PHYSICS
UNIT - 1
Chapter 2 : ULTRASONICS
Sound waves having frequency above the audible range, i.e. greater than 20,000 Hz are called
ultrasonics or ultrasonic waves (USW)
Production of Ultrasonic Waves
a) Magnetostriction Method
When a rod of ferromagnetic material such as iron or nickel is kept in a magnetic field parallel to
its length, the rod suffers a change in its length. This phenomenon is called magnetostriction.
The change in length of the rod is independent of the direction of the applied magnetic field
(whether parallel of anti-parallel) and depends only on the magnitude of the field and the
material of the rod.
Use of magnetostriction to generate USW
If a ferromagnetic rod is placed in an alternating magnetic field of frequency 𝑓, the rod will
change in length twice in each cycle resulting in vibrations of frequency 2𝑓. If 𝑓 is greater than
10,000Hz, the rod will vibrate at a frequency greater than 20,000Hz, thus producing USW.
Further, if the amplitude of the USW produced is to be large, the rod must be made to vibrate at
is natural frequency so that resonance will occur and the amplitude will be large.
The natural frequency 𝑓𝑛 of a rod of length 𝐿 made of material having density 𝜌 and Young’s
modulus 𝑌is given by the formula
1 𝑌
𝑓𝑛 = 2𝐿 √𝜌
Magnetostriction Oscillator
The circuit is
basically a Colpitt’s
oscillator which
generates a.c.
signals of any
desired frequency.
When the circuit is
switched on, the
tank circuit,
comprising of
inductor L and
variable capacitors
C3 and C4, generates
small a.c. signals. These are amplified by the transistor T which is biased to work as a positive
feedback amplifier. The current level of the signal is also amplified and it is then applied to a
solenoid inside which a ferromagnetic rod is placed. The frequency of the circuit is adjusted using
variable capacitors C3 and C4 to be exactly equal to half the natural frequency of the rod (𝑓𝑛 /2).
The rod will then vibrate at its natural frequency 𝑓𝑛 thereby producing USW of large amplitude.
Ultrasonics Prof. Harison Cota, Don Bosco College of Engineering, Fatorda Page 1
, Using this method USW of frequency upto about 100 KHz can be generated. (The length of the
rod puts a limit on the maximum frequency of USW that can be generated by this method.)
b) Piezoelectric Method
When certain crystals (called piezoelectric crystals) such as quartz, tourmaline, Rock salt,
Rochelle’s salt, etc. are subjected to mechanical pressure along a certain direction called the
mechanical axis, an electric potential is developed across the crystal along a perpendicular
direction called the electrical axis. This phenomenon is called direct piezoelectric effect.
The polarity of the electric potential developed depends on whether the applied pressure is
compressive or elongative.
Conversely, if an electric potential is applied to a piezoelectric crystal along its electrical axis, the
crystal will either expand or contract along the mechanical axis, depending on the polarity of the
applied potential. This phenomenon is called inverse piezoelectric effect.
Use of Inverse piezoelectric effect to generate USW
If an alternating electric potential of frequency 𝑓 is applied to a piezoelectric crystal along its
electrical axis, the crystal will expand and contract once in each cycle thus producing vibrations
of frequency 𝑓. If 𝑓 is greater than 20,000Hz, the crystal will also vibrate at the same frequency
and thus produce USW. Further, if the frequency 𝑓 matches with the natural frequency 𝑓𝑛 of the
crystal, the crystal will vibrate at resonance and produce USW of large amplitude.
The natural frequency 𝑓𝑛 of a crystal of thickness 𝑡 made of material having density 𝜌 and
Young’s modulus 𝑌is given by the formula
1 𝑌
𝑓𝑛 = 2𝑡 √𝜌
Piezoelectric Oscillator
The circuit is basically a
Hartley’s oscillator
which generates a.c.
signals of any desired
frequency. When the
circuit is switched on,
the tank circuit,
comprising of inductors
L1 and L2 and variable
capacitor C3, generates
small a.c. signals. These
are amplified by the
transistor T which is
biased to work as a
positive feedback
amplifier. The amplified a.c. signal is then applied to a piezoelectric crystal C which is kept
between two metal plates A and B. The frequency of the circuit is adjusted using variable
capacitor C3 to be exactly equal to the natural frequency of the crystal (𝑓𝑛 ). The crystal will then
vibrate at its natural frequency 𝑓𝑛 thereby producing USW of large amplitude.
Using this method USW of frequency upto about 500MHz can be generated.
