# Reflection, Refraction and Diffraction

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```					   Introduction to Acoustics

Reflection, Refraction and
Diffraction
University of Salford
Acoustics Audio and Video Group
Learning Outcomes
• Define reflection, refraction and diffraction
• Explain how refraction and diffraction occur
• Calculate angles of reflection and refraction

24-Mar-04          I2A: Reflection & Refraction   2
Reflection and transmission
• There is always some
oco    1c1
reflected and some
B
C                       transmitted sound
B 1c1   o co

A                                    A 1c1   o co

• the impedance seen by
the wave at the boundary
controls the amount of
reflected sound
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Absorption
p2
• From I  we find                     • Reflection coefficient:
ρc
Ir B2
 2
A2                    Ii A
• Incident:    Ii 
2  0co
• Absorption coefficient:
B2                          Ir   B2
• Reflected:   Ir                            1  1 2
2  0co                       Ii   A

24-Mar-04              I2A: Reflection & Refraction                4
Refraction: c2<c1
Incident
Wave                                      Reflected
Wave

i r
C1

C2<C1

t            Transmitted
Wave

24-Mar-04              I2A: Reflection & Refraction               5
Refraction: c2>c1
Incident                                  Reflected
Wave                                      Wave

i r
C1

C2>C1

t             Transmitted
Wave

24-Mar-04              I2A: Reflection & Refraction               6
Refraction equation
Incident                Reflected                  • Snell’s law:
Wave                    Wave
c1        c2

C1
i r                                      sin  i  sin  t 

C2>C1                                         • Two cases:
t      Transmitted
Wave
• c2<c1: always refraction
• c2>c1: refraction only for
small θi

24-Mar-04                          I2A: Reflection & Refraction                 7
Refraction in the atmosphere

24-Mar-04            I2A: Reflection & Refraction   8
Diffraction

24-Mar-04   I2A: Reflection & Refraction   9
Conclusions
• At a boundary, sound is both reflected and
transmitted - controlled by impedance
• Transmitted sound may be refracted - controlled
by sound speeds in the two regions
• Sound meeting an object may be diffracted -
controlled by size relative to wavelength

24-Mar-04         I2A: Reflection & Refraction   10

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