This document discusses acoustics and ultrasonics. It defines key terms like reverberation, absorption coefficient, and Sabine's formula. It describes how sound and ultrasound waves propagate and are produced. Common applications of ultrasound like non-destructive testing and sensors are explained. Production methods for ultrasound using piezoelectric and magnetostriction effects are summarized.

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Open Elective - Engineering Physics
Semester 2
Academic year 2021-21
G. S. Mandal’s
Maharashtra Institute of Technology, Aurangabad
(An Autonomous institute)
1/25/2022

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Unit II: Acoustics, Ultrasonic
•Acoustic terminology and definitions
•Reverberation and reverberation time
•Absorption coefficient
•Acoustic Wave Equation and its Basic Physical Measures
•Sabine’s formula (derivation not necessary) acoustics factor in architectural design.

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The facts and figures
•Sound propagates through air as a longitudinal wave
•Speed of sound:
343 m/s @ 20°C (331 m/s @ 0°C)
About 5 seconds per 1.61 km
•Human Audible wavelengths: 1.7cm to 17m
•Human Audible Frequency sensitivity roughly 20 Hz – 20kHz

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London’s St Paul’s Cathedral
Mutter a little something into the gallery wall and it can be heard on
the other side of the 33m diameter dome.
Just be careful what you say…

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The Hungarian Cave Theatre
Sound resonates within their
solid walls in an interplay with
light and shadow.

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Brunel Museum, UK
A pedestrian tunnel network beneath London’s Thames river

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‘Sound mirrors’ UK. These concrete forms, ranging from 20 to 200 feet
wide, were constructed in the 1920s as early warning devices for
approaching enemy planes.

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Forest megaphones, Estonia
To harness the sounds of the forest, planted amongst the trees

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Acoustics is the interdisciplinary science that deals with the study of
all mechanical waves in gases, liquids, and solids including topics
such as vibration, sound, ultrasound and infrasound.
Architectural Acoustics is a branch of acoustical engineering is the
science and engineering of achieving a good sound within a building.
Sound production, transmission, and effects so that good sound within
a building can be achieved is called as architectural acoustics.

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Echo versus Reverberation
An echo is a sound wave that has been reflected with sufficient
magnitude and delay to be detectable as a wave distinct from
that which was directly transmitted.
Reverberation is persistence of sound as a result of repeated
reflection or scattering after the sound source has stopped. So
while an echo is a distinct sound, reverberated sounds are
difficult to hear clearly because the reflections keep repeating.
The echo can not be heard if the distance between the sound
source and the reflecting surface is less than 17 metres
because the time between hearing the main sound and its
echo will be less than 0.1 of a second, and the human ear can
not distinguish between the two successive sounds.

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Direct Sound and Reflections

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Reverberation:
The persistence or prolongation of sound in a hall even though the sound source is
stopped.
Collection of reflected sounds from the surfaces in an enclosure
It is a desirable property of auditoriums as it helps to overcome the drop-off of
sound intensity in the enclosure.
If it is excessive, it makes the sounds run together with loss of articulation.
The sound becomes muddy, garbled.
To quantitatively characterize the reverberation, the parameter called the
reverberation time is used.

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The sound absorption coefficient of a surface is defined as the
ratio of the sound energy absorbed by the surface to that falling
on it.
Absorption coefficient of materials vary at the different
frequency.
 Absorption of sound by materials is due to the porosity,
compressibility and elasticity. The materials absorb sound when
the sound waves are dissipated into heat by friction in the narrow
pores.

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V is the volume of the room and Se the effective area
1 1 2 2 3 3 ...
surface area for a type of surface
absorption coefficient for this surface
e
i
i
S a S a S a S
S
a
= + + +
V/Se in m, RT60 in seconds
Sabine’s formula: Estimating the reverberation time
People or seats also can be given effective areas.

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perforated and slotted
softwood, sandwich
panels (fabrics
coverage with glass
wool, rock wool, or
foam), and different
densities of foam.

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 Conditions to provide good listening in a hall for speech and
music
1. Access to the direct sound for all people in the hall
2. To limit background noise level in the hall
3. To set the reverberation time appropriately in the room
4. To avoid or eliminate echo.
5. No focusing of sound in any part of the hall.
6. Sound proof walls to avoid the external noise in the hall.

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Unit II: Ultrasonic
•Properties
•Production of ultrasonic waves by piezo-electric method
•Production of ultrasonic waves magneto-striction method
•Engineering applications of ultrasonic waves: sensors, acoustic grating – Non
Destructive Testing – pulse echo system
https://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html

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Wit Aniwat

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The facts and figures
Infrasound
Ultrasound
Audible sound
0 - 20
20 – 20,000
> 20,000 to gigahertz
Earth quake
Speech, music
Bat, Quartz crystal
•Subsonic: speed lesser than that of sound. A speed of around 0.8 Mach
•Supersonic: speed greater than that of sound. A speed of around 2-3 Mach
•Hypersonic: speed greater than that of sound. A speed more than 5 Mach
Mach: speed of sound. If an aircraft is travelling at Mach 1, it is travelling at the speed of sound

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A visible pattern of sound waves. The focusing effect
of an acoustical lens on sound waves issuing from
the horn at extreme left. Wave pattern is produced
by a scanning technique.
Bell Telephone Laboratories photograph, from the
book The First Book of Sound: A Basic Guide to the
Science of Acoustics by David C. Knight, Franklin
Watts, Inc. New York (1960). p. 80.

