELOPMENT OF AN INSTRUMENT TO MEASURE SOUND
ABSORPTION
Niresh J1, Neelakrishnan. S2, Subha Rani. S3
Address for Correspondence
1,2
Department of Automobile Engineering, 3Department of Electronics and communication Engineering,
PSG College of Technology, Coimbatore, Tamilnadu, India – 641004.
ABSTRACT:
Impedance tube is an important device used to measure sound related properties of a material, for measuring the sound
absorption characteristics of a material. The most commonly used methods are the standing wave ratio method and transfer
function method. In this study a modified impedance tube with single microphone method is implemented. This type is
commonly used for determining the normal incidence sound absorption coefficient. The aim of this study is to determine the
accuracy of the sound absorption coefficient of various textile materials by comparing measurements from a commercially
available impedance tube with the developed tube using various test samples. The developed tube can measure sound
absorption coefficient in the frequency range within 80 Hz-1600 Hz. In this study various parameters such as frequency of
sound generated, distance between the sound generated and physical features of samples are studied, Two different samples,
Polypropylene and Coir fibers are tested in this tube and results are analyzed. The samples show significant amount of
absorption at frequencies around 1KHz. The modified circuitry provides better results at higher frequencies when compared
with the standard values.
KEY WORDS: Impedance Tube, Sound Absorption, Standing Wave, coir fibers, polypropylene.
1. INTRODUCTION
With the increase in public awareness and concern
for noise pollution, there is a need to control noise by
absorbing sound in various applications involving
sound producing elements and sound absorbing
materials. Many kinds of sound absorbing or
isolating materials, such as glass wool, polymeric
fibrous materials, and various types of foams are
currently used to absorb airborne noise and optimize
the transmission loss (TL). The factors that mainly
influence acoustic performance of sound absorptive
materials are fibre dimension, material thickness,
density, air flow resistance and porosity which will
be
changing
the
sound
absorbing
behavior[1],[2],[3],[4]. Various studies have proved
such as polyster[3], coir[4], wool[2], cotton[4].
Various research have proven that sound absorption
coefficient is more for the nonwoven have more fine
fabric as they have more chances to contact with the
sound waves[5]. Nonwovens materials are used in the
automobile sector for certain application eg foam and
warp knitted fabrics are used as upholstery or interior
trim materials. The present investigation deals with
the measuring sound absorption coefficient of various
materials in the closed tube. Automobiles come with
different noise producing elements whose frequency
ranges from 200 Hz to 3 kHz[6], and hence require
noise control materials with selective acoustic
properties for different frequency range of interest.
Measuring the acoustical properties and predicting
the noise control impact of these materials at the
design stage is important. The properties of various
textile materials can be determined either
theoretically using the empirical prediction[7] based
on regression analysis[8],[9] or experimentally using
impedance tube. The quantification of acoustical
impedance is an essential task in the design of
acoustical systems such as horns, acoustical pipes,
absorbent materials and silencers. The mechanical
impedance is obtained by dividing the applied force
to the particle velocity. In addition acoustical
impedance is the applied pressure divided by volume
velocity[10]. Therefore, the relationship between
mechanical and acoustical impedances is derived
from the force-pressure and volume-velocity.
However, most of the present acoustical impedance
Int J Adv Engg Tech/Vol. VII/Issue I/Jan.-March.,2016/264-267
quantification methods used today such as ISO10534-1[11] and ISO-10534-2[12] do not provide a
direct quantification of volume velocity. There are
two standardized methods for measuring the normal
sound absorption coefficient for small samples, one
using standing wave ratio standardized in ISO10534-1 and the transfer function method
standardized in ISO-10534-2. The standing wave
ratio (SWR) method is named as "classical kundt
duct" in ISO-10534-1. In this method, one end of the
kundt duct is closed while a speaker is placed at the
other end to generate a standing wave within the
duct. A microphone is moved along the duct to
quantify and locate the maximum and minimum
pressure amplitude points along the duct. The
standing wave ratio and the acoustical impedance are
then calculated accordingly[13].
