“There is definitely energy contained in that sound,” says David Cohen-Tanugi, vice
president of the MIT Energy Club and a John S. Hennessy Fellow in MIT’s Materials Science and
Engineering department. “But the density of the energy is very low, and there is no way to capture it
all. You’d have to have obscenely loud, continuous noise for harvesting to be worthwhile.” Sound
energy is the energy produced by sound vibrations as they travel through a specific medium.
Speakers use electricity to generate sound waves and now by using zinc oxide, the main ingredient of
calamine lotion, to do the reverse - convert sound waves into electricity. Piezoelectrics are materials
capable of turning mechanical energy into electricity, and can be substances as simple as cane sugar,
bones, or quartz. Much research in this field has been focused on transforming the movement of a
person running, or even the impact of a bullet, into a small electrical current, but although these
advanced applications are not yet available in consumer products, scientists have been using
piezoelectric materials in environmental sensors and speakers for years. Piezoelectrics create an
electrical charge under stress, and thus zinc oxide, the main ingredient of calamine lotion, was bent
into a field of nanowires sandwiched between two electrodes. The researchers subjected the
sandwich to sound waves of 100 decibels which produced an electrical current of about 50 millivolts.
Passing trains and subways aren’t only loud, but their surroundings rattle and vibrate as they pass,
and part of the thrill of a rock concert is feeling the whole auditorium shake. Piezo material
converts mechanical strain into electric energy this property of piezo material could be used to
make a device which would be able to sustainably convert the sound energy to electric energy as
piezo materials convert sound energy to electric energy. Transducer is also used to convert
Mechanical energy to electric energy i.e.it can convert sound energy to electric energy the simple
e.g. of use of transducer to convert sound to electric and vice versa is in speakers, headset also it
could be converted into electric energy.
CONCEPT
Suppose we create a very thin curtain like diphagram which will get fluctuated by the
oscillation and pressure created by the sound wave and a conductor will be attached to it which will
be placed between magnetic bars these fluctuation in the curtain will create a movement in conductor
which will affect the magnetic field of the magnet this will generate motional emf and will generate
voltage across it. As per faradays law generated emf is given by Generated voltage = Emf =velocity
of conductor X magnetic field X length of conductor thus the oscillation created by the sound wave
could be converted into electricity and as the frequency is high the movement will be fast due to it
we will get appreciable amount of electric energy.
Piezo electric materials are transducers its crystals could convert mechanical strain to
electricity, the crystals are formed naturally e.g. quartz, bone, DNA whereas artificially ZnO, lithium
niobatet Lead Metaniobate the sound energy could be converted into electricity using piezo electric
material. Let us see the properties of piezo electric material. Certain single crystal materials exhibit
the following phenomenon: when the crystal is mechanically strained, (here sound energy) or when
the crystal is deformed by the application of an external stress, electric charges appear on the crystal
surfaces; and when the direction of the strain reverses, the polarity of the electric charge is reversed.
This is called the direct piezo electric effect, and the crystals that exhibit it are classed as
piezoelectric crystal.
First let’s understand concept to produce current. When coil of aluminum comes in between two
magnets opposite polarity say P N pole, and some force is applied on coil to rotate on its axis it’ll
produce magnetic field and due to electromagnet flux charge/current flows. Its shows that to produce current, force or pressure are required or can say force is a main key
to produce current. This paper suggesting to utilizing sound vibration as an applied force to produce
current. Piezo electric material has ability to convert mechanical stress into electricity.
History of Piezoelectricity
The first scientific publication describing the phenomenon, later termed as piezoelectricity,
appeared in 1880. It was co-authored by Pierre and Jacques Curie, who were conducting a variety of
experiments on a range of crystals at the time. In those experiments, they cataloged a number of
crystals, such as tourmaline, quartz, topaz, cane sugar and Rochelle salt that displayed surface
charges when they were mechanically stressed.
Without any external stress Centers of charges coincide, charges are reciprocally cancelled
and formed electrical neutral unit cell.
Applied external stress Internal structure is deformed, separation of charge centers and dipoles are generated Poles inside material are mutually Cancelled and charge occurs on surface creates
polarization on the surface of material.
Mathematical modeling
Piezoelectricity is the combination of the materials electrical behavior:
D =ε E, And Hook’s laws = s T where D: electric displacement, ε: permittivity,
E: electric field strength, S: strain, s: compliance, T: stress
The coupled strain-voltage equation
S = sET + dtE converse piezoelectric effect
D= ε
T
E + dT direct piezoelectric effect
d ij , k = ∂Sij /∂Ek piezoelectric coefficient
When a poled piezoelectric material mechanically strained it became electrically polarized,
producing an electric charge on the surface of the material.
