Sound waves are mechanical compression waves that cause alternating high and low pressure regions in air or other materials. The speed of sound depends on the density and compressibility of the medium and follows the formula velocity=sqrt(elasticity/inertia). Humans can hear sounds between 20-20,000 Hz but some animals can hear infra or ultrasound beyond this range. Sound is measured on the decibel scale which is logarithmic to account for the wide range of sound amplitudes.

1. Sounds

2. 2
Sound
 Sound waves are mechanical,
compression waves which are in
general longitudinal in nature-meaning
that the particles vibrate parallel to the
direction of the wave’s velocity.

3. Sound Waves
A sound wave is a wave of alternating high-
pressure and low-pressure regions of air.

4. Properties of Sound Waves

5. 5
Speed of sound waves
 The speed of sound waves in a medium
depends on the compressibility and
density of the medium.
 The speed of all mechanical waves
follows an expression of the general
form
propertyinertial
propertyelastic
v

6. The frequency of sound
 We hear frequencies of sound as
having different pitch.
 A low frequency sound has a
low pitch, like the rumble of a big
truck.
 A high-frequency sound has a
high pitch, like a whistle or siren.
 In speech, women have higher
fundamental frequencies than men.

8. Complex sound

9. Loudness
Every increase of 20
dB, means the
pressure wave is 10
times greater in
amplitude.
Logarithmic
scale
Linear
scale
Decibels (dB) Amplitude
0 1
20 10
40 100
60 1,000
80 10,000
100 100,000
120 1,000,000

10. 10
Decibel (dB) scale
 The range of sound powers and sound pressures is very
wide.In order to cover this wide range while maintaining
accuracy, the logarithmic decibel (dB) scale was selected.
 Decibel is a dimensionless unit related to the logarithm
of the ratio of a measured quantity to a reference
quantity.
 Sound power level is the acoustical power radiated by a
source with respect to the standard reference of 10-12
watts.
SPL = 10 Log (W/Wre)
 The “w” subscript identifies the fact this equation deals
with power in units of watts.

11. Common Sounds and their Loudness

12. 15.2 Sound Waves
We know sound is a wave because:
1. Sound has both frequency and wavelength.
2. The speed of sound is frequency times wavelength.
3. Resonance happens with sound.
4. Sound can be reflected, refracted, and absorbed and also shows
evidence of interference and diffraction.

13. 15.2 Amplitude of sound
 The amplitude of a
sound wave is very
small.
 Even a loud 80 dB
noise creates a
pressure variation of
only a few millionths
of an atmosphere.

14. The wavelength of sound

15. The speed of sound
 The speed of sound in air is 343
meters per second (660 miles
per hour) at one atmosphere of
pressure and room temperature
(21°C).
 An object is subsonic when it is
moving slower than sound.

16. The speed of sound
 We use the term supersonic to describe motion at
speeds faster than the speed of sound.
 A shock wave forms where the wave fronts pile up.
 The pressure change across the shock wave is what
causes a very loud sound known as a sonic boom.

17. The speed of sound
 The speed of a sound
wave in air depends
on how fast air
molecules are
moving.
 The speed of sound
in materials is often
faster than in air.

18. Sound waves and boundaries
 Like other waves, sound
waves can be reflected by
surfaces and refracted as
they pass from one
material to another.
 Sound waves reflect from
hard surfaces.
 Soft materials can absorb
sound waves.

19. Fourier's theorem
 Fourier’s theorem
says any complex
wave can be made
from a sum of single
frequency waves.

20. Sound spectrum
 A complex wave is really a sum of component
frequencies.
 A frequency spectrum is a graph that shows the
amplitude of each component frequency in a complex
wave.

21. Classification of Sounds
Sounds
Audible
waves
Infrasonic
waves
Ultrasonic
waves

22. 22
Audible waves
 They are within the range of
sensitivity of the human ear.
 The range of human hearing
stretches between 20-20000 Hertz.
 They can be generated in a variety of
ways, such as by musical
instruments human voices, or loud
speakers

23. PH 0101 UNIT 1 LECTURE 5 23
Infrasonic waves
 These waves have frequencies
below the audible range, that is
less than 20 Hertz.
 Elephants can use infrasonic
waves to communicate with each
other, even when separated by
many kilometers.

24. 24
Ultrasonic waves
 They have frequencies above the
audible range, that is greater than
20000 Hertz.
 Some animals can emit these
sounds.
 Bats, for example, emit and hear
ultrasound waves, which they use
for locating prey and for
navigating.

25. Bats: Navigating with ultrasound
 Bats make high-pitched chirps which are
too high for humans to hear. This is called
ultrasound
 Like normal sound, ultrasound echoes off
objects
 The bat hears the echoes and works out
what caused them
 Dolphins also navigate with ultrasound
 Submarines use a similar method called
sonar
o We can also use ultrasound to look inside the
body…