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Properties of ultrasonic waves
•Longitudinal waves that produce alternate compressions and rarefactions.
•Vibrate at a frequency greater than the 20 KHz
•Smaller wavelengths. As a result, their penetrating power is high.
•Need medium to travel, they cannot travel through vacuum.
•Travel at the speed of sound in the medium. Higher velocities in a denser medium.
•In a homogeneous medium, they travel at a constant velocity.
•In low viscosity liquids, ultrasonic waves produce vibrations.
•Undergo reflection, refraction and absorption.
•Can be transmitted over a large distance without much loss of energy.
•Produce intense heat when they are passed through objects.

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Piezoelectric effect
The piezoelectric effect was discovered in 1880 by two French physicists, brothers
Pierre and Paul-Jacques Curie
Naturally occurring: quartz, tourmaline, and Rochelle salt (potassium sodium
tartrate).
Synthesized: Potassium niobate (KNbO3) and lead zirconate titanate (PZT
(Pb[ZrxTi1−x]O3 with 0 ≤ x ≤ 1))
Exhibit a more pronounced piezoelectric effect.
When a crystals like (calcite or quartz) under goes mechanical deformation along
the mechanical axis then electric potential difference is produced along the
electrical axis perpendicular to mechanical axis. This phenomenon is known as
piezoelectric effect.

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Principle: When the electric field is applied across the crystal its dimensions changes
and when alternating PD is applied across crystal then the crystal sets into elastic
vibrations
Working: A piezo-electric crystal like quartz is placed between two plates as shown in
figure. A suitable oscillator is connected across it. The electric oscillations along the
electric axis produce mechanical vibrations along the mechanical axis. The frequency
of oscillator is increased. At a particular frequency of oscillator, the oscillator
frequency becomes equal to natural frequency of vibration of crystal. Then the crystal
sets into resonance vibration and ultrasonic waves are produced.
S
Electric
oscillating
circuit
piezo-electric
crystal

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C
L
F
2
2
1
π
=
The frequency of this oscillating electricity
ρ
Y
t
k
F
2
=
Natural frequency of crystal
t = Thickness of crystal
Y = Young's Modulus
ρ = Density
k = 1, 2, 3, ... Integer
C = capacitance
S = switch
L, L1 , L2 = inductance

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Advantages:
Ultrasonic frequencies as high as 500MHz can be generated.
The output power is very high. It is not affected by temperature
humidity.
We get a stable and constant frequency of ultrasonic waves.
Disadvantages:
Cutting and shaping the crystal is quite complex
unwanted vibrations may creep in

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Production of ultrasonic waves magneto-striction method
https://youtu.be/MmopHPp4dQA

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Production of ultrasonic waves magneto-striction method
C
L
F
2
2
1
π
=
The frequency of this oscillating electricity
ρ
Y
l
k
F
2
=
Natural frequency of crystal

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Advantages:
1. Magnetostrictive materials are easily available and inexpensive.
2. Oscillatory circuit is simple to construct.
3. Large output power can be generated.
Disadvantages:
1. It can produce frequencies upto 3 MHz only.
2. It is not possible to get a constant single frequency, because rod depends
on temperature and the degree of magnetization.
3. As the frequency is inversely proportional to the length of the vibrating
rod, to increase the frequency, the length of the rod should be decreased
which is practically impossible.

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SOund NAvigation And Ranging (SONAR)
Developed during WW II
•High frequency, high energy ultrasonic waves can
travel over a long distance
•Sound pulses emitted reflected off surface
•Like radar and LIDER time of flight is measured to
determine distance
•Early sonar gave only distance and direction to
target
•Modern sonar used for mapping
•Navigation, submarine detection, depth detection,
commercial fishing, diving safety and
communication at sea
2
*t
v
d =

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Non destructive testing
To detect cracks and defects in parts and materials. It can also be used to determine a
material’s thickness, such as measuring the wall thickness of a pipe.
Ultrasonic waves are passed through an object or material using short pulse waves
with frequencies ranging from 0.1-15 MHz, although frequencies up to 50 MHz can be
used.
Pulse echo testing uses the same transducer emits and receives the signal. This
method uses echo signals at an interface, such as the back of the object or an
imperfection, to reflect the waves back to the probe.
Through-transmission testing uses an emitter to send the ultrasound waves from one
surface and a separate receiver to receive the sound energy that has reached the
opposite side of the object. Imperfections in the material reduce the amount of sound
that is received, allowing the location of flaws to be detected.

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LEFT: A probe sends a sound wave into a test material. There are two indications, one from
the initial pulse of the probe, and the second due to the back wall echo.
RIGHT: A defect creates a third indication and simultaneously reduces the amplitude of the
back wall indication.

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Ideal for detecting flaws and defects without damaging
the object or material being tested.
To check for corrosion or for growth of known flaws, and
thus potentially prevent to a failure of a part, component
or entire asset.
Used in a wide range of industries including aerospace,
automotive, construction, medical, metallurgy, and
manufacturing.

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Ultrasonic distance, level, and proximity sensors are commonly used
with microcontroller platforms like Raspberry Pi, Arduino, Beagle
Board, and more.
Ultrasonic sensors transmit sound waves toward a target and will
determine its distance by measuring the time it took for the
reflected waves to return to the receiver.
This sensor is an electronic device that will measure the distance of
a target by transmitting ultrasonic sound waves, and then will
convert the reflected sound into an electrical signal.