Example to explain concept
A human walking, for example is a low frequency event that can be captured in the form of
stress on a piezoelectric platform. A person walking across a room may complete 1-2 steps per
second. Each step introduces a stress in the floor of the room, and the frequency of that alternating
stress would be about 1-2 vibrations per second, and this waste vibration energy can be harvested.
Vibrations per second are a measure of frequency, often stated in Hertz (Hz). One vibration per
second is equal to 1 Hz. Two vibrations per second are equal to 2 Hz. The common United States
household’s electrical circuit carries electricity oscillating at 60 cycles per second, or 60 Hz, which is
evidenced by the low frequency buzz of an electric shaver.
To determine how much energy piezoelectric can produce, a few metrics need to be defined
that will be useful for the discussion. The first metric is power. Power is defined in Watts (W), which
is defined as units of energy per second. Power is an indication of how quickly energy can be
delivered. A powerful air conditioner can cool a room quickly, whereas a weakly powered heater may
require a long time to heat a room. Other examples include a solar panel which may be rated at 200 W
in peak sunlight at noon in the middle of a summer. The second metric is energy. Energy is defined in
many units. In standard units, energy is stated in Joules (J), but for electricity it is often most useful to
define energy in terms of watt-hours (W-h), for example, how many watts are produced in an hour. In
the examples above, the solar panel would produce 200 W-h from noon to 1 PM. The natural gas
power plant would produce 200 million watt-hours (200 megawatt-hours, or MWh) in the same hour.
Again, the two examples are different by a factor of one million.
One study used lead zirconate titanate (PZT) wafers and flexible, multilayer polyvinylidene
fluoride (PVDF) films inside shoes to convert mechanical walking energy into usable electrical
energy [1], [2]. This system has been proposed for mobile computing and was ultimately able to provide
continuously 1.3 mW at 3 V when walking at a rate of 0.8 Hz.
METHOD 1
Suppose we create a very thin curtain like diphagram which will get fluctuated by the
oscillation and pressure created by the sound wave and a conductor will be attached to it which will
be placed between magnetic bars these fluctuation in the curtain will create a movement in conductor
which will affect the magnetic field of the magnet this will generate motional emf and will generate
voltage across it. As per faradays law generated emf is given by Generated voltage = Emf =velocity
of conductor X magnetic field X length of conductor Thus the oscillation created by the sound wave
could be converted into electricity and as the frequency is high the movement will be fast due to it
we will get appreciable amount of electric energy. It would work similar as the working of turbine
this type of device could be made but its limitation will be that it will be efficient only in the place where
high decibel of sound is available, for example nuclear power plant, industries using huge and noisy
machines.
METHOD 2
In this method we could convert sound energy to heat energy as sound wave travel by
oscillating the particles of the medium so when sound energy travel through the medium it will
disturbs the particle of the medium these disturbance created by sound will be used to convert it into
heat energy as when the particles of the medium will be pushed by the sound wave it will collides
with adjacent particle of the medium this collision will result in production of heat energy the
production of heat energy will be more in the denser medium so for more heat production we will
need a material with very high density. This heat energy will be converted into electricity.
METHOD 3
Converting sound energy to electricity by piezo electric material (piezo electric materials are
the crystal which convert mechanical strain to electric energy) device could be made using piezo
electric material which will collect the sound wave which are travelling near it and that sound wave
will be used to cause a strain due to pressure created by its oscillation in the piezo crystal and that
will create the disturbance in its atoms resulting in the flow of electric charge on the surface of the
crystal thus sound energy could be converted into electricity as the piezo electric material convert
mechanical strain to electric energy. And thus this sound energy could be used to perform various
tasks by converting it into useful electric energy
conclusion
• As sound has enormous amount energy with it, it could be used by converting it into electric
energy for various purposes. Sound energy is a mechanical energy so according to law of
thermodynamics mechanical energy could be converted into electric energy.
• Sound energy could be converted by different methods:
Method 1- By creating apparatus using curtain (diphagram) magnet and conductor.
Methods 2- By converting Sound energy to heat energy and then heat energy to electric energy.
Method 3- By using transducers such as piezo electric material which converts mechanical strain
to electric energy and vice4 versa.
