The Scientific Bulletin of VALAHIA University MATERIALS and MECHANICS Nr.

6 (year 9) 2011

ACOUSTICAL DETERMINATIONS ON A COMPOSITE MATERIALS

( EXTRUDED POLYSTYRENE TYPE/ CORK)
Bratu P.1, Enchescu G. L.2, Roca M.C.,
Vasile O. 1, Iatan R. I. 2
1

I.C.E.C.O.N. Bucharest - icecon@icecon.ro, 2 Polytechnica University of Bucharest georgianaenachescu@yahoo.co.uk, r_iatan@yahoo.com

Abstract. In this article we obtained some experimental results of the extruded polystyreneand and cork. They are absorbtion
coefficient, reflection coefficient, impedance ratio. All these are necessary for extracting some properties of extruded
polystyrene and cork wich are using in auto industry, plane plates construction etc.
Keywords: absorbtion coefficient, reflection coefficient, impedance ratio. impedance ratio

1.

made on sound absorption coefficient, reflection

coefficient and impedance ratio for each card that is
found in the layer.

INTRODUCTION

As a result of activities and statistical analysis

conducted by the Environmental Protection Agency
concluded that there is a continuous upward dynamic
noise levels [1]. They are composed of all existing
sounds in the monitored traffic coming from either, or
at various other facilities in close vicinity. Based on
this analysis engineers looking to use appropriate
means annihilation or reduction of noise that cause
disease population.

Thus, this paper includes the characteristics mentioned

above, for samples of extruded polystyrene and cork.
Sound is a mechanical motion of matter. Without
matter there is no sound propagation. Sounds arise and
propagate not only in gases and fluids, but also in
solids. Thus, the oscillation propagates with sonic
speed, whose value is characteristic of the propagation
medium. Making an analysis of a snapshot of a wave of
sonic oscillation in the propagation molecules, it is
observed that the oscillation condition is repeated at a
definite distance. This distance is called wavelength,
(Fig. 1). The state of oscillation is repeated over time,
the number of oscillations per second is given by the
frequency and expressed in hertz (Hz), named after
physicist Hertz . Wavelength and frequency are linked
by sonic speed: = c f [2].
2.

The phenomenon in which sound energy surface air

encounters a delimitatoare in an enclosed space, which
is not reflected, but is (apparently) absorbed by the
(sd) is called sound absorption. It is characterized by
sound absorption coefficient which, in turn, is
given by the ratio, subunit between energy
(apparently) "absorbed E a and the energy
incident E i :

Figure 1. Graphical representation of wavelength

[3]

Figure. 2. Reading
impedance tube type
4206

THEORETICAL CONSIDERATIONS

This paper covers a study of

the sound insulation capacity
of the two analyzed materials
- expanded polystyrene and
cork. Creating a layer of
several boards such materials
may be one of the " pillars "
of reducing the intensity noise
base . To reach the final
result, first measurements are

Ea
.
(1)
Ei
Expression is performed on standardized frequency
absorption or classes [4].

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The Scientific Bulletin of VALAHIA University MATERIALS and MECHANICS Nr. 6 (year 9) 2011

It knows also that the phenomenon of reflection is

characterized by the coefficient of reflection or
acoustic reflection factor, which is given by the ratio
of reflected wave amplitude incident wave amplitude.
Sound wave is a pressure wave, the acoustic reflection
coefficient relationship can be written:
P
r= r .
(5)
Pi
Substituting into (4), the ratio of reflected pressure
wave amplitude, respectively, incident with (5) we get
a link between the coefficient of sound absorption and
sound reflection coefficient of the form [5]:
(6)
= 1 r2 .
Acoustic impedance, denoted as electrical impedance,
is equal to the ratio of acoustic pressure and velocity:
p
(7)
Z= .
v
Acoustic impedance is measured in the system and
Rayl (name given in honor of Lord Rayleigh) or
acoustic ohms [1 Rayl (1 Ohm Acoustic) =
1 kg m-2s-1].

