Download Sound Nature Of The Sound Waves: Longitudinal

Sound
SOUND
Sound is a form of energy which makes us hear. The nature
of sound produced depends upon the nature of material, the
number of vibrations the vibrating body makes in one second.
Sound is a wave motion.
Production Of Sound Waves
Sound is produced when an object vibrates.
medium. Sound travel 5 times faster in water and 15 times
faster in solid than air.
Factors On Which The Speed Of Sound Depends
1. Temperature
When the temperature increases, the speed of sound
increases.
2.. Humidity Of Air
As the humidity of air increases, the speed of sound through it
also increases.
Frequency Range Of Hearing In Humans
It is between 20 HZ to 20000 Hz.
Infrasonic Sound
A sound with frequency less than 20 Hz.
A vibrating object, which produces sound has a certain amount
of energy which travels in the form of sound waves. The
energy required to make an object vibrate and produce sound
is provided by some outside source like our hand, wind etc.
Sound can be produced by the following methods.
1. By vibrating membranes (in table)
2. By vibrating strings (in sitars)
3. by vibrating air (in flute)
4. by vibrating plates (in bicycle bell)
Propagation Of Sound
Sound waves are longitudinal waves. Sound waves in air
consist of compressions and rarefactions. When an object
vibrates then the air layers around it also start vibrating in
exactly the same way and carry sound waves from the sound
providing object to our ears.
In the transmission of sound through air, there is no actual
movement of air from the sound producing body to our ears.
Nature Of Sound Waves
Sound waves are mechanical waves i.e., they require a
material medium to propagate. Sound waves can not travel
through vacuum.
Movement Of Sound Through Solids, Liquids And Gases
The speed of sound is maximum in solids and minimum in
gases and intermediate in liquids. Actually because sound
wave are mechanical waves. They need material medium to
travel. So a substance in which there is density of particles
the speed of sound increases due to the increase in the
transformation of energy by a large number of particles of the
Ultrasonic Sound
A sound with frequency greater than 20000 Hz.
Nature Of The Sound Waves: Longitudinal
Sound waves are longitudinal waves. In the longitudinal waves
the individual particles of the medium move in a direction
parallel to the direction of propagation of the disturbance. The
particles do not move from one place to another but they
simply oscillate back and forth about their position of rest. This
is exactly how a sound wave propagates.
Compression
A compression is that part of a longitudinal wave in which the
particles of the medium are closer to one another than they
normally are. Or the portions of medium in which the particles
are pushed closer having a large number of particles per unit
volume. These are the regions where density as well as
pressure is high.
Rarefaction
A rarefaction is that part of longitudinal wave in which the
particles of the medium are farther apart than normal. Those
portions in which the density of the particles and the pressure
is lesser than normal.
Transverse Waves
There is another type of wave called transverse wave. In a
transverse wave the particles do not oscillate along the line of
wave propagation but oscillate up and down about their mean
position as the wave travels. Light is an example of transverse
wave in which the particles of the wave move in a direction
perpendicular to the direction of propagation of wave.
Characteristics Of The Sound Waves
The
(i)
(ii)
(iii)
(iv)
(v)
characteristics of sound waves are: Frequency
Amplitude
Velocity Of Wave
Wavelength
Time period
1. Wavelength
The distance between two nearest points in a wave which are
in the same phase of vibration is called wavelength. In other
words we can say that the distance between two nearest
crests of a wave or the distance between two nearest troughs
of a wave is called wavelength. Wavelength is denoted by
(Lambda). S.I. unit of wavelength is m.
3. Frequency
The number of waves produced per second. It is denoted by
υ (nu). The S.I. unit of frequency is Hertz (Hz). The frequency
of a work remains same in air, water, oil etc. Hertz is equal to
vibrations per second.
4. Time Period
The time taken to complete are vibration is called time period.
Time period is denoted by T.
Relation Between Time Period And Frequency
The time required to produce one complete wave is called time
period of the wave.