Ultrasonics Prof. Harison Cota, Don Bosco College of Engineering, Fatorda Page 2
UNIT - 1
Chapter 2 : ULTRASONICS
Sound waves having frequency above the audible range, i.e. greater than 20,000 Hz are called
ultrasonics or ultrasonic waves (USW)
Production of Ultrasonic Waves
a) Magnetostriction Method
When a rod of ferromagnetic material such as iron or nickel is kept in a magnetic field parallel to
its length, the rod suffers a change in its length. This phenomenon is called magnetostriction.
The change in length of the rod is independent of the direction of the applied magnetic field
(whether parallel of anti-parallel) and depends only on the magnitude of the field and the
material of the rod.
Use of magnetostriction to generate USW
If a ferromagnetic rod is placed in an alternating magnetic field of frequency 𝑓, the rod will
change in length twice in each cycle resulting in vibrations of frequency 2𝑓. If 𝑓 is greater than
10,000Hz, the rod will vibrate at a frequency greater than 20,000Hz, thus producing USW.
Further, if the amplitude of the USW produced is to be large, the rod must be made to vibrate at
is natural frequency so that resonance will occur and the amplitude will be large.
The natural frequency 𝑓𝑛 of a rod of length 𝐿 made of material having density 𝜌 and Young’s
modulus 𝑌is given by the formula
1 𝑌
𝑓𝑛 = 2𝐿 √𝜌
Magnetostriction Oscillator
The circuit is
basically a Colpitt’s
oscillator which
generates a.c.
signals of any
desired frequency.
When the circuit is
switched on, the
tank circuit,
comprising of
inductor L and
variable capacitors
C3 and C4, generates
small a.c. signals. These are amplified by the transistor T which is biased to work as a positive
feedback amplifier. The current level of the signal is also amplified and it is then applied to a
solenoid inside which a ferromagnetic rod is placed. The frequency of the circuit is adjusted using
variable capacitors C3 and C4 to be exactly equal to half the natural frequency of the rod (𝑓𝑛 /2).
The rod will then vibrate at its natural frequency 𝑓𝑛 thereby producing USW of large amplitude.
Ultrasonics Prof. Harison Cota, Don Bosco College of Engineering, Fatorda Page 1
, Using this method USW of frequency upto about 100 KHz can be generated. (The length of the
rod puts a limit on the maximum frequency of USW that can be generated by this method.)
b) Piezoelectric Method
When certain crystals (called piezoelectric crystals) such as quartz, tourmaline, Rock salt,
Rochelle’s salt, etc. are subjected to mechanical pressure along a certain direction called the
mechanical axis, an electric potential is developed across the crystal along a perpendicular
direction called the electrical axis. This phenomenon is called direct piezoelectric effect.
The polarity of the electric potential developed depends on whether the applied pressure is
compressive or elongative.
Conversely, if an electric potential is applied to a piezoelectric crystal along its electrical axis, the
crystal will either expand or contract along the mechanical axis, depending on the polarity of the
applied potential. This phenomenon is called inverse piezoelectric effect.
Use of Inverse piezoelectric effect to generate USW
If an alternating electric potential of frequency 𝑓 is applied to a piezoelectric crystal along its
electrical axis, the crystal will expand and contract once in each cycle thus producing vibrations
of frequency 𝑓. If 𝑓 is greater than 20,000Hz, the crystal will also vibrate at the same frequency
and thus produce USW. Further, if the frequency 𝑓 matches with the natural frequency 𝑓𝑛 of the
crystal, the crystal will vibrate at resonance and produce USW of large amplitude.
The natural frequency 𝑓𝑛 of a crystal of thickness 𝑡 made of material having density 𝜌 and
Young’s modulus 𝑌is given by the formula
1 𝑌
𝑓𝑛 = 2𝑡 √𝜌
Piezoelectric Oscillator
The circuit is basically a
Hartley’s oscillator
which generates a.c.
signals of any desired
frequency. When the
circuit is switched on,
the tank circuit,
comprising of inductors
L1 and L2 and variable
capacitor C3, generates
small a.c. signals. These
are amplified by the
transistor T which is
biased to work as a
positive feedback
amplifier. The amplified a.c. signal is then applied to a piezoelectric crystal C which is kept
between two metal plates A and B. The frequency of the circuit is adjusted using variable
capacitor C3 to be exactly equal to the natural frequency of the crystal (𝑓𝑛 ). The crystal will then
vibrate at its natural frequency 𝑓𝑛 thereby producing USW of large amplitude.
Using this method USW of frequency upto about 500MHz can be generated.
Ultrasonics Prof. Harison Cota, Don Bosco College of Engineering, Fatorda Page 2