2. MATERIALS AND METHODS:
Sample of different categories are made up of coir
fibre, polyester samples are provided by the PSG
Tech Centre of Excellence, Coimbatore, INDIA, the
specifications are listed in Table1.
Table1. Physical properties of samples
Physical
properties
of sample
Polyester
Coir Fiber
Straight
cross
section
Coir Fiber
Diagonal
cross
section
Sample
code
GSM
Thickness
cm
P1
C1
224
263
2.0
0.8
Air
permeability
Cc/cm2/s
2396.74
900.36
C2
271
0.9
898.76
2.1 System Implementation
The impedance tube set up contains a signal source
along with power amplifier section. The impedance
tube signal characteristics is improved by modifying
the signal source from generating monotone signals
into one that generates white noise signals The main
purpose of adding white noise is to provide a uniform
reference frame in the time-frequency space[14].
The noise circuitry is designed by adding the signals
generated from two similar signal generators, through
a resistive network. The signal thus generated, is
amplified using a power amplifier circuit and fed into
the impedance tube through a loud speaker.
Niresh et al., International Journal of Advanced Engineering Technology
Microphones that are placed inside the tube capture
the incident signal and reflected signal from the
sample. These signals are too weak to be transmitted
to recording devices and hence preamplifiers are used
to increase a microphone signal to line-level by
providing stable gain while preventing induced noise
that would otherwise distort the signal. A microphone
preamplifier increases the 0 to 100 microvolt range
level up by 40 dB, to approximately 0 to 10 volts.
The amplitude of the incident and reflected signals
vary according to the sound absorption of the sample.
Therefore by monitoring the amplitude variations of
both signals and taking the ratio of it, the sound
absorption coefficient of material is calculated. A
digital storage oscilloscope, spectrum analyzer or a
PC is used for storing and analyzing these signals.
2.2. Electronic circuit design
The power amplifier circuit is to have high gain and
signal filtering characteristics to achieve high signal
fidelity. The microphone probes are made of double
core shielded wires to eliminate noise interference.
The signal from the microphone is amplified by a
newly designed pre amplifier circuit. The circuit
takes the audio signal from the condenser
microphone and amplifies it, so that the microphone
output can be used as input to some device which
wouldn’t normally accept microphone level signals
(which are very low). The output of the microphone
is fed to a two stage amplifier. There is a current
series feedback and a voltage shunt feedback
configuration configured to pick up smaller sounds.
The output can be varied with a 10 kΩ potentiometer.
Since the microphone circuit is biased, the output is a
sinusoidal signal.
2.3 Measurement of Air permeability
This test method covers the measurement of the air
permeability the rate of air flow passing
perpendicularly through a known area under a
prescribed air pressure differential between the two
surfaces of a material of textile fabrics and is
applicable to most fabrics including woven fabrics,
air bag fabrics, blankets, napped fabrics, knitted
fabrics, layered fabrics, and pile fabrics. The fabrics
may be untreated, heavily sized, coated, resin-treated
or otherwise treated. It is measured by using Air
Permeability Tester FX 3300 Lab Air IV.
3. SYSTEM ANALYSIS
The complete experimental set up to measure the
acoustical characteristics in given in Fig.1, which
contains a power supply, amplifier and sound source,
microphone and pre-amplifier circuit, impedance
tube and data acquisition system (spectrum analyzer).
Fig 1 Schematic measuring setup
Two different materials are tested in the designed
impedance tube. The sample holder is removed from
Int J Adv Engg Tech/Vol. VII/Issue I/Jan.-March.,2016/264-267
E-ISSN 0976-3945
the impedance tube set up. The samples with
diameter equal to that of impedance tube are pasted
above the surface of the movable termination which
runs along the length of the sample holder. The signal
generator excites the loud speaker and the movable
termination is moved to a position in impedance tube
where maximum peak voltage captured from the
microphone occurs. The absorption coefficient is
calculated by taking average of repeated
measurements at the same frequency. The normal and
average absorption coefficient is calculated as in
equation (1) and (2).