Suppose time period of a wave is T seconds
Now,
In T Seconds, number of waves produced = 1
So,
1
In 1 second, number of waves produced =
T
But the number of waves produced in 1 second is called
frequency. This means that frequency of wave of time period
1
.
T will be
T
So, we can say that the frequency of a wave is the reciprocal
of its time-period, i.e
1
Or
f=
T
1
Frequency =
Time Period
5. Velocity Of Wave
The distance traveled by a wave in one second is called
velocity of wave. It is denoted by ν.
S.I. Unit
m/s
Or
Pitch, Loudness And Quality
Pitch
2. Amplitude
The maximum displacement of the particles of the medium
from their original undisturbed positions, when a waves passes
through the medium is called amplitude of the wave. This
describe the size of the wave. The S.I. unit is metre. It is
denoted by A.
ms −1
Pitch is the characteristic of sound, which distinguishes
between a shrill sound and a grave sound. Pitch is the
interpretation of frequency of the emitted sound by the brain.
Higher the pitch, higher is the frequency. A high pitch sound
corresponds to more number of compressions and rarefactions
passing through a point per unit time.
Example
Sound from a flute has high pitch whereas sound from a violin
is of low pitch.
Loudness
Loudness of the sound is defined as the degree of the
sensation produced on the ear.
The loudness or softness of a sound depends upon amplitude
of the sound. Amplitude of sound wave depends upon the
force with which an object is made to vibrate. When the table
is hit lightly soft sound is produced. When a table is hit hardly
a loud sound is produced. A loud sound can travel a larger
distance as they have more energy. As sound waves spread
out from its source its amplitude as well as its loudness
decreases.
Quality
The quality or timber of sound is that characteristic which
enables us to distinguish one sound from another having the
same pitch and loudness. The more the sound is pleasant, the
rich is its quality.
Tone
A sound of single frequency is called a tone.
Note
The sound which is produced due to the mixture of several
frequencies is called a note and is pleasant to listen to.
Intensity
The amount of sound energy passing each second through
unit area is called intensity of sound. Sometimes the terms
intensity and loudness is used interchangeably but they are
not same. Loudness is a measure of the response of the ear to
sound. Even when two sounds are of equal intensity, we may
hear one sound louder than other simply because our ear
detects it better.
Reflection Of Sound Waves
The returning back of sound waves after striking a hard
surface is called reflection of sound. Sound can be reflected
from any hard surface whether smooth or rough. The
reflection of sound causes echoes.
Echoes
The repetition of sound caused by the reflection of sound
wave is called on echo. The sensation of sound persists in our
brain for about .1 s. To hear a distinct echo the time interval
between the original sound and the reflected one must be at
least .1 s. For hearing distinct echoes the minimum distance of
the obstacle from the source of sound must be 17.2 m. This
distance will change with the temperature of air. Echoes may
be heard more than once due to successive or multiple
reflections.
Multiple Echoes
When the sound is reflected repeatedly from a number of
obstacles, more than one echoes are produced called multiple
echoes.
Minimum Distance For An Echo To Be Heard
We know,
Distance = Speed × Time
Now,
Speed of sound in air = 344 m/s
1
s)
Time taken = 0.1 s (
10
Because 0.1 is the minimum time interval between two sounds
to be heard clearly. Therefore,
Distance traveled = 344 × 0.1
= 34.4 m
This distance is the total distance traveled by sound. But, our
distance from the sound reflecting surface such as a wall to
hear an echo should be half of the total distance traveled by
sound, i.e.
34.4
= 17.2 m.
2
From this we conclude that the minimum distance from a
sound reflecting surface to hear an echo is 17.2 m.
Reverberation
The repeated reflection that results in the persistence of sound
is called reverberation. In an auditorium and a big hall
excessive reverberation is highly undesirable, therefore to
reduce it the walls of the auditorium are generally covered
with sound absorbing materials like compressed fireboard,
rough plaster or draperies. The seat materials are also
selected on the basis of their sound absorbing properties.
