15th International Conference on Experimental Mechanics
PAPER REF: 3174
ESTIMATION OF ACOUSTIC ENERGY HARVESTED FROM SOUND
USING ELECTROMAGNETIC TRANSDUCER
Nicolae Filip1(*), Gabriel Fodor2, Lucian Candale1
1
Dep. Automotive Eng. and Transportation, Tech. University of Cluj-Napoca, Faculty of Mechanics, Romania
2
Dep. of Mechanical Engi., Tech. University of Cluj-Napoca, Faculty of Machine Buildings, Romania
(*)
Email: nicolae.filip@aert.utcluj.ro
ABSTRACT
This paper presents the results of the work carried out to estimate the electricity
harvested from sound in laboratory conditions. Using a laboratory stand set up in this purpose
and a data acquisition system designed to collect the experimental results, the tests were
carried out to measure the electricity obtained from pure sound waves. The experimental
results showed the best conversion around the transducer resonant frequency, at 30 Hz.
Maximum electric values of 0,03105 A electric current and voltage of 0,304914 V were
obtained for 81,7 dB(A) sound pressure level of generated wave. The mathematical evaluation
of the results offers information to establish the distribution law from to acoustic into electric
conversion.
Keywords: electromagnetic, sound, wave, conversion, voltage, electric current, exponential,
coefficient.
INTRODUCTION
The environmental impact of the noise guides the researchers in the field of noise reduction
and noise harvest. Providing electric energy from so-called waste energy (noise), offers more
possibilities to assure new renewable energy for hand devices or for different applications.
The researches developed in the last decades lead to three conversion ways of the acoustic
energy: electromagnetic using the electromagnetic induction effect, electrostatic or capacitive
and piezoelectric. Tiruthani (2008) and Yldiz (2009) described the mentioned noise
harvesting ways presenting their advantages and disadvantages. The capability of
electromagnetic devices to harvest the energy was explored and tested by a few authors. J Cho
(2005) shows the fact that the electromechanical coupling coefficient is crucial for the
performance of piezoelectric energy conversion devices.
Richard (2008) proposed a new electro-magnetic transducer for converting ambient kinetic
energy into useful electrical power.
The piezoelectric way of conversion was the most explored. Sodano (2004) develops
conversion techniques using MEMS (Micro-Electro-Mechanical-Systems). Horowitz (2005)
demonstrates the capability of the vibrating energy conversion into electrical energy, starting
from the same MEMS.
Stewart (2008) develops the physical acoustics of energy harvesting for piezoelectric
transducers.
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Tenghsien (2008) proposed the design, fabrication, and characterization of an acoustic wave
actuated microgenerator for power system applications in mobile phones. Jae-yun Lee (2009)
using a resonator having a piezoelectric membrane wall, obtained up to 14V c.c. for
elementary waves generated in the 100 Hz to 1400 Hz range. Chon and Lee obtained 0,05 –
1,65 V from environmental sound using acoustical cones for transforming the spherical waves
into plane ones. Acoustical energy was also used as an intermediary step in converting
thermal energy into electrical one. This is the case of Synko (2007) and Loh (2010).
In this research an electromagnetic transducer was used in order to evaluate the harvesting
efficiency for pure generated waves.
EXPERIMENTAL STAND AND TEST SETUP
The experimental stand (fig. 1) consists of: generating waves device (loudspeaker) (1), closed
acoustic pipe (320 mm diameter and 1480 mm length) (2); N121 Environmental Noise
Analyzer (3), memory card (4), microphone NOR 1225 (5) placed in front of the
electromagnetic transducer SAL 3050 (6), Data Acquisition System (DAQ) (7), PC (8),
booster (9) and HM507 oscilloscope (10).
Fig 1. Experimental stand. 1 – tone generator (loudspeaker), 2 – acoustic pipe; 3 – N121 noise analyzer;
4 – flash card; 5 - microphone; 6 – electromagnetic transducer; 7 – data acquisition device; 8 – PC; 9 – booster;
10 – oscilloscope.
The input pure sound wave generated by a trial NGH Tone Generator, amplified via booster
was furnished to the loudspeaker (1). The acoustic pipe ensures a plane wave displacement.
The output electrical signal is discharged to the DAQ system having as input impedance a 10
Ώ resistor in parallel with a 470 µF capacitor (Filip and all 2001),. The acquisition frequency
is software controlled in steps of: 10, 51, 124 and 249 Hz.
The main characteristics of the electromagnetic transducer are: resonance frequency 29 Hz,
power 120 W, electrical impedance 8 Ω, and maximum SPL 92 dB.
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15th International Conference on Experimental Mechanics
For the acoustic pipe were calculated: the resonance frequency, the acoustic and mechanical
impedance.
To calculate the pipe resonance frequency the equation for plane waves is used [14]:
fr =
c⋅n
4 ⋅ (l + 0,4 ⋅ d )
(1)
where: c is the sound velocity 343, 26 m s-1 for the 200C ambient temperature and
corresponding to 1,2041 kg m -3 air density; n - natural odd number; l – length of the pipe; d the pipe diameter.
In laboratory conditions (200C) the calculated resonance frequency was: 52,7 Hz for
corresponding sound speed. The calculated acoustic impedance and the mechanical
impedance of the pipe were also found to be: 413,006 N s m-3 and 38,162 N s m-1.
