What is sound, different characteristics and some facts

1. Sound

2. Content
• Production of sound
• Propagation of sound
• Characteristics of a sound wave
• Reflection of sound
• Range of hearing
• Ultrasound
• Conclusion
• Facts

3. Production of sound
Vibrating tuning fork splashes water
and produces sound.
• Sound is produced by the vibration of objects.
• Vibration is the rapid to and fro movement of an
object.
• Sound of human voice is produced by vibrations
in the vocal cords.

4. Propagation of Sound
• The sound produced by a vibrating object travels through a medium to a listener.
• The medium can be solid, liquid or gas.
• When an object vibrates, the particles around the medium vibrates.
• The particle in contact with the vibrating object is first displaced from its equilibrium position.
• It then exerts a force on the adjacent particle and the adjacent particle is displaced from its position
of rest.
• After displacing the adjacent particle the first particle comes back to its original position.
• This process repeats in the medium till the sound reaches the ear.
• The disturbance produced by the vibrating body travels through the medium but the particles do not
move forward themselves.
• A wave is a disturbance which moves through a medium by the vibration of the particles of the
medium.
• So sound is considered as a wave. Sound waves are characterized by the motion of particles in the
medium and are called mechanical waves.

5. Propagation of sound in air
• Air is the most common medium through
which sound travels.
• When a vibrating object moves forward, it
pushes and compresses the air in front of it
forming a region of high pressure called
compression (C).
• The compression moves away from the
vibrating object.
• When the vibrating object moves backward, it
forms a region of low pressure called
rarefaction (R).
• As the object moves to and fro rapidly, it
produces a series of compressions and
rarefaction in the air which makes the sound
to propagate in the medium.

6. Characteristics of sound wave
• Wavelength
• Amplitude
• Frequency
• Time period
• Speed

7. Wave length
• Compressions are the regions of high pressure and density where the particles are crowded and are represented by the
upper portion of the curve called crest.
• Rarefactions are the regions of low pressure and density where the particles are spread out and are represented by the
lower portion of the curve called trough.
• The distance between two consecutive compressions (crests) or two consecutive troughs is called wave length. It is
represented by the symbol (λ).
• Its SI unit is metre.

8. Amplitude
• The amplitude of a sound wave is the measure of
the height of the wave.
• It depends on the amount of energy that started the
waves.
• It is usually represented by the letter A.
• It’s unit is density or pressure.
• The loudness or softness of a sound is determined
basically by its amplitude.
• The louder the sound, the larger the amplitude.

9. Frequency
• We know that when sound is propagated through a medium, the density of the medium oscillates
between a maximum value and a minimum value.
• The change in density from the maximum value to the minimum value, again to the maximum
value, makes one complete oscillation.
• The number of such oscillations per unit time is the frequency of the sound wave.
• It is usually represented by V.
• Its SI unit is hertz (Hz).

10. Time period
• The time taken by two consecutive compressions or rarefactions to cross a fixed point is
called the time period of the wave.
• In other words, we can say that the time taken for one complete oscillation in the density of
the medium is called the time period of the sound wave.
• It is represented by the symbol T.
• Its SI unit is second (s).
• Frequency and time period are related as : ν = 1/T.

11. Speed of sound
• The rate at which sound travels in a given medium is called
speed of sound.
• The speed of Sound depends on the nature of material
through which it travels. In general, sound travels slowest in
gases, faster in liquids and fastest in solids. Speed of sound in
air is 344 m/s at room temperature (20ºC). Speed of sound in
water is 1500 m/s. Speed of sound in iron is 5130 m/s.
• The speed of sound depends on the temperature of the
medium. Speed of sound in air at 0ºC is 332 m/s and at 20ºC, it
is 344 m/s.
• The speed of sound depends on the humidity of air. Speed
of sound in dry air is less but more in humid air.
• Speed= wavelength * frequency

12. Reflection of sound
• When sound travels in a given medium, it strikes the surface of
another medium and bounces back in some other direction, this
phenomenon is called the reflection of sound.
• Reflection of sound is more prominent if the surface is rigid and
smooth, and less if the surface is soft and irregular. Soft materials
absorb large amount of sound energy.
• The waves are called the incident and reflected sound waves. The
sound waves that travel towards the reflecting surface are called the
incident sound waves. The sound waves bouncing back from the
reflecting surface are called reflected sound waves.
• The following two laws of reflection of light are applicable to sound
waves as well:
• The incident wave, the normal to the reflecting surface and the
Hhreflected wave at the point of incidence lie in the same plane.
• The angle of incidence ∠i is equal to the angle of reflection ∠r.
•

