Based of acoustics. Lesson 1

1.

LESSON 1
BASED OF ACOUSTICS
The chapter of physics which studies the
generation, propagation and interaction of
elastic waves into elastic media is called
acoustics, and these waves are acoustic waves.

2.

Waves in elastic media
• The acoustic waves propagate only in elastic media.
The particles of the medium are displaced due to the
impact of external forces and return to their
reference position due to elastic properties.

3.

TYPES of GUIDED WAVES
The wave are longitudinal, if the direction of particles oscillation coincides with the
direction of wave propagation.
The waves are called transverse, if the direction of particles oscillation is perpendicular
to direction of wave propagation.

4.

TYPES of GUIDED WAVES
the plate wave, which propagates in a thin plate
A surface wave is a wave that propagates only on the surface of object

5.

Waves in elastic media
Antisymmetric mode

6.

7.

CLASSIFICATION OF ELASTIC WAVES
Infrasound
• hypersound
Sound
16 Hz
Hz
16
Ultrasound
20 kHz
hypersound
1 GHz
10 000 GHz

8.

WAVE VELOCITY INTO LIQUID
c
1
ρ- the density of the liquid (kilogram over stere)
β- the coefficient of adiabatic compressibility
1 V
P V
V
-change of the volume
V
P - change of the pressure

9.

WAVE VELOCITY INTO SOLID
E
C
P
E is Young modulus
P is the pressure
Wavelength:
f is frequency
λ =c/f

10.

Ultrasonic attenuation
The attenuation due to absorption of elastic wave by
environment, i.e. transformation of acoustic energy into other
forms of energy (heat energy).
A A0 e x
I I 0 e 2 x
The absorption coefficient in liquid is proportional to the square
of the frequency and viscosity
2 f 2
3 c 3

11.

REFRACTION and REFLECTION
of the longitudinal wave
• angle of incidence equals angle of reflection

12.

FIRST CRITICAL ANGLE

13.

REFLECTION and TRANSMISSION
COEFFICIENTS
1
Z1 1с1
Z 2 2 с2

14.

IMPEDANCE
• Z=ρc
ρ – density of medium
c – velocity of elastic wave into medium
Acoustic properties of some mediums
Meduim
velocity,
m/s
impedance,
кg/(s m²)
Blood
Adipose
Kidney
Liver
bone
water
1570
1450
1561
1549
4080
1480
1,61
1,38
1,62
1,65
7,80
1,48
absorption factor,
db/sm,
for f = 1 МHz
0,13
0,63
1,0
0,94
13
0,0022

15.

ACOUSTIC FIELD
• Acoustic pressure
P= ρcAω =ZVm
V sub m is the maximum amplitude of the particle velocity, ω circular frequency, A - the maximum amplitude of displacement
of particles, z is impedance.
2
W
P
• Intensity of acoustic wave I St 2 с
W – energy of acoustic wave, S – surface square exposed to
acoustic wave, t – time.
• Acoustic wave: S (t ) A sin( t kx 0 )
ω – cyclic frequency of oscillations; х – the position of a particle
in the direction of wave propagation; k– wave number; φ0 - the
epoch angle or the initial phase.
k = 2π/λ

16.

ACOUSTIC FIELD
• The amplitude of vibrational velocity:
dA
V (t )
A cos t V cos t
0
m
dt
Vm is the maximum amplitude of vibrational velocity:
Vm = ωA,
A - the maximum displacement of the particle relative to the
equilibrium position (the amplitude); ω – cyclic frequency of
oscillations;
• The acceleration amplitude:
d 2 A dVm
B
A 2 sin t
0
dt
dt 2

17.

DESIGN OF ULTRASONIC SENSOR
The design of the acoustic sensor
1 – piezo ceramics;
2 - damper (to reduce the duration of the
vibrations on impact (shot)), (material with
high absorption);
3 - protector (to protect the piezoelectric
element), (epoxy resin);
4 - contact lubricant layer;
5 - testing object;
6 - transducer body
7 – input cable
2d
d – the thickness of the piezoelectric ceramic

18.

ACOUSTIC FIELD OF ULTRASONIC
SENSOR
А
sin a
D
For a circular plate A = 1,22
for a square plate A = 1,
Z0 = d²/4λ
Fresnel zone
Fraunhofer zone
The interval from the radiator to Z0 called the near area, or Fresnel zone. The area
where Z> Z0, called the far zone, or Fraunhofer zone.
Z0=d²/4λ

19.

ACOUSTIC FIELD OF ULTRASONIC
SENSOR
Z0

20.

DIRECTIONAL CHARACTERISTIC
of single probe
The pattern is the
dependence of the
intensity of the ultrasonic
wave in the far zone from
the viewing angles in the
space

21.

• Thank you