1. Sound Waves Vibration of a tuning fork

2. Pressure Waves Sound waves is usually analyzed from the point of view
of pressure because the pressure is easier to measure
than displacement.
Compression (pressure higher)
Rarefaction (pressure lower)
Atmospheric pressure

3. Pressure and Displacement in Sound Waves
A sound wave can be described in terms of displacements
of particles in the medium or in terms of pressure fluctuations.
Displacement
Pressure fluctuation
Position
The displacement graph is π/2 out of phase with the
pressure graph.

4. Variation of the air pressure in an air column

5. Pressure and displacement in an air column
When the air is constrained to a node, the air motion will be alternately squeezing toward that point and expanding away from it, causing the pressure variation to be at a maximum.

6. Frequency Spectrum of Sound Waves
Frequency in Hz on a log scale
/Hz
20 kHz
20 Hz
Used in medical
diagnosis
Subsonic
Range of sound frequencies
heard by human ears
Ultrasound
Note that the audible range varies somewhat from one individual to another.

7. The Speed of Sound
Typically there are two essential types of properties which effect wave speed.
Inertial properties (e.g. mass density, )
The greater the inertia the slower the wave.
Elastic properties (e.g. Young modulus, E)
The higher the elasticity the faster the wave.
Speed of Sound in a long solid rod
Where E is the Young modulus and
 is the density of the solid rod.

8. Speed of Sound in air
The speed of a sound wave in air depends upon the properties of the air
the temperature
and the temperature will affect the strength of the particle interactions (an elastic property).
the pressure
The pressure of air (like any gas) will affect the mass density of the air (an inertial property).
v = TC (m s-1)

9. Measuring the Speed of Sound in Air (1)
The speed of sound in air can be measured by
Stationary waves pattern established between a loudspeaker and a metal reflector.
Using a resonance tube (or Kundt’s tube)
Glass Tube
Signal
generator

10. Measuring the Speed of Sound in Air (2)

11. Measuring the speed of sound using echoes

12. Speed of Sound in Various Materials
Speed of sound /m s-1
Air (0 oC)
331
Air (20 oC)
344
Helium (20 oC)
999
Hydrogen (20 oC)
1330
Water (0 oC)
1400
Water (20 oC)
1480
Mercury (20 oC)
1450
Aluminium
6420
Steel
5940
Lead
1960

13. Effect of temperature and relative humidity on the speed of sound in air

14. Loudness Loudness is a sensation in the consciousness of a human
being.
The assessment of loudness is controlled by
(1) The sound wave intensity, which depends on
(a) the square of the amplitude,
(b) the frequency,
(c) the speed,
(d) the density of the medium.
(2) The sensitivity of the hearer to the particular frequency
being sounded.

15. Energy Transport and the Amplitude of a Wave
The energy transported by a wave is directly proportional to the square of the amplitude of the wave.
E  A2

16. Sound Intensity
The sound intensity in a specified direction is the rate of sound energy flowing through a unit area normal to that direction. The sound intensity is normally measured in watt per square metre (W/m2). A
Energy flow
Unit : W m-2

17. Variation of Intensity with Distance (1)
The intensity decreases away from the source because
- the energy is distributed through a larger volume,
(For a point source and isotropic medium the intensity
will obey the inverse square law.)
- of attenuation: the wave energy is gradually converted
by an imperfectly elastic medium into the internal
energy of the medium’s molecules.

18. Variation of Intensity with Distance (2)
As the wave moves outward, the energy it carries is spread over a larger and larger area since the surface area of a sphere of radius r is 4r2. r
If the power output is P,
then the average intensity,
through a sphere with
radius r is

19. Variation of Intensity with Distance (3)
Intensity level
Distance
If a source of sound can be considered as a point,
where r is the distance from the source.

20. Intensity Level (Decibel)
The intensity level  of a sound wave is defined by
where I = the intensity of sound,
Io= 1 pW m-2 (Threshold of hearing)
Unit : decibel (dB)
The dB-scale is a logarithmic, the reason for measuring
sound this way is that our ears (and minds) perceive
sound in terms of the logarithm of the sound pressure,
rather than the sound pressure itself.

21. Difference in Sound Intensity Level
Consider two loudspeakers playing sounds with power P1 and P2 respectively.
The difference in decibels between the two is defined to be

22. Intensity of Various Sounds
Source of the Sound
Intensity Level/dB
Intensity/W m-2
Jet plane at 30 m
140
100
Threshold of pain
120 1
Loud indoor rock concert
120 1
Siren at 30 m
100
1 × 10-2
Automobile moving at 90 km h-1 75
3 × 10-5
Busy street traffic 70
1 × 10-5
Ordinary conversation, at 50 cm 65
3 × 10-6
Quiet radio 40
1 × 10-8
Whisper 20
1 × 10-10
Rustle of leaves 10
1 × 10-11
Threshold of hearing
1 × 10-12

23. Sensitivity of Human Ear as a Function of Frequency
Each curve represents sounds that seemed to be equally loud.
Note that the ear is most sensitive to sounds of frequency between 2 kHz and 4 kHz.

24. Sound Proofing (1)
Soundproofing of a room involves the isolation of that room
from audible sound from the outside and may be taken to
include the acoustic damping of the room itself.
Preventing the entrance of sound from the
outside is accomplished by
-sealing openings,
-making the walls absorbent of
sound, and
-minimizing the passage of
sound energy through the solid structures of the walls.

25. Sound Proofing (2)
Sound absorbing materials such as foam insulation in the walls help with both the sealing and absorbing of mid-range to high frequency sounds.