13. Echo
• Echo is a reflection of sound that arrives at the listener
with a delay after the direct sound.
• The delay is directly proportional to the distance of the
reflecting surface from the source and the listener.
• A true echo is a single reflection of the sound source.
• The human ear can not distinguish between two
successive sounds if the period between them is less
than 0.1 second. Thus, for hearing distinct echoes, the
minimum distance of the obstacle from the source of
sound must be 17.2 m.
• Echo can be used to calculate the speed of sound.
V = 2d/t.

14. Sample question
1- A person clapped his hands near a cliff and heard the echo after 2 s.
What is the distance of the cliff form the person if the speed of sound, v
is 346 m/s?
Solution-
Time taken= 2 s
Speed of sound= 346 m/s
Distance travelled= v*t
= 346*2= 692m
Distance between Person and cliff= 692/2= 346 m

15. Reverberation
• Reverberation is the phenomenon of overlapping of sound caused by multiple reflections until it is
reduced to a value where it is no longer audible.
• In an auditorium or big hall, reverberation is highly undesirable. To reduce reverberation, the roof and
walls of the auditorium are generally covered with sound-absorbent materials like compressed
fiberboard, rough plaster or draperies.
• The phenomenon of reverberation is utilized by the producers of living or recorded music in order to
enhance sound quality.
• Reverberation is the persistence of sound after the sound source has been stopped. It results from a
large number of reflected waves which can be perceived by the brain as a continuous sound. On the
other hand, an echo occurs when a pulse of sound can be heard twice.

16. Range of Hearing
• The audible range of sound for human beings extends
from about 20 Hz to 20000 Hz.
Children under the age of five and some animals, such as
dogs can hear up to 25 kHz.
• As people grow older their ears become less sensitive to
higher frequencies.
• Sounds of frequencies below 20 Hz are called infrasonic
sound or infrasound.
• Rhinoceroses communicate using infrasound of frequency
as low as 5 Hz.
• Frequencies higher than 20 kHz are called ultrasonic
sound or ultrasound.
• Ultrasound is produced by dolphins, bats, tortoises, etc.

17. Ultrasound
• Ultrasonic sound is used to clean objects like electronic
components. The components to be cleaned are kept in a
cleaning solution and ultrasonic waves are sent into the
solution. Due to the high frequency, the dirt particles get
detached from the components.
• Doctors commonly use ultrasound to study a developing fetus
(unborn baby), a person's abdominal and pelvic organs,
muscles and tendons, or their heart and blood vessels.
• Ultrasonic sound is used to detect cracks in metal blocks.
Ultrasonic waves are sent through the metal blocks and if there
are cracks, the waves are reflected back and the cracks can be
detected.
• Ultrasonic sound is used in ultra sound scanners for getting
images of internal organs of the human body.
• Ultrasonic sound is used to break small stones formed in the
kidneys into fine grains so that they are removed through the
urine.

18. Conclusion
• Sound is produced due to vibration of different objects.
• Sound travels as successive compressions and rarefactions in the medium.
• In sound propagation, it is the energy of the sound that travels and not the particles
of the medium.
• Sound cannot travel in vacuum.
• The change in density from one maximum value to the minimum value and again to
the maximum value makes one complete oscillation.
• The distance between two consecutive compressions or two consecutive rarefactions
is called the wavelength, λ.
• The time taken by the wave for one complete oscillation of the density or pressure of
the medium is called the time period, T.
• The number of complete oscillations per unit time is called the frequency (ν),1/T
• The speed, frequency, and wavelength of sound are related by the equation, v = λν.
• The speed of sound depends primarily on the nature and the temperature of the
transmitting medium.

19. Facts
• Sound travels 4 times faster in water than air, having a speed of 1,482 m/s
• Sound is used by many animals to detect danger, warning them of possible attacks before
they happen.
• The scientific study of sound waves is known as acoustics.
• The loudest natural sound on earth is caused by an erupting volcano.
• Horror films like to use infrasound, which is below the range of human hearing. It creates
shivering and anxiety.