The pure sound waves are generated in the frequency range of 25 Hz up to 80 Hz using a 5 Hz
step. The reason of the range of the generated wave setup was to cover both resonance
frequencies (of the transducer and of the pipe) within an acceptable area. Increasing the
amplitude of the wave, the voltage and electric current were measured for each frequency
step. The sound pressure level (SPL) was also registered by an N121 analyzer.
EXPERIMENTAL RESULTS
The measurements carried out are detailed in voltage and electric current values for generated
waves depending upon the sound pressure level. Each measurement consists of an acquired
sample of about 2500 values with maximum acquisition frequency of 249 Hz. The registered
data were statistically analyzed, figures 2 and 3 showing the calculated mean values.
0.030
Electric current [A]
0.025
0.020
0.015
0.010
25
30
35
40
45
50
55
60
65
70
75
80
Hz
Hz
Hz
Hz
Hz
Hz
Hz
Hz
Hz
Hz
Hz
Hz
0.005
0.000
62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100
SPL [dB(A)]
Fig. 2. The voltage harvested for different sound waves frequencies.
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0.30
0.25
Voltage [V]
0.20
0.15
0.10
25 Hz
30 Hz
35 Hz
40 Hz
45 Hz
50 Hz
55 Hz
60 Hz
65 Hz
70 Hz
75 Hz
80 Hz
0.05
0.00
62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98100
SPL [dB(A)]
Fig. 3. The electric current harvested for different sound waves
frequencies.
The voltage and electric current distribution have the highest values measured for low
frequencies around to the resonance frequency of the transducer. The influence of the pipe
resonance frequency consists of maintaining high values of the sound conversion also for the
50 Hz and 55 Hz generated wave. Maximum mean values were 0,03105 A corresponding to
0,304914 V for 30 Hz wave frequency and 81,7 dB(A) sound pressure level (SPL). The
generated sound pressure level was limited by the transducer (inverted loudspeaker)
characteristics.
All measurements show the existence of an inferior limit of the sound pressure from which
the energy conversion begins. Increasing the sound pressure level, we can observe an
exponential distribution of the electrical signal harvested both in voltage and in electric
current.
THE EXPERIMENTAL RESULTS EVALUATION
The experimental results evaluation consists of identifying a conversion law to precisely
describe the energy recovered in terms of the harvested electrical energy values (in voltage
and electric current).
Having found a similar pattern of the acoustical into electrical energy conversion process for
all the tests carried out, the identification of a probability law was needed. Following the
regression of the experimental results, presented in table 1, we found that the best
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approximating distribution law for the tested electromagnetic transducer is of exponential
growth type, having the equation (fig. 4):
x
y = y0 + A ⋅ e t
(2)
where: y0 is the initial value (the origin); A and t the experimental regression coefficients and
x the generated sound pressure level.
Fig. 4. The exponential growth regression for the experimental results obtained for
harvested electric current and voltage.
For all fitted regressions the R square test results were up 0,9477 to 0,9895 and that fact
showing an acceptable conformity of the results: approximately 94% of the variations in the
response variable can be explained by the exponential growth distribution law (Nagelkerke
1991 and Balakrishnan 2011). The remaining 6% can be explained by unknown, lurking
variables or inherent variability.
Table 1. The exponential growth regression coefficients resulted for voltage harvested.
Frequency
[Hz]
25
30
35
40
45
50
55
60
65
70
75
80
A
t
coefficient coefficient
5.60 10 -10 3.90994
5.96 10 -9
4.65371
-10
5.26 10
4.19229
4.38 10 -11 3.87122
8.6110 -12
3.75171
4.07 10 -12 3.75604
5.55 10 -11 4.33263
9.06 10 -13 3.75927
4.65 10 -14 3.44742
2.23 10 -14
3.4334
-17
1.32 10
2.79337
4.80 10 -20 2.45399
R
square
0.99049
0.99126
0.99504
0.98062
0.97317
0.97412
0.96524
0.96322
0.95397
0.94148
0.962128
0.95408
The next step of the results evaluation consists of explaining the constant terms of the
equation (1). Considering the A coefficient variation depending on the frequency of the
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generated wave a normal (Gauss) distribution resulted, with the 29,68 mean value near to the
resonant frequency of the transducer (29 Hz) (fig. 5).
Fig. 5. The A coefficient Gauss regression fit for experimental results.
Analyzing in the same way the t coefficient it was appreciated that it represents the rate of
growth of the harvested signal in voltage or electric current, depending on the generated wave
frequency (fig. 6). In the case of t coefficient, a second peak was observed around the pipe
resonant frequency too.
Fig. 6. The t coefficient depending on the wave frequency and its polynomial regression.
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15th International Conference on Experimental Mechanics
CONCLUSIONS
The tests carried out show that the conversion of sound energy into electricity is possible even by
using common loudspeakers as electromagnetic transducers.
The existence of a lower pressure sound limit of the conversion yields from the experimental
results. The regression of the experimental results enabled the identification of the distribution
law which states the predictability of the conversion process as an exponential growth type,
sustained also by R square test results values (in all cases over 94%).
Regarding the coefficients of the identified distribution law, these explain the conversion
conditions: around the transducer resonance frequency the harvesting efficiency is most
favorable, corresponding to an optimum rate of conversion.
ACKNOWLEDGEMENT
This work was supported by CNCSIS –UEFISCDI, project number PNII – IDEI code
2531/2008.
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