Figure. 4. Amplifier
Figure. 3. Transformer

Figure. 5. PC with
data acquisition software
p 1( f ) = p + ( f ) + p ( f )

Acoustic impedance expression can be written in

another form, depending on wave speed and density of
the environment :

p 2 ( f ) = p + ( f ) a e j k s + p ( f ) a e j k s
R( f

R( f ) =

(f)

= j

Z=

p ( f

)
p+( f )

(8)

where B = c , and therefore:

(9)
2c 2 = c .
Under well defined conditions of pressure and
temperature, the impedance of a medium density 0
is called the characteristic impedance, denoted by Z 0
[6]:
Z 0 = 0c.
(10)
Z=

H 12 ( f ) e j k s
e j k s H 12( f )

H 12 ( f ) sin k l sin k ( l s )

cos k ( l s ) H 12 ( f ) cos kl

where: p = puls; ( f ) = frequency;

impedance [2].

Z ( f )=
3. PREREQUISITES EXPERIMENTS

Fig. 6. Scheme brief reading impedance tube with 2

microphones [7]

To achieve the purpose of the work, used the following

components: reading impedance tube with two
microphones, type 4206 (Fig. 2), transformer (Fig. 3),
amplifier (Fig. 4), PC data acquisition software Brel
& Kjer (fig. 4) [7].

Energy conservation law in the separation of the two

media is given by:
(2)
E i = E a + Er ,

Reading impedance tube, type 4206, comprises two

microphones placed so as to measure sound absorption
coefficient, reflection coefficient of acoustic and
acoustic impedance ratio in the frequency range 50 Hz
6.4 kHz [7].

E r wave energy is reflected in that. Dividing both

members of equation (2) with E i and considering

equation (1) we get:
E
=1 r .
(3)
Ei
It is known that the energy of a wave is proportional to
the square amplitude periodic motion of material
point, because:

In Figure 6 are given input and output data for

determining the absorption coefficient, reflection
coefficient and impedance ratio, by reading the

P
= r
Ei Pi
Er

B ,

impedance tube, type 4206 [7].

(4)

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The Scientific Bulletin of VALAHIA University MATERIALS and MECHANICS Nr. 6 (year 9) 2011

In Figure 7 are given the form and appearance of

specimens taken from extruded polystyrene and cork
(Fig. 8) have been subject to measurement.

Figure 7. Test piece

extruded polystyrene

Figure 8. Test piece of

cork

4. EXPERIMENTS - SETTINGS PRIOR

DETERMINATION OF MATERIAL CHARACTERISTICS

Figure 11. Generation settings: noise and

signal waveform

Figures 9 - 14 are presented in succession, the settings

that were used during the reading of experimental
results.

Figure 12. View the settings for the

environment: pressure,
temperature, relative humidity,
speed of sound, air density and
characteristic impedance of air

Figure 9. Selecting the type and parameters

for determining the impedance tube

Figure 10. Measurement parameters:

frequency, frequency resolution and
measurement time

Figure 13. Alert settings, units and

during the drive pulse

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The Scientific Bulletin of VALAHIA University MATERIALS and MECHANICS Nr. 6 (year 9) 2011

cycles, high durability, high resistance to vapor

diffusion, low density and ease of handling, easy to cut
with simple tools, strength deterioration under the
action of roots (for underground placement).

4. 1. EXPERIMENTAL RESULTS

Interpreting acoustic absorption curve - Fig. 15, we can

draw the following conclusion: when the speaker
switched during the first phase penetration of sound
waves, which restored the negative values up to - 1.66
E-01, then a sharp increase followed by a decrease until
reaching a state of continuous growth, the average
absorption rate increases with raising. This finding may
support the hypothesis that sound-absorbing material is
extruded polystyrene.

Characteristic measurements were made for the two

materials above.
4. 1. 1. Extruded polystyrene

Figure 14. Signals of both microphones

and signal generator

Fig. 16. Acoustic reflection coefficient chart for the

first specimen taken from the extruded polystyrene

Figure. 15. The results for the average sound

absorption for the first specimen taken from AE
extruded polystyrene
Figure 17. AE medium impedance ratio for the
first specimen taken from the extruded polystyrene

According to data sheet for plates of extruded

polystyrene, which were taken specimens that acoustic
measurements are made, they have a uniform structure
of closed cells, which confers certain specific technical
features, such as low conductivity value heat, long
term, excellent mechanical strength, lack of capillary,
high moisture resistance, resistance to freeze-thaw

Comparing the two curves of the coefficient of

reflection - fig. 16 - see that was a reflection of the first
real value to 1.00753041, at a frequency of 16 Hz,
while the imaginary value of 0.129094 occurred at the
same frequency, which makes us specimen is taken to
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The Scientific Bulletin of VALAHIA University MATERIALS and MECHANICS Nr. 6 (year 9) 2011

say that the reflection coefficient, increased. Partial

return of sounds in the environment in which the
specimen came to the meeting area, initially due to
high reflection coefficient just this, then the behavior of
real reflection coefficient is in a similar shape, the
imaginary.