=
(1)
αavg= ( α1+ α2+ α3+ α4+ α5+ α6+ α7) /7
(2)
Three different materials Polyester and coir fibers,
are tested in impedance tube. The coir fiber materials
are tested with two different configurations. The
sample holder is removed out from the impedance
tube set up. The samples with the diameter equal to
that of impedance tube are pasted above the surface
of the movable termination which runs along the
length of the sample holder. The signal generator is
made to excite the loud speaker and the movable
termination is moved to a position in impedance tube
where maximum peak voltage captured from the
microphone occurs.
3.1 Evaluation of samples
The sample of polyester sample is shown in fig.2.
Which is tested in the modified impedance tube and
the existing impedance tube.
Fig. 2. Polyester sample
The absorption coefficient is calculated by taking
average of all the measurements. The normal and
average absorption coefficient is calculated from the
equation and the values are tabulated in Table 2
Table 2. Experimental results for Polyester
F(Hz)
Amplitude
Without
Sample(v1)
Amplitude
With
sample(v2)
Absorption
coefficient
(α)
Existing
Impedance
tube (α)
411
554
870
1190
1380
1540
0.8
1.8
1.6
0.9
0.84
1
0.72
1.7
1.12
0.4
0.72
0.6
0.10
0.06
0.30
0.56
0.14
0.40
0.16
0.09
0.33
0.57
0.19
0.45
Fig 5 shows the absorption characteristics of
Polyester with thickness of 20cm. The average
absorption coefficient for Polyester is 0.33. It is seen
from the figure that the Polyester material shows
higher absorption at 2 KHz region. Because of long
time, large surface to volume ratios and high heat
conductivity of fibers, heat exchange takes place
isothermally at low frequencies. So the absorption is
low at low frequencies. At the same time in the high
frequency region compression takes place
adiabatically. In the frequency region between these
isothermal and adiabatic compression, the heat
exchange results in loss of sound energy. This loss is
high if the sound propagates parallel to the plane of
fibers and may account up to 40% of sound
attenuation. It is suitable for higher frequencies than
Niresh et al., International Journal of Advanced Engineering Technology
lower frequencies for noise absorption. The first peak
occurs at around 1 KHz and the second peak is
observed at same bandwidth. The experimental
results are comparable to actual result at the
frequency of above 1 KHz.
The other sample coir fiber with straight cross section
and diagonal section which is used for the testing
purpose is shown in fig 3 and fig 4.
E-ISSN 0976-3945
Fig. 4. Coir fiber with diagonal cross section
The experimental results for coir fiber diagonal cross
section is shown in table 3.
Table 4. Experimental results for Coir Fiber Diagonal
cross section
Fig. 3. Coir fiber with straight cross section
The average absorption coefficient of coir fiber with
diagonal cross section is 0.13 .The coir fiber with
diagonal cross section is less absorbing than the coir
fiber with straight cross section. The primary factors
causing sound attenuation when the sounds contact a
sound absorber are frictional losses, momentum
losses and temperature fluctuations[15].