Uses Of Multiple reflection Of Sound
1.
Megaphones and musical instruments like trumpet and
shehnais have a tube followed by conical opening which
reflects sound successively.
2.
In stethoscope the sound of the patient’s heart beat
reaches the doctor’s ears by multiple reflections.
SONAR : Sound Navigation And Ranging
Sonar is a device which is used to find the depth of a sea or to
locate the under water things.
Working Of SONAR
In SONAR ultrasonic waves are sent out in all directions from
the ship and reflected waves are received. By measuring the
time between sending the sound waves and receiving the
reflected waves the distance of the under water object from
the ship can be calculated.
3.
Generally the ceilings of the concert halls, conference
halls and cinema halls are curved so that the sound after
reflection reaches all corners of the hall.
Apparatus Of SONAR
The SONAR apparatus consists of two parts.
1. A transmitter which emits ultrasonic waves.
2. A receiver which revives the reflected ultrasonic waves.
Range Of Hearing
The audible range of sound for human beings extends from
about 20 Hz to 20,000 Hz. Children under the age of five and
some animals such as dogs can hear upto 25,000 Hz.
Ultrasound
The sound wave having frequency greater than 20000 Hz
which can not be heard by human beings are called ultrasonic
sound or ultrasound. Dogs, bats and dolphins can hear
ultrasound. Due to its very high frequency ultrasound has a
greater penetrating power.
Supersonic
Supersonic refers to sped of an object which is grater than
speed of sound. Nowadays so many jet aeroplanes have been
developed which have the speed 5 to 7 times greater than
sound.
Applications Of Ultrasound
(i) Ultrasound is used to investigate inside the human body.
(ii) Ultrasound is used in sonar to measure the depth of sea
or ocean and to locate under water objects like shoals of
fish, ship works, submarines, sea rocks and hidden ice
bergs in sea.
(iii) Ultrasound is used for finding the level of a liquid in a
metal tank.
(iv) Ultrasound is used in the industry for detecting flaws
(faults) in metal blocks or sheets.
(v) Ultrasound is used in the treatment of muscular pain and
in treatment of disease called arthritis.
(vi) Ultrasound may be employed to break small stones
formed in the kidneys into fine grains. These grains later
get flushed out with urine.
(vii) Ultrasonography is used for the examination of the foetus
during pregnancy to detect congenial defects and growth
abnormalities.
(viii) Ultrasound is generally used to clean parts located in hard
to reach places. For example spiral tube, odd shaped
parts, electronic components etc.
(ix) Ultrasonic waves are used for ‘echocardiography’.
(x) Ultrasound is used by bats to find their prey.
Infrasound
The sound frequencies below 20 Hz are called infrasonic
sounds or infrasound. For example the vibrations of a
pendulum.
Structure And Working Of Human Ear
The ear allows us to convert pressure variations in air with
audible frequencies into electric signals that travel to brain via
the auditory nerve. Human ear converts sound energy to
mechanical energy. It allows us to perceive the loudness of
sound by detection of the wave’s amplitude.
The
1.
2.
3.
human ear consists of three basic parts: The outer ear
The middle ear
The inner ear
The outer ear is called ‘pinna’. It collects the sound from the
surroundings. The collected sound passes through the auditory
canal. At the end of the auditory canal there is a thin
membrane called the ear drum or tympanic membrane. When
a compression of the medium reaches the ear drum the
pressure on the outside of the membrane increases and forces
the eardrum inward. Similarly an eardrum moves outward
when a rarefaction reaches. In this way the eardrum vibrates.
The middle ear consists of three bones the hammer, anvil and
stirrup. These three bones amplify the vibrations several
times.
The inner ear consists of cochlea. The middle ear transmits the
amplified pressure variations received from the sound waves
to the inner ear. In the inner ear the pressure variations are
turned into electrical signals by the cochlea. These electr8ical
signals are sent to the brain via the auditory nerve, and the
brain interprets them as sound.