4. 1. 2. Cork
We know that cork is the bark of cork oak and has a
number of characteristics (low density, elasticity
convenient, waterproof, low conductivity, good
resistance to wear), which makes it increasingly used in
various industrial purposes and not only [8]. For a
specimen taken from the cork were performed acoustic
measurements of properties as for polystyrene
materials [7] and extruded polystyrene. Absorption
coefficient of the graph you can see the texture of
sound penetration specimen during increasing
frequency from 16-40 Hz oscillations that occur after
the lower curve of absorption, followed by a
continuous increase, followed by a regression, in the
negative, this state shows that there is a sound energy
absorption, but not as good as in the range of
frequencies from 500 Hz to 1500 Hz. This phase of
growth followed by decline, repeated as the frequency
is higher.

From equation (1), we see that the minimum ratio is

lower than if the imaginary real, when training curve,
as of 1030 kPa pressure, which means that there has
been an increase in acoustic velocity. This was due to
disturbance intensity transformer DC recorded during
experimental measurements. An increase in the number
of oscillations per second occurs near the two curves,
as much to confuse, which means that the report
reaches the desired impedance and thus extruded
material is a good receiver sound pressure.

Figure 18. The results for sound absorption

average AE for the first specimen taken from the
cork
Figure 20. AE medium impedance ratio for the
first specimen taken from the cork
After the value of 3100 Hz frequency, the absorption
curve is found as in the initial phase, which shows that
the specimen thickness (3mm) can not absorb sound
energy by 3150 Hz. Comparing the graphs in Figures
318 p.m. we can see that in both cases the first phase
sounds can penetrate into fabric, then absorption occurs
in a proportion lower or higher.
In Figure 19 we can see that the reflection coefficient
curve for the real and imaginary part to have an
approximately similar shape and is ending upward
curve, which shows that the partial return of the
original sounds in the environment is possible.

Figure 19. Acoustic reflection coefficient and

graphs to the average for the first specimen taken
from the cork
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The Scientific Bulletin of VALAHIA University MATERIALS and MECHANICS Nr. 6 (year 9) 2011

[2] Anghel, V., Acoustic (course)

[3]http://ro.wikipedia.org/wiki/Lungime_de_und%C4
%83
[4]http://www.ursa.ro/_files/normativ_privind_protecti
a_la_zgomot.pdf
[5]http://newton.phys.uaic.ro/data/pdf/INTERFER.pdf
-reflection and absorption
6]http://www.scritube.com/biologie/biofizica/UNDEACUSTICE - I - UNDE SONORE 242182022.
[7] Bratu, P., ed. A., Acoustical Determinations on a
Composite Material Type Expanded Polystirene, The
9th International Conference, OPROTEH 2011, Bacau,
May 24-26, 2011
[8] http://www.produsepluta.ro/articol/ro/pluta-12/ceeste-pluta--31

As with extruded polystyrene impedance ratio in the

real comes to be confused with that of the imaginary,
which is desirable - Figure 20.
5. CONCLUSIONS
This paper addresses research absorbing characteristics
of extruded polystyrene and cork, through experimental
tests on three samples of each material. Since
experimental results are close both in terms of forms of
variation in sound frequency, and value in the article
are exposed only to the first sample diagrams.
Should be noted that restricting the time of testing
equipment, in terms of thickness specimens, at present
cannot give values on the absorption of sound energy
in the case of laminated plates. This can be done
properly, the carrier further research, properly equipped
premises in structure and scope of testing.
6. REFERENCES
[1] Arghir, M, S. A., monitoring road traffic noise,

Didactic and Pedagogical Publishing House, Bucharest,

2008

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