Table 3. Experimental results for Coir Fiber Straight
cross section
F
(Hz)
Amplitude
Without
Sample(v1)
Amplitude
With
sample(v2)
Absorption
coefficient
(α)
Existing
Impedance
tube (α)
411
554
870
1190
1380
1540
0.92
1.6
1.55
0.76
0.84
1
0.84
1.55
1.45
0.66
0.8
0.88
0.09
0.03
0.06
0.13
0.05
0.12
0.12
0.06
0.09
0.15
0.05
0.15
F(Hz)
Amplitude
Without
Sample
(v)
Amplitude
With
sample
(v)
Absorption
coefficient
(α)
Existing
Impedance
tube
(α)
411
554
870
1190
0.92
1.6
1.55
0.76
0.84
1.55
1.25
0.68
0.09
0.03
0.19
0.11
0.12
0.06
0.22
0.12
1380
1540
0.84
1
0.82
0.9
0.02
0.10
0.06
0.11
The same samples are tested using the commercial
tube available in the market and a comparison is
made between both the readings to ensure the
efficiency of the designed tube in measuring the
absorption coefficient. The difference between the
modified tube and the existing tube is projected in the
figure which show that for all samples, the developed
tube provides a near value of absorption coefficient
to that of the existing tube.
Fig 6 shows comparatively lower absorption than the
Polyester. It is showing lowest absorption for
frequencies lower than 500 Hz. The average
absorption coefficient of coir fiber with straight cross
section is 0.12.The different absorption results at
different frequencies are attributed to the frequency
dependence of the sample physical material
properties like thickness, porosity, and flow
resistivity.
Fig.5 Relation between Frequency and sound
absorption coefficient of materials
.
0.6
0.5
0.4
0.3
0.2
0.1
0
Polyester
Absorption
coefficient (α)
Polyester Existing Coir Fiber Straight Coir Fiber Straight Coir Fiber Diagonal Coir Fiber Diagonal
cross section
cross section
cross section
cross section
Impedance tube (α)
Absorption
Existing Impedance
Absorption
Existing Impedance
coefficient(2)
tube(2)
coefficient(3)
tube (3)
Fig. 6 Comparison of sound absorption for materials tested
It is observed that the values measured using the
proposed tube deviate from the original tube by an
average of 4.55% for most of the frequency ranges.
The different absorption results at different
frequencies are attributed to the frequency
dependence of the sample's physical properties like
thickness, porosity, and flow resistivity. The
amplitude vs frequency for two different samples is
plotted in figure 7. The observed error may be due to
the lower sensitivity of the microphone and offer
Int J Adv Engg Tech/Vol. VII/Issue I/Jan.-March.,2016/264-267
electronic components used for the developed tube.
Further the error can be considered as minimum
when compared to the existing tube. However the
low cost tube can be implemented for measuring the
sound absorption coefficient of the material by the
industries at the initial stage of the material. Also,
calibration of the developed tube may lead to
decrease in the error.
Niresh et al., International Journal of Advanced Engineering Technology
[8]
[9]
[10]
[11]
[12]
Fig. 7. Amplitude vs Frequency for different
materials
4. CONCLUSION
A low cost impedance tube is designed and
developed with modified electronic circuitry for the
measurement of absorption characteristics of fifteen
sound absorbing materials encountered in vehicles.
Standing wave method of measurement is used
according to ISO 10534-1 regulation. The results of
the absorption characteristics of various materials
were compared with the existing tube and the results
prove the efficiency of the designed tube as the error
rate is only around 4.5% for all the samples. The cost
of the impedance tube is reduced to around 35% of
the commercially available impedance tube and
hence the developed tube is cost effective. This paper
discussed about the modified implementation of the
electronic circuitry for the measurement of
absorption characteristics of the various porous
materials. The results of the absorption
characteristics of coir fiber and the Polyester are
compared. The experimental results for polyester are
matching with standard values above 1 KHz. The
Polyester shows higher absorption with absorption
coefficient of 0.6 than coir fiber material for
frequencies around 2 KHz. The coir fiber with
straight cross section provides better absorption at 0.6
whereas the coir fiber with diagonal cross section
provides lesser absorption at 0.5. Thus it is concluded
that the coir fiber with straight cross section is
compatible with the Polyester.
5. ACKNOWLEDGMENT
The support extended by PSGTECHS Centre of
Excellence for Industrial Textiles for carrying out
this study is acknowledged.
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