SOUND and its properties

2.  Sound is a mechanical wave that is
an oscillation of pressure transmitted through
a solid, liquid, or gas, composed
of frequencies within the range of hearing
and of a level sufficiently strong to be heard,
or the sensation stimulated in organs of
hearing by such vibrations.

3.  Sound is a longitudinal wave propagating in
an elastic medium
 The sound wave is pressure /medium motion
oscillations
 Pressure and particle velocity are in phase in
the propagating sound wave

4.  Sound is a sequence of
waves of pressure which
propagates through
compressible media such
as air or water. (Sound can
propagate through solids
as well, but there are
additional modes of
propagation).
 During their propagation,
waves can be reflected,
refracted, or attenuated by
the medium.

5. The behavior of sound propagation is generally affected by
three things:
•A relationship between density and pressure. This
relationship, affected by temperature, determines the speed
of sound within the medium.
•The propagation is also affected by the motion of the
medium itself. For example, sound moving through wind.
Independent of the motion of sound through the medium, if
the medium is moving, the sound is further transported.
•The viscosity of the medium also affects the motion of
sound waves. It determines the rate at which sound is
attenuated. For many media, such as air or water, attenuation
due to viscosity is negligible.
•However When sound is moving through a medium that
does not have constant physical properties, it may be
refracted (either dispersed or focused).

6.  Sound reflection depends on the
difference between the characteristic
impedance of the medium on both sides
of the boundary.
 Large impedance differences=large
fraction of sound energy is reflected, small
fraction is transmitted

7.  Reflection from
a soft-hard
boundary
 Reflection from
a hard-soft
boundary

8.  An upward oriented
velocity gradient
(lower ground
temperatures)
produces downward
deflection:
 A downward oriented
gradient (higher
ground temperatures)
produces upward
deflection

9.  Objects interact with the sound wave in the
following ways:
 Objects that are smaller than 1/6th
wavelength are ’transparent’ to sound
 Objects with sizes comparable to the
wavelength scatter or diffract the sound
wave
 Objects with sizes more than 5-10
wavelengths reflect the sound wave

10.  The effect of wind currents is to bend the
sound wave
 In addition to the drop of sound pressure
due to spreading, sound can be
absorbed/scattered by the medium
 Especially humidity effects on ultrasound

11.  Since sound velocity
depends on temperature,
c=331 + 0.6 T in air
temperature gradients
result in a velocity gradient.
 A velocity gradient
produces refraction of the
sound wave according to
Snell’s law:
2
2
1
1 sin
sin
c
c




12.  For humans,
hearing is
normally limited to
frequencies
between about
20 Hz and
20,000 Hz
(20 kHz).
 The sound waves
cause pressure
changes against
our ear drum
sending nerve
impulses to our
brain.
Perception of sound

13.  The vibration
disturbs the air
around it.
 This makes changes
in air pressure.
 These changes in air
pressure move
through the air as
sound waves.

14.  The mechanical vibrations that can be
interpreted as sound are able to travel
through all forms of matter,
gases, liquids, solids, and plasmas. The
matter that supports the sound is called
the medium. Sound cannot travel through
a vacuum
•1 Longitudinal and transverse waves
•2 Sound wave properties and characteristics
•3 Speed of sound
• 5 Noise

15.  Compressions
The close together part of the wave
 Rarefactions
The spread-out parts of a wave
Wave particles vibrate back
and forth along the path that
the wave travel
1 Longitudinal and transverse waves
Longitudinal or compression waves.

16.  Crests
Highest part of a wave
 Troughs
The low points of the
wave

17. Frequency= waves/time

18. Properties of Sound

19.  Frequency: the number of wave
cycles/second (f).
 Wavelength: The distance (in m)
between two wave maxima
 Phase: time offset of wave zero
crossing compared to reference

20. Amplitude-
is the maximum distance
the particles in a wave
vibrate from their rest positions.
Wave Velocity - is the speed with which a wave
crest passes by a particular point in space
It is measured in meters/second.
Wave Velocity = Frequency  Wavelength

21.  Pitch (how high or low)
 Loudness (volume)
 Timbre (tone color)

22.  The vibration patterns of
some sounds are
repetitive.
 Vibration patterns are
also called waveforms.
 Each repetition of a
waveform is called a
cycle.
 We can hear frequencies
between 20 hertz or
cycles (vibrations) per
second (low pitches)to
20 kilohertz, i.e. 20,000
Hz (high pitches).

23.  When the frequency of a
sound doubles we say
that the pitch goes up
an octave.
 We can hear a range of
pitches of about ten
octaves.
 Many animals can make
sounds and hear
frequencies that are
beyond what we can
hear.

24.  To create vibrations
energy is used.
 The greater amount
of energy used the
louder the sound.
 The strength of the
changes in air
pressure made by
the vibrating object
determines
loudness.

25.  As the distance
from the source
increases the
amount of
power is spread
over a greater
area.
 The amount of
power per
square meter is
called the
intensity of the
sound.

26. Sound Loudness (dbs) Hearing
Damage
Average Home 40-50
Loud Music 90-100 After long
exposure
Rock Concert 115-120 Progressive
Jet Engine 120-170 Pain

27. Ultrasound
- sound waves with frequencies above the
normal human range of hearing.
Sounds in the range from 20-100kHz
Infrasound
- sounds with frequencies below the
normal human range of hearing.
Sounds in the 20-200 Hz range

28. is the specific property of sound
that enables us to determine
the different types of sound
produced.

29.  Medium velocity (m/s)
 air (20 C) 343
 air (0 C) 331
 water (25 C) 1493
 sea water 1533
 diamond 12000
 iron 5130
 copper 3560
 glass 5640
The speed of sound depends on the medium the waves pass
through, and is a fundamental property of the material.

30. Noise is defined as
“Sound or a sound that is
loud, unpleasant,
unexpected, or
undesired.”

31.  the result of two or more sound
 waves overlapping

32. Different sounds that you hear include
(A) noise, (B) pure tones, and (C)
musical notes.

33. Doppler Effect
is the apparent change in the
frequency of a sound caused by
the motion of either the listener
or the source of the sound.

35. A moving sound source
 ’compress’ the sound
waves in front of the
object in the motion
direction
-Increased frequency
 Rarify the sound waves
behind the object
-Decreased frequency

36.  Sounds from Moving Sources.
› A moving source of sound or a
moving observer experiences an
apparent shift of frequency called
the Doppler Effect.
› If the source is moving as fast or
faster than the speed of sound, the
sound waves pile up into a shock
wave called a sonic boom.
› A sonic boom sounds very much
like the pressure wave from an
explosion

37. Thank you