1. ACOUSTICS
The Science of Sound

2. Imagine our world without
sound.

3. Sound brings things to life.

4. WHAT THEN IS
SOUND?

5. Sound Defined
 All sounds are produced by the
vibrations of material objects.
 It is a longitudinal wave that carries
energy produced by vibrations of matter
and is propagated through a
medium such as solid, liquid or gas.

6. Not all sounds are audible
 Audible range for humans is between 20
Hz to 20000 Hz
 Lower than this range is classified as
INFRASONIC
 Higher than this range is classified as
ULTRASONIC

7. The reason for hearing

8. Some animals can detect
ULTRASONIC frequencies
50,000 Hz
100,000 Hz

9. How about INFRASONICS?
 Sources include earthquakes, thunder,
volcanoes and waves produced by
vibrating heavy machinery
 May pose damage to human body including
irritation of internal organs

10. Sound is a pressure wave
Compression
Rarefaction

11. THE SPEED OF
SOUND

12. Prelude
 Sound is a mechanical wave.
 Its propagation depends on the properties
of a medium.

13. Can you hear me?

14. How about here?

15. Factors Affecting Sound
Speed
 Phase of the medium
 Density and elasticity of the medium
 Temperature

16. Phase of the Medium

17. Density and Elasticity of the
Medium (Solid)
 The denser and more elastic the medium,
the faster the transmission of sound.
v – velocity of sound
Y – elastic modulus of
the solid medium
ρ – density of the solid
medium

18. Sample Problem 1
 Find the speed of sound in an aluminum rod
at a temperature of 20°C. The Young’s
modulus for aluminum is 70 x 109 N/m2 and
its density is 2.7 x 103 kg/m3.

19. Density and Elasticity of the
Medium (Liquid)
 The denser and more elastic the medium,
the faster the transmission of sound.
v – velocity of sound
B – bulk modulus of the
solid medium
ρ – density of the liquid
medium

20. Sample Problem 2
 Find the speed of sound in water at a
temperature of 20°C. The bulk modulus of
water is 2.2 x 109 N/m2 and its density is 1.0 x
103 kg/m3.

21. The Speed of Sound in Various
Solid Media
Medium
Speed (m/s)
Aluminum
5100
Copper
3500
Iron
4500
Glass
5200
Polystyren
e
1850

22. The Speed of Sound in Various
Liquid Media
Medium
Speed (m/s)
Alcohol
1125
Mercury
1400
Water
1500

23. Sound Speed in Air and
Temperature
 As temperature increases, sound speed in
air increases at a definite rate.
 Vsound = (Vsound at 0°C + 0.6T°C)m/s
 Vsound at 0°C = 331 m/s

24. Sample Problem 3
 In the cold months of December and
January, the temperature in Baguio can go
down to 10°C. What is the speed of sound in
air?

25. Sample Problem 4
 An automatic focus camera is able to focus on
objects by use of an ultrasonic sound wave.
The camera sends out sound waves that
reflect off distant objects and return to the
camera. A sensor detects the time it takes for
the waves to return and then determines the
distance an object is from the camera. If the
temperature of the air is 20°C and the
infrasonic frequency returns to the camera
0.150 seconds after leaving the camera, how
far away is the object?